Contrasting temperature responses in seasonal timing of cercariae shedding by Rhipidocotyle trematodes

Abstract

Global warming is likely to lengthen the seasonal duration of larval
release by parasites. We exposed freshwater mussel hosts, Anodonta
anatina, from two high-latitude populations to high, intermediate, and
low temperatures throughout the annual cercarial shedding period of the sympatric trematodes Rhipidocotyle fennica and R. campanula, sharing the same transmission pathway. At the individual host level, under warmer conditions, the timing of the cercarial release in both parasite species shifted towards seasonally earlier period while its duration did not change. At the host population level, evidence for the lengthening of larvae shedding period with warming was found for R. fennica. R.campanula started the cercarial release seasonally clearly earlier, and at a lower temperature, than R. fennica. Furthermore, the proportion of mussels shedding cercariae increased, while day-degrees required to start the cercariae shedding decreased in high-temperature treatment in R. fennica. In R. campanula these effects were not found, suggesting that warming can benefit more R. fennica. These results do not completely support the view that climate warming would invariably increase the seasonal duration of larval shedding by parasites, but emphasizes species-specific differences in temperature-dependence and in seasonality of cercarial release.

Full text

Parasitology
cambridge.org/par
Research Article
Cite this article: Taskinen J, Choo JM,
Mironova E, Gopko M (2022). Contrasting
temperature responses in seasonal timing of
cercariae shedding by Rhipidocotyle
trematodes. Parasitology 149,1045–1056.
https://doi.org/10.1017/S0031182022000518
Received: 28 November 2021
Revised: 7 April 2022
Accepted: 11 April 2022
First published online: 16 May 2022
Key words:
Bucephalidae; cercaria; climate change;
Digenea; mollusc; parasite phenology;
temperature; transmission
Author for correspondence:
Jouni Taskinen,
E-mail: jouni.k.taskinen@jyu.fi
© The Author(s), 2022. Published by
Cambridge University Press
Contrasting temperature responses in seasonal
timing of cercariae shedding by Rhipidocotyle
trematodes
Jouni Taskinen1, Jocelyn M. Choo1, Ekaterina Mironova2
and Mikhail Gopko2
1
Department of Biological and Environmental Science, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä,
Finland and
2
A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Leninskij prosp., 33,
119071 Moscow, Russia 3
Abstract
Global warming is likely to lengthen the seasonal duration of larval release by parasites. We
exposed freshwater mussel hosts, Anodonta anatina, from 2 high-latitude populations to high,
intermediate and low temperatures throughout the annual cercarial shedding period of the
sympatric trematodes Rhipidocotyle fennica and R. campanula, sharing the same transmission
pathway. At the individual host level, under warmer conditions, the timing of the cercarial
release in both parasite species shifted towards seasonally earlier period while its duration
did not change. At the host population level, evidence for the lengthening of larvae shedding
period with warming was found for R. fennica.R. campanula started the cercarial release sea-
sonally clearly earlier, and at a lower temperature, than R. fennica. Furthermore, the propor-
tion of mussels shedding cercariae increased, while day-degrees required to start the cercariae
shedding decreased in high-temperature treatment in R. fennica.InR. campanula these effects
were not found, suggesting that warming can benefit more R. fennica. These results do not
completely support the view that climate warming would invariably increase the seasonal dur-
ation of larval shedding by parasites, but emphasizes species-specific differences in tempera-
ture-dependence and in seasonality of cercarial release.
Introduction
Marked seasonal fluctuation in temperature is a characteristic of high-latitude ecosystems.
Such seasonal temperature variation can affect trematode parasites in different ways, including
the timing of seasonal production of infective stages (miracidia and cercariae), since their
release primarily occurs during the warm summer months in temperate and boreal zones
(e.g. Chubb, 1979; Taskinen, 1998a; Karvonen et al., 2004). Indeed, most experimental and
field studies (e.g. Fingerut et al., 2003; reviewed in Poulin, 2006; Thieltges and Rick, 2006;
Studer et al., 2010; Shim et al., 2013; Rosen et al., 2018; Selbach and Poulin, 2020;
Vyhlídalová and Soldánová, 2020) have reported increased release of cercariae with a moderate
temperature rise. However, in some cases, this effect can be transient (Paull et al., 2015), absent
or even negative, most probably because cercariae emergence rates are tended to decrease at
the threshold temperatures and drop with temperature rise (Koprivnikar and Poulin, 2009a,
2009b; Morley and Lewis, 2013). A common expectation is that the predicted climate warming
(IPCC, 2014) will increase the seasonal duration of larval release by parasites as a consequence
of a longer thermal growing season (longer summer) (Marcogliese, 2001; Harvell et al., 2009;
Lõhmus and Björklund, 2015; Prokofiev et al., 2016; Galaktionov, 2017). Such a lengthening of
the cercarial release period has been observed in water bodies receiving thermal effluents
(e.g. Aho et al., 1982). However, experimental long-term manipulations of temperature
conditions over the seasonal cercarial release period are rare. To our knowledge, the only over-
season experimental long-term study, by Paull and Johnson (2014), indicates a seasonal shift,
rather than lengthening, in the timing of cercariae emergence. This shift to a seasonally clearly
earlier occurrence of cercariae emission led to a decrease in parasite transmission and reduced
parasite-induced host pathology due to temporal mismatch between cercariae and their target
host (Paull and Johnson, 2014). Furthermore, whether such lengthening of the seasonal
cercarial shedding period would be a result of a longer cercarial shedding at the individual
host level or a result of the seasonal variation between host individuals has remained unex-
plored. In addition, most experimental studies of cercarial release were conducted at constant
–though different –temperatures, while in natural conditions the temperature fluctuates
during the cercariae shedding period. Finally, as most of the studies have been performed
with snail hosts and marine species, only little is known about temperature effects in fresh-
water bivalve–trematode associations (however, see Morley and Lewis, 2013; Choo and
Taskinen, 2015). This gap demands attention since bivalves are important parts of freshwater
ecosystems providing valuable ecosystem services (e.g. Vaughn, 2018), and are very commonly
infected by trematodes, frequently with high prevalence of infection (Müller et al., 2015).
Bivalves can release a huge amount of cercariae (Taskinen, 1998a) which infect many central

