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Secondary sexual characteristics in codfishes
(Gadidae) in relation to sound production, habitat
use and social behaviour
Jon Egil Skjæraasen a b , Justin J. Meager a c & Mikko Heino a b d
a Department of Biology, University of Bergen, Bergen, Norway
b Institute of Marine Research, Bergen, Norway
c Faculty of Science, Health and Education, University of the Sunshine Coast, Australia
d International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria
Available online: 09 Feb 2012
To cite this article: Jon Egil Skjæraasen, Justin J. Meager & Mikko Heino (2012): Secondary sexual characteristics in
codfishes (Gadidae) in relation to sound production, habitat use and social behaviour , Marine Biology Research, 8:3,
201-209
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INVITED REVIEW
Secondary sexual characteristics in codfishes (Gadidae) in relation to
sound production, habitat use and social behaviour
JON EGIL SKJÆRAASEN
1,2
*, JUSTIN J. MEAGER
1,3
& MIKKO HEINO
1,2,4
1
Department of Biology, University of Bergen, Bergen, Norway,
2
Institute of Marine Research, Bergen, Norway,
3
Faculty of
Science, Health and Education, University of the Sunshine Coast, Australia, and
4
International Institute for Applied Systems
Analysis (IIASA), Laxenburg, Austria
Abstract
Little is known about the reproductive biology of the codfishes (Gadidae). Lacking direct observations, the study of
secondary sexual characteristics can provide cues to their reproductive biology and behaviour. We reviewed here published
accounts on sexual dimorphisms in 25 gadids in light of their general lifestyle, i.e. pelagic or demersal, and social behaviour.
In addition, complementary data on fin lengths and drumming muscle size in haddock (Melanogrammus aeglefinus), saithe
(Pollachius virens), blue whiting (Micromesistius poutassou) and cod (Gadus morhua) are presented. Capacity for sound
production occurred in almost half of the studied species, but was most prevalent in demersal species, where it is probably
used in resource contests and to attract mates. For semi-pelagic gadids, we postulate that sound production may be linked
to the formation of male-biased spawning shoals and the attraction of females towards such shoals; we identify candidate
species to further test this hypothesis. Although rarely studied, sexual fin dimorphisms occur in several gadids. Cod, saithe
and blue whiting males have longer pelvic fins than females, whereas no such dimorphism was observed in haddock. In cod
and haddock, males use pelvic fins during courtship of females and agonistic encounters with other males. Pelvic fins
probably also have a similar function in other gadids. The hitherto available information on sexually dimorphic traits and/or
courtship behaviour in seven gadid species suggests that complex mating systems and non-random mate choice occurs
frequently in this important group of exploited fishes.
Key words: Codfishes, fin dimorphisms, mating systems, sexual selection, sound production
Introduction
Sexual selection, typically through female choice of
certain male traits, can lead to the emergence of
secondary sexual characteristics or sexual dimorph-
isms, provided there is a genetic component to the
variation in said trait (Ryan 1997). Such traits are
thought to give females either a direct, e.g. more
offspring produced, or indirect, e.g. higher quality
offspring, benefit. Sexual selection can result in
sexual dimorphism in size, body structure or colour
(e.g. Kodric-Brown 1990; Hendry & Berg 1999;
Gardner 2010). Teleost examples abound and in-
clude the genus Xiphophorus where males develop
‘swordtails’ or elongated caudal fins (Basolo 1990a)
and females show preference for males with longer
swords (Basolo 1990b), whereas dominant black
goby Gobius niger Linnaeus, 1758 males have a
distinct dark nuptial colouration (Mazzoldi &
Razzotto 2002). Given that sexual dimorphisms are
commonly closely linked to reproductive behaviour,
they may therefore provide important insights
into the mating systems of species that are poorly
understood.
The codfishes (family Gadidae) comprise numer-
ous species of which many are of significant com-
mercial and cultural importance and have been
harvested for thousands of years (Cohen et al.
1990). Despite this, little is known about their
mating behaviour and reproductive systems, because
most spawn in the ocean at depths where direct
observations of spawning behaviour are difficult.
*Correspondence: Jon Egil Skjæraasen, Department of Biology, University of Bergen, Box 7803, N-5020 Bergen, Norway.
E-mail: jon.skjaeraasen@bio.uib.no
Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory,
University of Copenhagen, Denmark
Marine Biology Research, 2012; 8: 201209
(Accepted 19 October 2011; Published online 6 February 2012; Printed 24 February 2012)
ISSN 1745-1000 print/ISSN 1745-1019 online #2012 Taylor & Francis
http://dx.doi.org/10.1080/17451000.2011.637562
Downloaded by [Jon Egil Skjæraasen] at 03:58 10 February 2012
Only Atlantic cod (Gadus morhua Linnaeus, 1758)
and haddock (Melanogrammus aeglefinus (Linnaeus,
1758)) have been subject to close scientific scrutiny.