prey fish species and are transmitted to the key predatory fishes of
freshwater ecosystems (Taskinen et al., 1991; Cribb et al., 2001).
Therefore, a long-term study on temperature effects in a fresh-
water bivalve–trematode association, with temperature treatments
mirroring the natural seasonal fluctuations in temperature condi-
tions should be considered necessary.
Cercarial larvae of trematodes emerge over species-specific
temperature conditions both in the laboratory and field studies
(e.g. Fingerut et al., 2003). However, interspecific comparisons
of cercarial production in varying temperature conditions that
can reveal species-specific responses have been utilized quite
rarely and mainly focus on short-term temporal variability in cer-
carial emergence (e.g. diurnal rhythms) rather than on large-scale
seasonal patterns (de Montaudouin et al., 2016; Prokofiev et al.,
2016; Vyhlídalová and Soldánová, 2020; see, however, field obser-
vations by Fingerut et al., 2003; Koprivnikar and Poulin, 2009a).
In addition, in most previous studies, the influence of temperature
on cercariae shedding has been compared between parasite spe-
cies with different transmission pathways, while species-specific
differences in the closely related sympatric parasites sharing the
same hosts are rarely studied (Vyhlídalová and Soldánová,
2020). In the present study, we used 2 closely related, sympatric
trematodes, Rhipidocotyle campanula and R. fennica, in their
shared first intermediate host, freshwater mussel Anodonta ana-
tina. The parasites also have the same second intermediate host,
the cyprinid fish Rutilus rutilus (Taskinen et al., 1991; Gibson
et al., 1992).
In addition, though the importance of studying temperature-
dependent cercariae output in high-latitude areas (>60°) has
been highlighted earlier (Morley and Lewis, 2013; Studer and
Poulin, 2014; Galaktionov, 2017), experimental investigations
are still scarce (Prokofiev et al., 2016). They would be timely
since impacts of the climate change are predicted to be the
most pronounced at high latitude regions. For example, climate
models predict an increase in annual temperature from 2 to 7°C
by the 2080s compared to a 1961–1990 baseline period in the cur-
rent study region Finland at 60–70°N (Jylhä et al., 2004), and in
the temperate lakes of the northern hemisphere in general
(Sharma et al., 2007).
Production of cercariae is an important component of the
complex life cycle of trematodes, underpinning transmission to
the next host and hence influencing the fitness of the parasite.
Cercariae of trematodes are transmitted to a variety of hosts
and, in addition to being an infectious agent, they can serve as
a valuable food source for many aquatic organisms (Johnson
et al., 2010; Orlofske et al., 2012; Mironova et al., 2019,2020;
McKee et al., 2020). Thus, cercariae play an important role in
the functioning of aquatic ecosystems (Kuris et al., 2008;
Thieltges et al., 2008; Preston et al., 2013). Therefore, the seasonal
timing and duration of the cercarial shedding period can affect
different trophic levels in aquatic ecosystems by changing parasite
burden and food availability.
In the present long-term (5 months) experiment, we investi-
gated the seasonal cercarial shedding traits using 3 different tem-
perature levels reflecting the natural temperature variation over
the distribution range of the bivalve host A. anatina in the current
study region, Finland (60–68°N). We suggest that higher tempera-
ture will accelerate parasite development, leading to the earlier
seasonal start of cercariae emission. This is based on the general
view that parasites receive a competitive advantage over the host
under high temperature (e.g. see Lõhmus and Björklund, 2015;
Marcogliese, 2016 for the discussion) and on the previous obser-
vation of the short-term temperature effect on R. fennica cercarial
release (Choo and Taskinen, 2015). We were specifically inter-
ested in disentangling the seasonal duration of cercarial shedding
at the individual host and host population levels. Our hypotheses
were that under higher temperatures, mussels will start to emit
cercariae of both Rhipidocotyle species seasonally earlier, will
emit cercariae for a longer period, and need fewer day-degrees
to start cercariae shedding than under lower temperatures.
Materials and methods
Study species
The bivalve mollusc host, A. anatina, is a common European
freshwater mussel with a maximum life span >10 years, age of
maturation 2–4 years and maximum length of 12 cm (Taskinen
and Valtonen, 1995). Anodonta anatina serves as the first inter-
mediate host of the bucephalid trematodes Rhipidocotyle campan-
ula and R. fennica (Taskinen et al., 1991,1997; Gibson et al.,
1992; Müller et al., 2015). Cercariae are produced asexually by
sporocysts located in the gonads of the mussel. The prevalence
of infection by R. campanula in natural populations is usually
less than 10% (Taskinen et al., 1991; Müller et al., 2015), whereas
that by R. fennica can be up to 50% (Taskinen et al., 1994).
Pronounced seasonality in the developmental stages of cercariae,
shedding of cercariae, developmental stages of sporocysts and
quantity of sporocysts of Rhipidocotyle species in A. anatina
was observed (Taskinen et al., 1994; Taskinen, 1998a). Both para-
site species have been linked to decreased growth, survival and
reproduction of A. anatina (Taskinen and Valtonen, 1995;
Taskinen, 1998b; Jokela et al., 2005; Müller et al., 2015). The
second intermediate host of these trematodes is the cyprinid
fish R. rutilus, in which R. fennica metacercariae encyst in the
fins and R. campanula metacercariae –in the gills (Taskinen
et al., 1991; Gibson et al., 1992). The definitive hosts for R. cam-
panula are the percid fishes Perca fluviatilis and Sander lucioperca
and the definitive host for R. fennica is the esocid fish Esox lucius
(Taskinen et al., 1991; Gibson et al., 1992).
Experimental set-up
Altogether 281 A. anatina mussels were collected from the River
Kuusaankoski (17 May 2011; 62°25′N, 26°00′E) and 290 mussels
from the River Haajaistenjoki (22 May 2011; 63°63′N, 26°99′E),
Finland. These 2 sampling sites were chosen for comparison of
cercariae emergence traits in different mussel populations. They
were located quite far from each other (distance > 140 km) and
differed by environmental conditions, which allowed the collect-
ing of mussels and parasites of various phenotypes and life stories
for the experiment. At the Konnevesi Research Station, University
of Jyväskylä, the mussels were individually marked and measured.
Average shell length ± S.E. for the River Haajaistenjoki mussels was
61.9 ± 0.6 (range 33.0–92.6 mm), and for the River Kuusaankoski
mussels 77.7 ± 0.6 (range 38.8–101.7 mm). There was a significant
size difference between mussel populations (estimate ± S.E.=16.9±
1.1, t= 16.08, P< 0.0001), while the mussels shedding and non-
shedding cercariae did not differ in their size (estimate ± S.E.=
1.83 ± 1.3, t=1.45, P= 0.15) (Supplementary Fig. S1, Tables S1
and S2). Twelve mussels were infected with both species of parasites
and were excluded from the subsequent analyses, except for the ana-
lysis of probability of cercariae shedding in different temperature
treatments (see section Data analysis). Data about sizes of mussels
taking into account parasitic species are presented below.
From the date of collection to 25th June, mussels were kept in
the laboratory in 2 tanks (1 population per tank) under flow-
through conditions (open tanks that allow a constant flow of
new water). The infection status of collected mussels was
unknown at this stage (it became clear later during experimental
monitoring of cercariae emission), thus infected and uninfected
mussels were maintained together. Each 163-l tank was filled
1046 Jouni Taskinen et al.

with 5 cm of sand at the bottom and supplied with water from the
hypolimnetic zone (9 m depth) of Lake Konnevesi at a rate of up
to 10 L min
−1
. Water temperatures in both tanks were similar
throughout this period ranging from 10.5°C on 31 May to
11.7°C on 25 June (Fig. 1).
On 25 June, the mussels were randomly assigned to 1 of the 3
temperature treatments –high, intermediate and low temperature
(see below) –with 2 replicate tanks per treatment. Mussels from
both populations and from all size groups were distributed evenly
to each of the 6 tanks (for mussel numbers per tank, see Table 1).
The average water temperatures from 25 June to 28 October, when
the experiment was terminated, were 18°C (range 7–24°C), 15°C
(range 7–20°C) and 13°C (range 6–18°C) in high, intermediate
and low temperatures, respectively. There was no length difference
between mussels allocated to temperature treatments (2-way ana-
lysis of variance; F
2, 553
= 0.056, P= 0.945) and no interaction
between population and treatment (F
2, 553
= 0.697, P=0.499).
Average shell length of mussels (±S.E.) in high-temperature tanks
was 70.0 ± 1.0 mm (range 33.0–101.7 mm), in intermediate
temperature tanks 69.4 ± 1.0 mm (range 37.9–100.0 mm) and in
low-temperature tanks 69.8 ± 0.9 mm (range 37.4–101.5 mm). See
Table S3 for the treatment per population descriptive statistics.
The water temperature ranges in the different temperature
treatments corresponded to the natural water extreme tempera-
ture variations currently occurring throughout the distributional
area of A. anatina in Finland, from 60 to 68°N and represent
the maximum summer temperatures varying from about 17 to
24°C, respectively (Kuha et al., 2016). The number of days
when the average daily water temperature in the different treat-
ments was ⩾15°C, a measure of the length of the warm/growing
season, was 74, 71 and 20 days in high, intermediate and low-
temperature treatments, respectively.
The temperature treatments were established as follows. (1)
High-temperature tanks were placed in outside shelter and supplied
with running water –using a pump –from the littoral zone (<2 m
depth) of Lake Konnevesi. (2) Intermediate-temperature tanks were
kept indoors and supplied with heated hypolimnetic water pumped
from Lake Konnevesi. Water was heated in a separate tank
with aquarium heaters before delivery to mussel tanks. (3)
Low-temperature tanks were kept indoors and supplied with
Fig. 1. The daily (mean ± S.E.) cercarial release of Rhipidocotyle campanula (R. c) and R. fennica (R. f) from the mussel host Anodonta anatina, water temperature
profile at mean 3-day intervals and the total duration of cercarial release by mussels from the River Haajaistenjoki (straight horizontal line) and from the River
Kuusaankoski (dotted horizontal line), from 31 May to 28 October in the high- (A), intermediate- (B) and low-temperature treatments (C). An asterisk represents
the day when the mussels were assigned to the different temperature treatments. Note the different scales on the y-axes.
Parasitology 1047

hypolimnetic water pumped from Lake Konnevesi. Anodonta mus-
sels are filter-feeders utilizing phytoplankton, bacteria and fine
organic particles (Jorgensen et al., 1984), thus a continuous flow
of lake water was necessary to provide the mussels with food.
Due to logistic constraints, differences other than temperature
existed between the treatments. Mussels in the high-temperature
treatment were subject to a larger daily fluctuation of temperature
than those in the intermediate- or low-temperature treatments
(Fig. 1), as the littoral water and outdoor tanks were used. In add-
ition, the daily/seasonal profile varied such that the daily water
temperature in the high-temperature treatment tanks peaked in
late July (24°C), while in the intermediate (19°C) and low-
temperature tanks (17°C), it peaked in early September (Fig. 1).
The indoor tanks were illuminated by artificial light with the
photoperiod set to correspond with the natural rhythm. The out-
door tanks received natural light but the shelter above the tanks
provided effective cover against direct sunlight. However, during
the 24 h cercarial release monitoring period, similar artificial
light was used for all mussels to provide equal light conditions
(see below). Water flow into the holding tanks was adjusted
such that it was higher in the intermediate and low temperatures
(10 L min
−1
) than in the high-temperature tanks (5 L min
−1
). This
was to compensate for the probable higher food density in the
high-temperature tanks that received littoral water, than the
intermediate- and low-temperature tanks that received hypolim-
netic water. A submersible temperature logger was placed in 1
replicate tank per treatment to measure water temperature every
4 h from 25 June to 28 October (end of the experiment).
Temperature in the high-temperature treatment (outdoor tanks)
varied much stronger during the day than in other treatments
(Fig. 1). However, results by Roushdy (1984) indicate that cercar-
ial release does not differ between constant and diurnally variable
temperatures.
Cercarial release from each mussel was followed over a period
of 20 weeks by counting cercariae released per A. anatina at
roughly 2-week (12–15 days each) intervals between 31 May
and 28 October, during a total of 12 monitoring sessions. On
each monitoring day, individual mussels were placed in a 4-l
transparent plastic box (length 26.5 cm, width 19 cm and height
13.6 cm) filled with 2 L of filtered lake water for 24 h (possible
dead mussels were removed at this stage) and then returned to
their respective holding tanks. The water temperatures in the
monitoring boxes during the 24 h period of cercarial shedding
were adjusted to correspond with those in the respective holding
tanks and, when necessary, a temperature-controlled room was
used. Light conditions during the monitoring were also set to
correspond with the natural day length and rhythm because the
cercarial release of Rhipidocotyle species is diurnal (Taskinen
et al., 1991). The chosen 24 h incubation period allows excluding
the effects of circadian rhythms, considerably influencing the
results on cercariae shedding (Hannon et al., 2017). The number
of cercariae in the box was counted visually (at low densities, <20
cercariae), or microscopically from a 50 mL of the mixed sub-
sample (at high densities, >20 cercariae). The only 1 sample
was used for cercarial counts since we were interested in estimat-
ing cercariae emergence at different temperatures (but not cer-
cariae emergence per mussel per day), so all mussels from a
certain temperature treatment at a certain day can be considered
a sample, while each individual mussel is a subsample.
The experiment was terminated on 28 October 2011, when
cercarial release approached zero in practically all treatments.
For Rhipidocotyle species, the cercarial shedding in the field and
in the laboratory has been reported to occur between late May
and early October (Taskinen et al., 1994,1997; Taskinen, 1998a).
Data analysis
Statistical analyses and plots preparation were performed using
PASW Statistics 18 and R (R Core Team, 2020). Plots were
drawn using ggplot2 (Wickham, 2016), cowplot (Wilke, 2020)
and gridExtra packages (Baptiste, 2017), multiple comparisons
for GLMs were, when possible, done using multcomp package
machinery (Hothorn et al., 2008), while when impossible (e.g.
with zero-inflated models) Bonferroni corrections were used.
Fisher test was used to compare the proportion of mussels
shedding cercariae in different treatments and independence of
infections. In the former case, double-infected mussels were
added to both R. campanula and R. fennica columns. When
double-infected mussels were excluded, results were similar (see
the Supplementary material). For all other analyses, mussels
that did not shed cercariae and double-infected mussels were
not included in the statistical analyses. Data from replicate
tanks were combined, as prior tests revealed no differences
between replicates for any measured variable.
To check whether mussels from different treatments and
populations differ in their shedding start date, day-degrees
required for the start, stop date and the mean duration of cercarial
emergence, we used the following strategy. First, we tried
Table 1. Total numbers of A. anatina mussels (N) and numbers of mussels shedding cercariae (N
s
;nshedding R. fennica/nshedding R. campanula) from the River
Haajaistenjoki and the River Kuusaankoski kept in high- (HT), intermediate-(IT) and low-temperature (LT) treatments
NN
s
Start date Stop date Duration Start °C Stop °C
R. fennica R. campanula R. fennica R. campanula R. f. R. c. R. f. R. c. R. f. R. c.
River Haajaistenjoki
HT 96 33/8 12 Jul–8 Aug 31 May–27 Jun 27 Jul–18 Sep 14 Jun–18 Sep 10 16 21.5 10.5 16.0 16.0
IT 97 19/11 12 Jul–3 Oct 31 May–4 Sep 4 Sep–14 Oct 12 Jul–3 Oct 14 18 15.5 10.5 9.0 12.0
LT 97 6/20 4 Sep–3 Oct 31 May–8 Aug 18 Sep–14 Oct 14 Jun–3 Oct 6 18 16.5 10.5 11.0 11.0
River Kuusaankoski
HT 93 37/3 12 Jul–8 Sep 31 May–14 Jun 8 Aug–18 Sep 14 Jun–27 July 10 8 21.5 10.5 16.0 23.0
IT 93 13/6 12 Jul–14 Oct 31 May–12 Jul 4 Sep–28 Oct 12 Jul–18 Sep 16 16 15.5 10.5 8.0 15.7
LT 95 2/8 4 Sep–3 Oct 31 May–8 Aug 4 Sep–3 Oct 12 Jul–4 Sep 4 14 16.5 10.5 11.0 16.5
Start and stop dates represent the range between the earliest and latest observations of cercarial emergence, respectively. Duration indicatesthe length (weeks) of the cercarial release at the
host population level, from the first to the last observation of shedding. Start °C and Stop °C represent the water temperatures (°C) on the dates when the first and last cercariae emerged,
respectively.
R. f. and R.c. =Rhipidocotyle fennica and R. campanula, respectively.
1048 Jouni Taskinen et al.