Laboratory studies have demonstrated that during
the reproductive period, male cod and haddock
court females and there is pronounced aggression
between males (Brawn 1961a, b; Hutchings et al.
1999; Hawkins & Amorim 2000). In cod, these
behaviours appear to be energetically costly (Skjær-
aasen & Hutchings 2010; Skjæraasen et al. 2010a)
and linked to individual male reproductive success
(Rowe et al. 2008), suggesting the presence of female
choice (Rowe et al. 2008; Skjæraasen et al. 2010b).
Concurrent with these displays, male cod and
haddock also produce sound (Brawn 1961a; Haw-
kins & Amorim 2000). In the field, male cod are
observed to form dense sex-biased shoals (Morgan &
Trippel 1996; Nordeide 1998), which females ap-
pear to visit at the time of mating (Robichaud &
Rose 2001; Meager et al. 2009, Meager et al. 2010).
The cod mating system has therefore been suggested
to resemble a lek (Hutchings et al. 1999; Nordeide &
Folstad 2000; Windle & Rose 2007; Meager et al.
2010).
Cod and haddock vocalize during reproductive
displays with ‘drumming muscles’, large pairs of
striated muscles attached to the swim bladder
(Brawn 1961c; Hawkins & Amorim 2000; Nordeide
et al. 2008). These muscles are larger in cod and
haddock males than females during the reproductive
period (Hawkins 1993; Engen & Folstad 1999).
Hawkins & Rasmussen (1978) examined sound
production and drumming muscles in nine gadid
species and found that they were present in all sound
producing species, but not in ‘silent’species. Simi-
larly, the pelvic fins of male cod are also used in both
courtship and aggressive behaviours (Brawn 1961a,
b) and are larger in males than females (Skjæraasen
et al. 2006). Examination of secondary sexual
dimorphisms in combination with insights into
shoaling dynamics and lifestyle thus represents a
useful tool for making inferences about the repro-
ductive behaviour and thereby sexual selection of
species difficult to observe in the field.
Here we review the literature on the presence of
sound production and sexual dimorphisms in gadids
(subfamilies Gadinae, Lotinae and Gaidropsarinae,
cf. Endo 2002) in the light of their general lifestyle,
i.e. pelagic or demersal, and, where such information
was available, shoaling behaviour during reproduc-
tion (Table I). In addition, we present new data on
fin lengths and drumming muscle size in four
common North Atlantic gadids: haddock, saithe
(Pollachius virens (Linnaeus, 1758)), blue whiting
(Micromesistius poutassou (Risso, 1827)) and cod.
Material and methods
All fish sampled for the purpose of the present study
were sourced from the Institute of Marine Research
surveys conducted between February and April
2007. Northeast Arctic (NEA) haddock and saithe
(70838?N, 20850?E) were caught in the Barents
Sea. Cod were sourced from catches at the main
spawning grounds for NEA cod in Lofoten
(67838?N, 1830?E). Blue whiting were caught in the
FaroeShetland Channel ( 59848?N, 07843?W).
See Table II for further information on sample sizes.
All fish were frozen to 308C immediately upon
capture, and subsequently transported to a freezer
room (308C) at the University of Bergen until they
were measured in June - July 2008.
Laboratory measurements of fin lengths and drumming
muscle size
Fish were first thawed for approximately 1620 h
before total length (91 cm) and body weight (91
g) was measured. We then measured the length of
the longest pelvic and pectoral fin ray from the base
of the fin to the tip of the ray with callipers (91
mm). For the three dorsal and two anal fins we
followed the procedure of Engen & Folstad (1999)
and measured the length of the third fin ray along the
length of the spine, counting in a head-to-tail
direction. This is usually the longest fin ray. The
only exception to this procedure was the first dorsal
fin of haddock, where we measured the length of the
first fin ray, which is the longest fin ray for haddock.
Only whole, undamaged fin rays were measured.
Fish were then gutted and sexed based on macro-
scopic examination of the gonads. All drumming
muscles were subsequently removed using forceps.
These were placed in numbered aluminium trays
and dried at 608C and weighed daily (90.0001 g)
until the weight remained stable and no more weight
loss occurred to obtain muscle protein weight and
exclude water. We took the utmost care to remove
and only weigh the drumming muscle itself and not
any connective fibres or swimbladder tissue.
Data analyses
In addition to the results of the present study, we
examined published research on gadids for records
of sexual dimorphisms, drumming muscles, sound
production, reproductive behaviour, spawning shoal-
ing dynamics and habitat association.