generalized linear models (GLMs) with the Gaussian error struc-
ture and identity link function. We checked the residuals visually
on Q-Q plots and using the Shapiro–Wilks test. Ordinary GLMs
(hereinafter GLMs) were preferred due to their easier interpret-
ability. When necessary, response variables were log-transformed.
If models’assumptions were still violated, we used negative bino-
mial models since in most cases our data were strictly positive
with a heavy right tail. In a few cases, we used zero-inflated mod-
els to account for high numbers of zero values (Zeileis et al.,
2008). Finally, several times we had to switch to quasi-Poisson
models due to the convergence problems with negative binomial
models.
We always started with models including mussel length, mus-
sel origin (River Haajaistenjoki or River Kuusaankoski popula-
tion) and temperature treatment as predictors. Mussel lengths
were centred since zero length is not biologically sensible. We
considered it biologically relevant to include mussel size since it
is often positively correlated with cercariae emergence rate (e.g.
Morley et al., 2010). We also included all double interactions,
while higher-order interactions were not included due to a lack
of a priori hypotheses and interpretation problems.
Initial models were simplified using Akaike’s information cri-
terion (AIC) (Symonds and Moussalli, 2011). For quasi-Poisson
models, which do not have AIC in the output, nested models
were compared using the F-test.
To compare survival of mussels shedding cercariae in different
temperature treatments, we started with a logistic regression,
where treatment and mussel’s length were predictors (interaction
was included), and then simplified it.
To check the influence of temperature on the cercariae emis-
sion rate, we calculated mean temperature and mean cercariae
emission at each sampling day in each treatment. Under tempera-
tures below 9–11°C, mussels rarely emit cercariae, which was pre-
dictable since for many species of trematodes such temperatures
are likely to be threshold ones for shedding infective stages
(Morley and Lewis, 2013). In the subsequent analysis, we used
only non-zero emission data. Models, where zero-emission data
were included, are presented in the Supplementary material.
Qualitatively, they are similar to non-zero ones; however, we sug-
gest that they mainly reflect a large bulk of points with
zero-emission at low temperatures rather than a biologically sens-
ible relationship. One point in R. campanula data looked like an
obvious outlier (see the Supplementary material) and was
excluded from the dataset before the analysis.
We fitted GLMs where temperature and the day of the obser-
vation were predictors (interaction included). A seasonality fac-
tor (number of days from the beginning of the experiment,
which roughly represents the time since the beginning of shed-
ding season) was added since, during the exploratory analysis,
we noticed that under similar temperatures, cercariae emissions
were lower at the end of shedding period than at the beginning
of it. Temperatures were centred since a cercariae emission rate
at 0°C has no biological sense, while at mean temperature does
have.Theresponsevariable(meansheddingrate)was
log-transformed.
To check between-species differences in shedding traits, we
used a similar analytical strategy; however, instead of the popu-
lation factor, we added the species factor in our models. We did
not include population since our analysis of separate species did
not find substantial differences in cercariae shedding traits in
mussels from different populations. Since data on the tempera-
ture emission start looked very different for different parasitic
species and residuals looked ‘fishy’in any model we tried, a
robust regression was used for the data analysis. ‘f.robtest’func-
tion from the sfsmisc (Maechler, 2021) package was used to
obtain Pvalues.
Results
Shedding probability
In total, 178 out of 571 A. anatina mussels shed cercariae in the
course of observations. Among those, 110 mussels were infected
only with R. fennica and 56 only with R. campanula, whereas
12 were infected with both species (excluded from most of data
analyses, see above). The empirical probability of co-infection
(0.021) was almost equal to the expected one assuming the inde-
pendence of infections (0.019) (Fisher’s exact test: P= 0.53). The
proportions of mussels shedding R. fennica significantly differed
between temperature treatments with higher share at higher tem-
peratures. The general test on 3 × 2 contingency table and all
paired comparisons between treatments were significant even
after applying the Bonferroni correction (Fisher’s exact test: P<
0.0002, Table S4). For R. campanula, no significant differences
between treatments were found (P> 0.2 in all cases, Table S4;
Fig. S2). When double infections were included, results were simi-
lar (Table S4).
Cercarial release at the total host population level
At the host population level, the total cercarial emergence period
(from the first to the last observation of emerged cercariae) of
R. fennica ranged from 12 July to 28 October, whereas that of
R. campanula ranged from 31 May to 3 October (Table 1;
Fig. 1). The total period of cercarial shedding by R. fennica lasted
for 10 weeks in the high, and 14–16 weeks in the intermediate, but
for only 4–6 weeks in the low-temperature treatment (Table 1;
Fig. 1). The total period of cercarial shedding by R. campanula
in the low-temperature treatment was clearly longer than that of
R. fennica, as it ranged from 14 to 18 weeks (Table 1). In other
temperatures, differences in shedding duration between the
parasites were not consistent (Table 1). At host population
level, duration of cercarial shedding in R. campanula decreased,
rather than increased, by temperature (Table 1). Water tempera-
ture at the time of the first emergence of R. fennica cercariae var-
ied from 16.5 to 21.5°C, which was clearly higher than 10.5°C
observed for R. campanula (Table 1;Fig. 1). The exact timing
of the cercariae emergence for each mussel shedding cercariae is
given in Figs S7 and S8.
Seasonal cercarial release with respect to temperature
Quantitatively the peak cercarial release by R. fennica co-occurred
with the seasonal thermal maximum, but that of R. campanula
clearly outran it (Fig. 1). Cercarial shedding by R. fennica
increased substantially at temperatures above 15°C (Fig. 2A and
B; Fig. S3) but high numbers of R. campanula cercariae were
released as soon as the temperature exceeded 10°C (Fig. 2C and
D; Fig. S3). For R. fennica a‘raw’relationship between tempera-
ture and cercariae production (Fig. 2A) shows abnormally low
values of the cercariae emission at intermediate temperatures.
These values can be explained by the fact that such temperatures
could be met twice in a shedding season: in spring and in autumn.
The GLM showed that temperature had a strong positive influ-
ence on the number of released cercariae (Table 2). The main
effect of the season (i.e. day from the start of the experiment)
was also positive at the average temperature (Table 2). However,
significant interaction suggests that with time the influence of
temperature decreased. The amount of variance explained by
the model was remarkably high (R
2
= 0.83). The residual plot
(Fig. 2B) showed that after accounting for the season, the relation-
ship between temperature and cercariae production in R. fennica
became almost linear. A similar model (Table S6) for R. campan-
ula did not explain data better than the intercept-only model.
Parasitology 1049

However, exclusion of 1 outlier value (see Fig. S4B) resulted in a
model where both main effects of seasonality and temperature
were significant. With the increase in temperature, the emission
of R. campanula also increased; however, the number of emitted
cercariae falls with time (Table 2;Fig. 2C and D). There was no
interaction between these 2 predictors (F
1
= 0.38, P= 0.54). For
the similar models including zero values, see the Supplementary
material (Tables S5 and S7; Figs S3 and S4A).
Cercarial release at the individual host level at different
temperatures
For the sake of brevity, we present only the most important quali-
tative results here (summarized with statistics in Table 2), while
the full statistical models are available in the Supplementary
material. At high temperature, emission of R. fennica cercariae
started (Table S8a; Fig. 3A and B) and ended (Table S8b;
Fig. 3C and D) earlier than at low and intermediate temperatures
(Table 2). The differences between low and intermediate tempera-
ture treatments were not significant (Tables S8a and S8b, respect-
ively) after correcting for the multiple comparisons. Such a shift
in the emission start and end did not result in a substantial emis-
sion duration change. Interestingly, in larger mussels, R. fennica
cercariae emission started (Table S8a) and stopped (Table S8b)
later than in smaller ones. However, although treatment × length
interactions were non-significant, the estimates were negative
and it seems that the size effect was reliable only in the high-
temperature treatment in case of cercariae shedding start
(Table S8a; Fig. 3B). Since many mussels appeared to emit cer-
cariae only once during the observations, the emission duration
for them was considered a zero, which demands using the
zero-inflated model for emission duration analysis (see above).
Although R. fennica cercarial release lasted for longer at the
host population level in higher temperatures (see above), evidence
for this was not sufficient at the individual host level. There was a
tendency towards a slightly longer R. fennica cercariae emission
duration in low-temperature treatment (Table S8c), but this did
not hold after the Bonferroni correction. Importantly, tempera-
ture treatment had a substantial effect on the number of day-
degrees needed to start the R. fennica cercariae release, with less
day-degrees required under high-temperature treatment com-
pared to intermediate and low ones (Fig. 4; Table S8d). Again,
the difference between the 2 lower temperature treatments was
not significant (Table S8d). The larger mussels needed more day-
degrees to start R. fennica cercariae release (Table 2); however, this
effect was reliably seen only in the high-temperature treatment.
Twenty-five out of 56 mussels releasing R. campanula cer-
cariae had been already emitting them when the experiment
started. Therefore, zero-inflated model was used to evaluate the
influence of predictors on the emission start date in R. campan-
ula. After the stepwise removal of non-significant terms, we
found that the intercept-only model is the most parsimonious
one. However, the AIC of the model, where the temperature is
the only predictor, differs from the AIC of the intercept-only
one by less than 2 points and, therefore, can be considered
informative (Symonds and Moussalli, 2011). Therefore, we
decided to present the results of the above-mentioned model
(Table S8e) though they should be treated with great care.
Essentially, in lower temperatures, R. campanula cercariae emis-
sion started later than in high temperature (estimate ± S.E. = 0.70
± 0.27 and 0.59 ± 0.25, z= 2.65 and 2.34, P= 0.008 and 0.019
for low and intermediate temperature, respectively). The only pre-
dictor left in the model explaining the duration of cercariae emis-
sion in R. campanula was host length. Larger mussels tended to
Fig. 2. Relationship between mean temperature during each 3-week monitoring period and release of Rhipidocotyle fennica (A, B) and R. campanula (C, D) cercariae
by Anodonta anatina mussels (2 study populations combined). The average cercariae release by mussels from different temperature treatments indicated with dot
(high), triangle (intermediate) and square (low temperature). (A, C) ‘Raw’data. (B, D) Residual plot accounting for the seasonality.
1050 Jouni Taskinen et al.

release cercariae for a shorter time though the relationship was
only marginally significant (P= 0.048). This is probably because
the larger mussels stopped cercariae emission earlier than smaller
ones (Table S8f). Similar to R. fennica, emission of R. campanula
cercariae tended to stop later under intermediate and low tem-
peratures; however, only the difference between low- and high-
temperature treatments was statistically significant (Table S8f ).
A substantial share of mussels started to shed R. campanula cer-
cariae simultaneously already in the beginning of the experiment,
thus having the same number of day-degrees (115) to start shed-
ding. To account for it, we fitted a zero-inflated model using 115
day-degrees as a zero value to explain variation in day-degrees
required for start of R. campanula cercariae shedding. After
simplification, the model contained only the host population as
a predictor, while no significant effects were found (Table S8g).
Survival of the mussels through the experiment was 64.2% (see
Figs S5, S7 and S8). The mortality of mussels shedding cercariae
was significantly higher in the high-temperature treatment, while
in other 2 treatments mortality was equal (Table S8h). More
detailed survival analysis will be published elsewhere.
Cercarial release at the individual host level and differences
between parasite species
There was a significant difference between the parasite species
with respect to the 5 cercarial shedding traits studied: start date,
water temperature and number of day-degrees at the start, stop
date and duration of cercarial release (Table S9). However, the
temperature effect on these traits was generally similar for both
parasite species. First of all, the release of cercariae by R. fennica
Table 2. Comparison of cercarial shedding traits between Rhipidocotyle species; 2 populations of the host mussel Anodonta anatina combined
R. fennica (RF) R. campanula (RC) Statistics (estimate ± S.E.)
Host population level
nof shedding mussels 110 56
Shedding period, days 108 (12 Jun–28 Oct) 125 (31 May–03 Oct)
Shedding duration (H)
a
, weeks 10 8–16
Shedding duration (I)
a
, weeks 14–16 16–18
Shedding duration (L)
a
, weeks 4–614–18
Seasonal peak shedding During peak T °C Less tied to T °C
Nof released cercariae vs T °C Positive correlation Positive correlation GLM (RF): 1.23 ± 0.20, z= 5.99, P= 0.00002;
GLM (RC): 0.162 ± 0.045, z= 3.60, P= 0.001
Nof released cercariae vs time
b
Positive correlation Negative correlation
c
GLM (RF): 0.043 ± 0.01, z= 4.54, P= 0.0003;
GLM (RC): −0.012 ± 0.004, z=−2.96, P= 0.007
Nof released cercariae: T °C × time
interaction
Temperature effect decreases
with time
Interaction not
significant
GLM (RF): −0.006 ± 0.002, z=−3.14, P= 0.006
GLM (RC) (nested models comparison): F
1
= 0.38,
P= 0.54
Host individual level
Emission start (time/T °C) Later (mainly >15–20°C) Earlier (about 10–12°
C)
Zero-inflated model: 1.04 ± 0.10, z= 10.01,
P< 0.0001, Table S9d
Robust linear model: 10.13 ± 0.44, F= 570.47,
P< 0.0001, Table S9d
Emission start (effect of T °C) Earlier at higher T °C Earlier at higher T °C See Tables S8a and S8e for all details
Proportion of shedding mussels Higher at high T °C No effect Fisher’s exact test: P< 0.0002, Table S4
Emission start (effect of host size) Later for large mussels
d
No effect GLM with log(DV) (RF): 0.011 ± 0.003, z= 3.70,
P= 0.0003, Table S8a
Emission end (time/temperature) Later (at similar T °C) Earlier (at similar T °C) GLM: 32.6 ± 4.6, t= 7.1, P< 0.0001. Table S9b/ns,
Table S9f
Emission end (effect of T °C) Earlier at higher T °C Earlier at higher T °C See Table S8b, for all details
Emission end (effect of host size) Later for large mussels Earlier for large
mussels
GLM (RF): 0.34 ± 0.14, t= 2.50, P= 0.014;
GLM (RC): −0.58 ± 0.27, t=−2.13, P= 0.038,
Tables S8b and S8f
Emission duration Shorter Longer Zero-inflated GLM: −0.28 ± 0.0021, z= 0.47,
P< 0.0001, Table S9c
Emission duration (effect of T °C) Absent Absent
Emission duration (host size effect) No effect Negative GLM (RC): −0.66 ± 0.33, t=−2.02, P= 0.048
Emission start (day-degrees) More Less GLM with log(DV): 1.58 ± 0.10, t= 15.7, P< 0.0001
E. start (day-degrees, effect of T °C) Less needed at higher T No effect See Tables S8d and S8g for all treatment
comparisons
a
H, I, L = high-, intermediate- and low-temperature treatments, respectively.
b
Time = days from the beginning of experiment.
c
Exclusion of 1 outlier value (see Fig. S4B) resulted in a model where effects of seasonality were significant.
d
This relationship did not hold for intermediate temperature (estimate ± S.E.=−0.011 ± 0.002, z=−5.16, P< 0.0001, Table S9a).
Parasitology 1051

started seasonally significantly later than that by R. campanula
(Table S9a) with the treatment-specific difference varying from
42 to 87 days. In fact, this difference is likely to be even more pro-
nounced since many R. campanula-infected mussels had already
started cercariae emission at the beginning of the experiment.
Lower temperatures led to later seasonal start of cercariae emis-
sion in both species (Table S9a). Rhipidocotyle fennica stopped
cercariae release generally later than R. campanula, but lower tem-
perature caused a later stop of cercariae release in both species
(Table S9b). Interestingly, the interaction between seasonal timing
–and partly also the duration –of cercariae release and the host’s
length differed in the 2 parasite species. Larger mussels infected
with R. campanula ended release earlier (leading to a negative
trend between host size and duration of cercariae emission;
Table S9c), while those infected with R. fennica both started
and ended cercariae release later than the smaller ones, at least
in high temperature (leading to mainly positive trend between
host size and duration of cercariae emission; Table S9c). The
Fig. 3. (A, C) Boxplots for number of days needed to start (A) and stop (C) cercariae release in Rhipidocotyle fennica under different temperature treatments (2 study
populations combined). Width of box reflects the sample size, height of box denotes limits of upper and lower quartiles, horizontal line is the median of vales,
whiskers mark the highest and lowest values within the 1.5 × interquartile range and dots indicate values outside that range. (B, D) Respective plots accounting
for size of the mussel host, Anodonta anatina, where high temperature marked with dots and continuous line, intermediate temperature with triangles and dotted
line, and low temperature with squares and broken line.
Fig. 4. Sum of day-degrees needed to start release of
Rhipidocotyle fennica cercariae by the mussel host
Anodonta anatina (2 study populations combined); box-
plot for different temperature treatments (A) and
respective plot accounting for size of host (B). For
explanation of boxplot details and plot symbols, see
Fig. 3.
1052 Jouni Taskinen et al.