For the new data, we tested for sexual dimorph-
isms by comparing pelvic-fin length or drumming-
muscle mass between sexes using ANCOVAs. We
controlled for the effect of body size by including
202 J. E. Skjæraasen et al.
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total length as the covariate for analyses involving fin
length as the response variable, and total weight in
analyses where drumming muscle mass was the
response variable. The initial models also contained
an interaction term between the categorical variable
sex and slope. If this parameter was not significant,
i.e. slopes were homogenous, a standard ANCOVA
analysis was applied. All mass and length data, i.e.
both the response and covariate variables, were log
e
-
transformed to meet the assumption of normality
and to linearize allometric relationships. We also
investigated the variability in pelvic-fin length and
Table I. Summary table of information on adult habitat, social behaviour and sexual dimorphism in codfishes (Gadidae); maximum size
(total length, cm); reproductive behaviour (RB); presence of, and sexual dimorphism in drumming muscles (D, drumming muscle present,
SD, sexually dimorphic drumming muscle), and the presence of sexual pelvic-fin length dimorphism (PFD) sorted according to habitat.
Under ‘Habitat’, D denotes demersal, P denotes pelagic, and BeP denotes bentho-pelagic. Under ‘RB’, A denotes the presence of
aggressive behaviour and C courtship behaviour during reproduction. Under D/SD the first Y denotes the presence of drumming muscles
and * indicates that actual sound production of the species has been recorded. The second Y indicates that the drumming muscle have been
shown to be sexually dimorphic. ** Juvenile but not adult saithe possess a drumming muscle. N denotes the absence of a drumming
muscle/pelvic-fin dimorphism. ‘’ Indicates that the species in question has not been examined for this particular trait. Data on habitat and
maximum size (cm) were obtained from the FAO species catalogue (Cohen et al. 1990).
Species Habitat Size RB D/SD PFD References
Melanogrammus aeglefinus
(Linnaeus, 1758)
D 100 AC Y*/Y N Hawkins & Rasmussen 1978; Hawkins & Amorim
2000; this study
Raniceps raninus
(Linnaeus, 1758)
D30Y*/Hawkins & Rasmussen 1978
Gadus ogac Richardson, 1836 D 70 Y/ Hawkins & Rasmussen 1978
Boreogadus saida
(Lepechin, 1774)
D40Y/Hawkins & Rasmussen 1978
Molva molva (Linnaeus, 1758) D 200 Y/Hawkins & Rasmussen 1978
Molva dypterygia
(Pennant, 1784)
D 155 Y*/ Hawkins & Rasmussen 1978 only examined in males
Brosme brosme
(Ascanius, 1772)
D 110 Y/Hawkins & Rasmussen 1978
Gaidropsarus mediterraneus
(Linnaeus, 1758)
D50Y*/Almada et al. 1996
Gaidropsarus vulgaris
(Cloquet, 1824)
D60NHawkins & Rasmussen 1978
Ciliata mustela (Linnaeus, 1758) D 25 NHawkins & Rasmussen 1978
Enchelyopus cimbrius
(Linnaeus, 1776)
D41NHawkins & Rasmussen 1978
Microgadus proximus
(Girard, 1854)
D30NHawkins & Rasmussen 1978
Eleginus navaga (Walbaum, 1792) D 42 NHawkins & Rasmussen 1978
Eleginus gracilis (Tilesius, 1810) D 55 NHawkins & Rasmussen 1978
Pollachius virens
(Linnaeus, 1758)
P 130 N** Y Hawkins & Rasmussen 1978, this study
Gadiculus argenteus
Guichenot, 1850
P15NHawkins & Rasmussen 1978
Micromesistius poutassou
(Risso, 1827)
P/BeP 50 N Y Andersen & Ja´kupsstova 1978; Hawkins & Rasmussen
1978; this study
Pollachius pollachius
(Linnaeus, 1758)
P/BeP 75 Y*/ Jonatan Nilsson, Institute of Marine Research,
Bergen, Norway, pers., comm
Trisopter us esmarkii
(Nilsson, 1855)
P/BeP 20 NHawkins & Rasmussen 1978
Trisopter us minutus
(Linnaeus, 1758)
BeP 40 NHawkins & Rasmussen 1978
Trisopter us luscus
(Linnaeus, 1758)
BeP 45 NHawkins & Rasmussen 1978
Gadus morhua Linnaeus, 1758 BeP 200 AC Y*/Y Y Brawn 1961a,b; Morgan & Trippel 1996; Engen &
Folstad 1999; Skjæraasen et al. 2006
Merlangius merlangus
(Linnaeus, 1758)
BeP 70 AC N Y Hawkins & Rasmussen 1978; Milic & Kraljevic 2011
Theragra chalcogramma
(Pallas, 1811)
BeP 80 C Y/Hawkins & Rasmussen 1978; Baird & Olla 1991;
Onuki & Somiya 2006
Gadus macrocephalus
Tilesius, 1810
BeP 100 N N Sakuri & Hattori 1996
Secondary sexual characteristics in codfishes (Gadidae) 203
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drumming-muscle mass by comparing the coeffi-
cient of variance (CV) for each trait, because theory
suggests that sexually selected characters exhibit
large individual variation (Andersson 1994).