total duration of cercarial release was shorter for R. fennica than
for R. campanula (Table S9c) and this difference is likely to be
underestimated taking into account that a large share of R.
campanula-infected mussels had already started cercariae shed-
ding in the beginning of the experiment.
The water temperature at the start of the seasonal release of
cercarial was higher for R. fennica (15–20°C) and lower for R.
campanula (10–12°C) (Fig. S6; Table S9d); however, this effect
became less pronounced under intermediate and low tempera-
tures. For both species, the number of day-degrees needed to
start cercariae shedding was larger in intermediate- and low-
temperature treatments comparing to the high-temperature one
(Table S9e; Fig. S6). Finally, R. fennica needed much more day-
degrees to start the cercariae release but there were no interspe-
cific differences in the temperatures at which cercariae release
stopped in any temperature treatments (Fig. S6; Table S9f).
Discussion
Ongoing and predicted increases in global temperatures and in
duration of the growing season will have important implications
for many host–parasite systems, e.g. changes in the timing of
parasite life-cycle stages (Marcogliese, 2001; Mouritsen and
Poulin, 2002; Kutz et al., 2005; MacDonald et al., 2021). A com-
mon expectation is that the seasonal larval release by parasites will
start earlier and become more prolonged as a consequence of
increased thermal growing season (Marcogliese, 2001; Harvell
et al., 2009; Nikolaev et al., 2020).
The present study on the variation in the seasonal cercarial
shedding patterns of 2 sympatric parasites gives mixed evidence
both for and against the aforementioned hypothesis. At the indi-
vidual host level, high temperature caused a marked shift in the
cercariae release period towards earlier start and end of cercariae
emission, but –against the expectation –did not result in a longer
seasonal duration of cercariae release within an individual host.
Such a shift can have important consequences in natural habitats
leading to a temporal mismatch in occurrence between hosts and
parasites (Lõhmus and Björklund, 2015; Cohen et al., 2017;
Gehman et al., 2018), thus, reducing the infection load (Paull
and Johnson, 2014; McDevitt-Galles et al., 2020). However, the
total period of cercarial shedding by R. fennica at the host popu-
lation level, from the first to the last observation of emergence,
was much longer in high and intermediate-temperature treat-
ments compared with the low-temperature one –as expected by
the general hypothesis. Such a population-level effect was not
observed in R. campanula probably because about a half of
R. campanula-infected mussels had already started shedding
cercariae when the experiment begun. In addition, though the
temperature did not influence the duration of cercariae release
at the individual level, it positively correlated with the numbers
of produced cercariae in both species, thus, potentially giving
the parasite an advantage in the competition with a host for the
host’s resources. Importantly, under similar temperatures, fewer
cercariae were emitted in the end compared with the beginning
of the emission season, which is likely to be a result of exhaustion
of the host and/or parasite. Therefore, the present results support
the view that climate warming would increase the duration of lar-
val shedding, and lengthen the transmission period by parasites,
but that such lengthening is produced by increased variation
between host individuals, rather than due to a lengthened individ-
ual shedding period.
The results also indicated that even closely related, sympatric
parasite species that share the same transmission pathway can
respond differently to temperature change (see also Selbach and
Poulin, 2020). High variability in cercariae shedding patterns
(e.g. daily rhythms) have been found previously both within a
single trematode genus (Vyhlídalová and Soldánová, 2020) and
species (Théron and Combes, 1988; Riley and Uglem, 1995;Le
Clec’het al., 2021). Rhipidocotyle fennica brought forward the
start of seasonal cercarial release and started cercarial release
with lower day-degrees in the high-temperature treatment, but
for R. campanula this was not found. Furthermore, R. campanula
clearly started the seasonal cercarial release earlier, at a lower tem-
perature than R. fennica (10 and 15°C, respectively), with fewer
day-degrees, stopped the seasonal cercarial release earlier, and
had a markedly longer total seasonal duration of cercarial emis-
sion. The peak release of R. fennica cercariae occurred during
the warmest weeks in concordance with field observations
(Taskinen et al., 1994), and also the proportion of mussels shed-
ding R. fennica cercariae was higher in higher temperature treat-
ments, while in R. campanula this phenomenon was not found.
Based on these observations, one might predict that the projected
longer summers and higher temperatures associated with climate
warming (Tietäväinen et al., 2010; Ruosteenoja et al., 2011) would
benefit especially R. fennica, which requires higher temperatures
(as well as more day-degrees) to start cercariae release. On the
other hand, it can be predicted that R. campanula could thrive
better than R. fennica in colder, more northern, short-summer
environments, where the early onset of cercarial release is
presumably advantageous.
The earlier start of the seasonal cercarial release by R. campan-
ula is difficult to explain by the transmission dynamics, as the 2
species share the same current (the bivalve A. anatina) and
next (the fish R. rutilus) host in their life cycles (Taskinen
et al., 1991; Gibson et al., 1992). It is also worth noting that
both R. fennica and R. campanula are specific only to A. anatina
as their first intermediate host in the study area (Taskinen et al.,
1991; Gibson et al., 1992). The definitive hosts of R. fennica and
R. campanula are the predatory fishes northern pike (E. lucius)
and perch/pikeperch (P. fluviatilis/S. lucioperca), respectively
(Taskinen et al., 1991; Gibson et al., 1992). Thus, it is also possible
that the timing of cercarial shedding could be an adaptation to
increase transmission to the final hosts, such as the differential
seasonal feeding of the final hosts on roach that we are not
aware of. However, it is difficult to believe that the earlier start
of cercarial release by R. campanula could be an adaptation
only to northern conditions (although it might facilitate occur-
rence there) because both R. campanula and R. fennica occur as
far south as Ukraine (Taskinen et al., 1991; Petkevičiūtėet al.,
2014; Stunžėnas et al., 2014; Müller et al., 2015).
We propose that the mechanism enabling the early onset of cer-
carial release by R. campanula is that they have their cercarial pro-
duction machinery ‘on standby’throughout the year (Taskinen
et al., 1994). Fully developed, mature cercariae are found in R. cam-
panula sporocysts in high proportions in all seasons, readily avail-
able for shedding when a suitable temperature is attained (Taskinen
et al., 1994). In R. fennica, mature, ready-to-emerge cercariae are
only found during the cercarial shedding period (Taskinen et al.,
1994). This probably means that it takes a relatively long time for
R. fennica to respond to increasing water temperature in terms of
cercarial production, as the growth of sporocyst starts from practic-
ally zero in spring (Taskinen et al., 1994). Cercarial release by
Rhipidocotyle spp. can also be triggered outside the natural shed-
ding period by transfering infected mussels to high temperature
in the laboratory, but also in that case the time needed for
R. campanula to start shedding cercariae is much shorter than
for R. fennica (Taskinen et al., 1991).
Although our results about the effect of temperature on shed-
ding start and proportion of mussels shedding cercariae predict
that warming is likely to benefit R. fennica more than R. campan-
ula, the infection success of parasites may also depend on cercar-
ial output, their survival and infectivity. Temperature effects on
Parasitology 1053

these traits can partly compensate for each other, sometimes
resulting in similar transmission efficiency at different tempera-
tures (Poulin, 2006). However, it is impossible to account for all
these effects in our study.
Two studied parasitic species also differed in their relation-
ships to the host size. Larger R. fennica-infected mussels tended
to start cercariae release later and shed them for a longer time,
while in R. campanula no clear relationships were seen. Such a
difference hints again that even closely related sympatric parasitic
species infecting the same host can differ in strategies of host
exploitation. Surprisingly, there were almost no differences in cer-
cariae shedding traits in parasites from different populations,
which suggests that our study comprises a reasonable amount
of generality.
It is important to notice that, although we explained the
obtained results by temperature and seasonality effects, there
were differences between the treatments also in terms of water
flow and water source (littoral vs hypolimnetic), light conditions,
temperature fluctuation and seasonal temperature profile.
Whereas the high-temperature tanks were kept in an outdoor
shelter and were subject to a diurnal temperature fluctuation
and natural light, the intermediate- and low-temperature tanks
were kept in an indoor tank hall and illuminated with artificial
light. However, the photoperiod was equal in all treatments and
corresponded to the natural rhythm. In addition, the cercarial
release was shown to be similar at constant and diurnally variable
temperatures (Roushdy, 1984). According to Choo and Taskinen
(2015), a short-term (1 h) temperature increase triggers cercariae
emission of R. fennica from A. anodonta, but on the other hand a
similar decrease in temperature results in an equivalent decrease
in cercariae release. Therefore, the higher daily fluctuation of tem-
perature in the high-temperature treatment probably did not
influence the net daily cercariae release. In addition, the monitor-
ing of cercariae release was performed at constant temperature
(no variation within a day) in all temperature treatments.
Importantly, the mussels in high-temperature treatment supplied
with littoral water could receive more food than those in colder
treatments, supplied with water from the lake hypolimnion,
although we aimed at compensating this by doubling the water
flow in the cold- and intermediate-temperature treatments (see
section Methods).
Since food deprivation of host can constrain cercariae release
(e.g. Seppälä et al., 2015), higher cercariae output in high-
temperature treatment obtained in our study could be, e.g. a result
of higher food availability for mussel host. However, our data on
mussel mortality and weight change during the experiment (i.e.
higher mortality in high-temperature treatment, no increase in
mussel weight) do not indicate the importance of nutritional
differences for the interpretation of the results. Even though we
cannot completely rule out confounding factors other than
temperature, we do not believe that the difference in water and
light source, or temperature fluctuation, could explain the
observed contrasting responses in the seasonal cercarial release
by R. fennica and R. campanula between the temperature treat-
ments. New infections of mussels during the experiment, via
miracidia from unfiltered lake water, were unlikely due to the sea-
sonal maturing of Rhipidocotyle trematodes in late autumn
(Taskinen et al., 1991). Thus, the present results should reliably
indicate temperature responses in the seasonal timing of cercarial
shedding by R. fennica and R. campanula.
Previous studies investigating the seasonal dynamics of trema-
tode cercarial release include field observations showing a signifi-
cant increase in cercarial emergence during summer months
(Taskinen et al., 1994; Taskinen, 1998a; Fingerut et al., 2003), a
longer seasonal shedding period in water bodies receiving thermal
effluents (Aho et al., 1982) and experimental evidence on the role
of temperature in controlling daily cercariae output (Koprivnikar
and Poulin, 2009a; Vyhlídalová and Soldánová, 2020), the start
and the duration of cercariae emergence (Taskinen et al., 1991;
Fingerut et al., 2003; Paull and Johnson, 2014; Prokofiev et al.,
2016). Long-term experimental studies like the present one are
still scarce (Paull and Johnson, 2014).
The results of this study partly support the idea that climate
warming would increase the seasonal duration of larval shedding
by parasites, but emphasize species-specific differences in the sea-
sonal cercarial release and transmission with respect to warming
(Marcogliese, 2001; Harvell et al., 2009). Research on the geo-
graphic distribution of the species is needed to determine whether
the observed temperature differences in cercarial shedding traits
affect the current distribution and relative abundance of
Rhipidocotyle species at the northern boundary of their occur-
rence. Due to the contrasting species-specific temperature-
dependence, the A. anatina–Rhipidocotyle spp. association offers
a unique system to study the effects of the ongoing and predicted
climate warming on host–parasite relationships at high latitudes.
Supplementary material. The supplementary material for this article can
be found at https://doi.org/10.1017/S0031182022000518.
Data. Data can be requested from the authors.
Acknowledgements. We thank Nebiyu Girgibo and Waidi Alabi for assist-
ance in the field and in the laboratory. Katja Pulkkinen, Anssi Karvonen and
Roger Jones provided valuable comments on the manuscript. Roger Jones
kindly checked the English of the manuscript. Konnevesi Research Station
of JYU provided the facilities and necessary assistance for the experiment.
Author contribution. J. T. conceived the ideas and designed the experiment;
J. C. collected the data; J. T. and M. G. analysed the data. All authors contrib-
uted to the writing of the manuscript and gave final approval for publication.
Financial support. The research was supported by the JYU Rector’s grants
for doctoral studies (J. M. C.), the Emil Aaltonen Foundation (J. M. C.), the
Biological Interactions Graduate School travel grants for doctorate students,
University of Jyväskylä (J. M. C.), the Academy of Finland (J. T., grant number
260704) and the Russian Science Foundation (M. G., grant 19-14-00015).
Conflict of interest. None.
Ethical standards. None.
References
Aho JM, Camp JW and Esch GW (1982) Long-term studies on the population
biology of Diplostomulum scheuringi in a thermally altered reservoir.
Journal of Parasitology 68, 695–708.
Baptiste A (2017) gridExtra: miscellaneous functions for ‘Grid’graphics. R package
version 2.3. Available at https://CRAN.R-project.org/package=gridExtra.
Choo JM and Taskinen J (2015) Effect of short-term temperature change on
cercarial release by Rhipidocotyle fennica (Trematoda, Bucephalidae) from
the freshwater bivalve host, Anodonta anatina.Ecological Parasitology and
Immunology 4, 235932.
Chubb JC (1979) Seasonal occurrence of helminths in freshwater fishes. Part
II. Trematoda. Advances in Parasitology 17, 141–313.
Cohen JM, Venesky MD, Sauer EL, Civitello DJ, McMahon TA, Roznik EA
and Rohr JR (2017) The thermal mismatch hypothesis explains host sus-
ceptibility to an emerging infectious disease. Ecology Letters 20, 184–193.
Cribb TH, Bray RA and Littlewood TJ (2001) The nature and evolution of
the association among digeneans, molluscs and fishes. International
Journal for Parasitology 31, 997–1011.
De Montaudouin X, Blanchet H, Desclaux-Marchand C, Lavesque N and
Bachelet G (2016) Cockle infection by Himasthla quissetensis –I. From cer-
cariae emergence to metacercariae infection. Journal of Sea Research 113,
99–107.
Fingerut JT, Zimmer CA and Zimmer RK (2003) Patterns and processes of
larval emergence in an estuarine parasite system. Biological Bulletin 205,
110–120.
1054 Jouni Taskinen et al.

Galaktionov KV (2017) Patterns and processes influencing helminth parasites
of Arctic coastal communities during climate change. Journal of
Helminthology 91, 387–408.
Gehman AM, Hall RJ and Byers JE (2018) Host and parasite thermal ecology
jointly determine the effect of climate warming on epidemic dynamics.
PNAS 115, 744–749.
Gibson DI, Taskinen J and Valtonen ET (1992) Studies on bucephalid digen-
eans parasitising molluscs and fishes in Finland. II. The description of
Rhipidocotyle fennica n. sp. and its discrimination by principal components
analysis. Systematic Parasitology 23,67–79.
Hannon E, Calhoun D, Chadalawada S and Johnson P (2017) Circadian
rhythms of trematode parasites: applying mixed models to test underlying
patterns. Parasitology 145,1–9.
Harvell D, Altizer S, Cattadori IM, Harrington L and Weil E (2009) Climate
change and wildlife diseases: when does the host matter the most? Ecology
90, 912–920.
Hothorn T, Bretz F and Westfall P (2008) Simultaneous inference in general
parametric models. Biometrical Journal 50, 346–363.
IPCC (2014) Climate Change 2014: Synthesis Report. Contribution of Working
Groups I, II and III to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change. [Core Writing Team, Pachauri RK and Meyer LA
(eds.)]. Geneva, Switzerland: IPCC.
Johnson PT, Dobson A, Lafferty KD, Marcogliese DJ, Orlofske SA, Poulin
R and Thieltges DW (2010) When parasites become prey: ecological and
epidemiological significance of eating parasites. Trends in Ecology and
Evolution 25, 362–371.
Jokela J, Taskinen J, Mutikainen P and Kopp K (2005) Virulence of parasites
in hosts under environmental stress: experiments with anoxia and starva-
tion. OIKOS 108, 156–164.
Jorgensen CB, Kiorboe T, Mohlenberg F and Riisgard HU (1984) Ciliary
and mucus-net filter feeding, with special reference to fluid mechanical
characteristics. Marine Ecology Progress Series 15, 283–292.
Jylhä K, Tuomenvirta H and Ruosteenoja K (2004) Climate change projec-
tions for Finland during the 21st century. Boreal Environmental Research 9,
127–152.
Karvonen A, Seppälä O and Valtonen ET (2004) Parasite resistance and
avoidance behaviour in preventing eye fluke infections in fish.
Parasitology 129, 159–164.
Koprivnikar J and Poulin R (2009a) Interspecific and intraspecific variation
in cercariae release. Journal of Parasitology 95,14–19.
Koprivnikar J and Poulin R (2009b) Effects of temperature, salinity, and
water level on the emergence of marine cercariae. Parasitology Research
105, 957–965.
Kuha J, Arvola L, Hanson PC, Huotari J, Huttula T, Juntunen J, Järvinen
M, Kallio K, Ketola M, Kuoppamäki K, Lepistö A, Lohila A, Paavola R,
Vuorenmaa J, Winslow L and Karjalainen J (2016) Response of boreal
lakes to episodic weather-induced events. Inland Waters 6, 523–534.
Kuris AM, Hechinger RF, Shaw JC, Whitney KL, Aguirre-Macedo L, Boch
CA, Dobson AP, Dunham EJ, Fredensborg BL, Huspeni TC, Lorda J,
Mababa L, Mancini FT, Mora AB, Pickering M, Talhouk NL, Torchin
ME and Lafferty R (2008) Ecosystem energetic implications of parasite
and free-living biomass in three estuaries. Nature 454, 515–518.
Kutz SJ, Hoberg EP, Polley L and Jenkins EJ (2005) Global warming is chan-
ging the dynamics of Arctic host-parasite systems. Proceedings of the Royal
Society Biological Sciences 272, 2571–2576.
Le Clec’h W, Chevalier FD, McDew-White M, Menon V, Arya GA and
Anderson T (2021) Genetic architecture of transmission stage production
and virulence in schistosome parasites. Virulence 12, 1508–1526.
Lõhmus M and Björklund M (2015) Climate change: what will it do to fish–
parasite interactions? Biological Journal of the Linnean Society 116, 397–
411.
MacDonald H, Akçay E and Brisson D (2021) The role of host phenology for
parasite transmission. Theoretical Ecology 14,1–21.
Maechler M (2021) sfsmisc: Utilities from ‘Seminar fuer Statistik’ETH Zurich.
R package version 1.1-11. Available at https://CRAN.R-project.org/
package=sfsmisc.
Marcogliese DJ (2001) Implications of climate change for parasitism of
animals in the aquatic environment. Canadian Journal of Zoology 79,
1331–1352.
Marcogliese DJ (2016) The distribution and abundance of parasites in aquatic
ecosystems in a changing climate: more than just temperature. Integrative
and Comparative Biology 56, 611–619.
McDevitt-Galles T, Moss W, Calhoun D and Johnson P (2020) Phenological
synchrony shapes pathology in host–parasite systems. Proceedings of the
Royal Society B: Biological Sciences 287, 20192597.
McKee KM, Koprivnikar J, Johnson PTJ and Arts MT (2020) Parasite infec-
tious stages provide essential fatty acids and lipid-rich resources to fresh-
water consumers. Oecologia 192, 477–488.
Mironova E, Gopko M, Pasternak A, Mikheev V and Taskinen J (2019)
Trematode cercariae as prey for zooplankton: effect on fitness traits of pre-
dators. Parasitology 146, 105–111.
Mironova E, Gopko M, Pasternak A, Mikheev V and Taskinen J (2020)
Cyclopoids feed selectively on free-living stages of parasites. Freshwater
Biology 65, 1450–1459.
Morley NJ and Lewis JW (2013) Thermodynamics of cercarial development
and emergence in trematodes. Parasitology 140, 1211–1224.
Morley N, Adam M and Lewis J (2010) The effects of host size and tempera-
ture on the emergence of Echinoparyphium recurvatum cercariae from
Lymnaea peregra under natural light conditions. Journal of Helminthology
84, 317–326.
Mouritsen KN and Poulin R (2002) Parasitism, climate oscillations and the
structure of natural communities. OIKOS 97, 462–468.
Müller T, Czarnoleski M, Labecka AM, Cichy A, Zając K and
Dragosz-Kluska D (2015) Factors affecting trematode infection rates in
freshwater mussels. Hydrobiologia 742,59–70.
Nikolaev K, Levakin I and Galaktionov K (2020) Seasonal dynamics of
trematode infection in the first and the second intermediate hosts: a long-
term study at the subarctic marine intertidal. Journal of Sea Research 164,
101931.
Orlofske SA, Jadin RC, Preston DL and Johnson PTJ (2012) Parasite trans-
mission in complex communities: predators and alternative hosts alter
pathogenic infections in amphibians. Ecology 93, 1247–1253.
Paull SH and Johnson PT (2014) Experimental warming drives a seasonal
shift in the timing of host-parasite dynamics with consequences for disease
risk. Ecology Letters 17, 445–453.
Paull SH, Raffel TR, LaFonte BE and Johnson PTJ (2015) How temperature
shifts affect parasite production: testing the roles of thermal stress and accli-
mation. Functional Ecology 29, 941–950.
PetkevičiūtėR, Stunžėnas V and StanevičiūtėG(2014) Differentiation of
European freshwater bucephalids (Digenea: Bucephalidae) based on karyo-
types and DNA sequences. Systematic Parasitology 87, 199–212.
Poulin R (2006) Global warming and temperature-mediated increases in
cercarial emergence in trematode parasites. Parasitology 132, 143–151.
Preston DL, Orlofske SA, Lambden JP and Johnson PTJ (2013) Biomass and
productivity of trematode parasites in pond ecosystems. Journal of Animal
Ecology 82, 509–517.
Prokofiev V, Galaktionov KV and Levakin IA (2016) Patterns of parasite
transmission in polar seas: daily rhythms of cercarial emergence from inter-
tidal snails. Journal of Sea Research 113,85–98.
R Core Team (2020) R: A Language and Environment for Statistical
Computing. Vienna, Austria: R Foundation for Statistical Computing.
Available at https://www.R-project.org/.
Riley M and Uglem G (1995) Proterometra macrostoma (Digenea: Azygiidae):
variations in cercarial morphology and physiology. Parasitology 110, 429–
436.
Rosen R, Blank S, Akabogu F, Hauschner R, Meneses S, Slater O, Melton A,
Tetidrick C and Gosnell W (2018) Effect of temperature on the release
of Proterometra macrostoma (Trematoda: Digenea) cercariae from their
snail intermediate host, Pleurocera semicarinata (Gastropoda:
Pleuroceridae) at North Elkhorn Creek, Kentucky. Comparative
Parasitology 85, 202–207.
Roushdy MZ (1984) The effect of diurnal fluctuating temperature on the
development of Schistosoma haematobium in Bulinus truncatus.Journal
of the Egyptian Society of Parasitology 14, 507–514.
Ruosteenoja K, Räisänen J and Pirinen P (2011) Projected changes in ther-
mal seasons and the growing season in Finland. International Journal of
Climatology 31, 1473–1487.
Selbach C and Poulin R (2020) Some like it hotter: trematode transmission
under changing temperature conditions. Oecologia 194, 745–755.
Seppälä O, Louhi KR, Karvonen A, Rellstab C and Jokela J (2015) Relative
reproductive success of co-infecting parasite genotypes under intensified
within-host competition. Infection, Genetics and Evolution: Journal of
Molecular Epidemiology and Evolutionary Genetics in Infectious Diseases 36,
450–455.
Parasitology 1055

Sharma S, Jackson DA, Minns CK and Shuter BJ (2007) Will northern fish
populations be in hot water because of climate change? Global Change
Biology 13, 2052–2064.
Shim KC, Koprivnikar J and Forbes MR (2013) Variable effects of increased
temperature on a trematode parasite and its intertidal hosts. Journal of
Experimental Marine Biology and Ecology 439,61–68.
Studer A and Poulin R (2014) Analysis of trait mean and variability versus
temperature in trematode cercariae: is there scope for adaptation to global
warming? International Journal for Parasitology 44, 403–413.
Studer A, Thieltges DW and Poulin R (2010) Parasites and global warming:
net effects of temperature on an intertidal host–parasite system. Marine
Ecology Progress Series 415,11–22.
Stunžėnas V, PetkevičiūtėR, StanevičiūtėG and BinkienėR(2014)
Rhipidocotyle fennica (Digenea: Bucephalidae) from Anodonta anatina
and pike Esox lucius in Lithuania. Parasitology Research 113, 3881–3883.
Symonds MRE and Moussalli A (2011) A brief guide to model selection, multi-
model inference and model averaging in behavioural ecology using Akaike’s
information criterion. Behavioral Ecology and Sociobiology 65,13–21.
Taskinen J (1998a) Cercarial production of the trematodes Rhipidocotyle fen-
nica kept in the field. Journal of Parasitology 84, 345–349.
Taskinen J (1998b) Influence of trematode parasitism on the growth of a
bivalve host in the field. International Journal of Parasitology 28, 599–602.
Taskinen J and Valtonen ET (1995) Age-, size-, and sex-specific infection of
Anodonta piscinalis (Bivalvia: Unionidae) with Rhipidocotyle fennica
(Digenea: Bucephalidae) and its influence on host reproduction.
Canadian Journal of Zoology 73, 887–897.
Taskinen J, Valtonen ET and Gibson DI (1991) Studies on bucephalid digen-
eans parasitizing molluscs and fishes in Finland I. Ecological data and
experimental studies. Systematic Parasitology 19,81–94.
Taskinen J, Valtonen ET and Mäkelä T (1994) Quantity of sporocysts and
seasonality of two Rhipidocotyle species (Digenea: Bucephalidae) in
Anodonta piscinalis (Mollusca: Bivalvia). International Journal of
Parasitology 24, 877–886.
Taskinen J, Mäkelä T and Valtonen ET (1997) Exploitation of Anodonta pis-
cinalis (Bivalvia) by trematodes: parasite tactics and host longevity. Annales
Zoologici Fennici 34,37–46.
Théron A and Combes C (1988) Genetic analysis of cercarial emergence
rhythms of Schistosoma mansoni.Behavior Genetics 18, 201–209.
Thieltges DW and Rick J (2006) Effects of temperature on emergence,
survival and infectivity of cercariae of the marine trematode
Renicola roscovita (Digenea: Renicolidae). Diseases of Aquatic Organisms
73,63–68.
Thieltges DW, de Montaudouin X, Fredensborg B, Jensen KT, Koprivnikar
J and Poulin R (2008) Production of marine trematode cercariae: a poten-
tially overlooked path of energy flow in benthic systems. Marine Ecology
Progress Series 372, 147–155.
Tietäväinen H, Tuomenvirta H and Venäläinen A (2010) Annual and
seasonal mean temperatures in Finland during the last 160 years based
on gridded temperature data. International Journal of Climatology 30,
2247–2256.
Vaughn CC (2018) Ecosystem services provided by freshwater mussels.
Hydrobiologia 810,15–27.
Vyhlídalová T and Soldánová M (2020) Species-specific patterns in cercarial
emergence of Diplostomum spp. from snails Radix lagotis.International
Journal for Parasitology 50, 1177–1188.
Wickham H (2016) ggplot2: Elegant Graphics for Data Analysis. New York,
USA: Springer Verlag.
Wilke C (2020) cowplot: Streamlined Plot Theme and Plot Annotations for
‘ggplot2’. R package version 1.1.1. Available at https://CRAN.R-project.
org/package=cowplot.
Zeileis A, Kleiber C and Jackman S (2008) Regression models for count data
in R. Journal of Statistical Software 27,1–25.
1056 Jouni Taskinen et al.