We then used partial correlation to measure
correlation between pelvic-fin length and drumming
muscle mass while correcting for total body length.
This test determined if there was a trade-off between
pelvic-fin length and drumming-muscle size, i.e. do
males with large pelvic fins have small drumming
muscles after controlling for fish size (e.g. Engen &
Folstad 1999)? All three variables were log
e
-
transformed to linearize relationships.
Sexual dimorphism in morphological
characters
Drumming muscles
Sound production has now been described in more
than 800 teleosts worldwide (Kaatz 2002). Sound
can be produced by various means such as extruding
gas through the cloaca (Wilson et al. 2004) or
rubbing fins together (Fine et al. 1996), but the
most common mechanism for making sound in
teleosts is by contracting muscles attached to the
swimbladder wall, i.e. the ‘drumming muscles’
(Ladich & Fine 2006).
In accordance with the results of Hawkins &
Rasmussen (1978), drumming muscles were present
in cod and haddock, but not in saithe and blue
whiting (Table I). For haddock, there was a strong,
significant difference in drumming muscle size
between males and females with males having bigger
muscles (F
(1,75)
251, PB0.0001, Figure 1). Over-
all, the slopes of the drumming muscle sizebody
size relationships did not differ between sexes
(P0.05), but this result was strongly influenced
by a single point: a male with a very small drumming
muscle (Figure 1A). If this male was excluded from
the analysis, the difference in drumming-muscle
mass between the sexes increased with size
(F
(2,73)
251, PB0.0001, Figure 1A). For a given
body weight, male haddock also had larger drum-
ming muscles than male cod (F
(1,95)
80.5,
PB0.0001, Figure 1AB, Table II). The partial
correlation coefficients did not indicate that males
with bigger drumming muscles had shorter pelvic
fins for either cod (r0.108, P0.47) or haddock
(r0.096, P0.53).
Cod mating sounds consist of calls of short
duration, i.e. grunts and hums (Brawn 1961c;
Finstad & Nordeide 2004), whereas haddock can
produce long pulses lasting for several seconds with a
number of ‘knocks’(Hawkins & Amorim 2000). The
larger drumming-muscle size of male haddock
compared to similarly sized cod (Table II; Hawkins
1993) thus concurs with their larger vocal repertoire.
The observed sexual dimorphism in haddock drum-
ming muscle was also noted by Hawkins (1993).
Sound production may vary between populations
(Mann & Lobel 1998; Parmentier et al. 2005;
Amorim et al. 2010) and individuals (Amorim et
al. 2011); for cod, such differences in the frequency
of vocalizations are positively associated with drum-
ming muscle mass (Rowe & Hutchings 2006).
The presence of drumming muscles has, to our
knowledge, been examined in 25 different gadids
(family Gadidae) to date (Table I). Eleven species
possess well-developed drumming muscles in the
adult stage; eight of these have a predominantly
demersal lifestyle. The only clear exceptions were the
bentho-pelagic Atlantic cod, walleye pollock
(Theragra chalcogramma (Pallas, 1811)) and the
pelagic/bentho-pelagic pollack (Pollachius pollachius
(Linnaeus, 1758)) (Table I). It seems further likely
Table II. Summary of morphological measurements. Mean (M) and coefficient of variation (CV) of total length (TL, cm), total weight
(TW, g), drumming-muscle dry weight (DR, g) and lengths (cm) of the first (D1), second (D2), and third dorsal (D3) fin, pectoral (PF),
pelvic (PL), pectoral (PF), and first (A1) and second (A2) anal fin (all lengths are given in cm). Numbers in parentheses indicate sample
size. Not all fin measurements were conducted on each sample.
Blue whiting Haddock Saithe Cod
ß(10) à(25) ß(50) à(27) ß(56) à(57) ß(48) à(2)
M CV M CV M CV M CV M CV M CV M CV M CV
TL 24.9 0.06 25.8 0.05 49.8 0.16 53.1 0.13 65.0 0.21 60.6 0.21 70.9 0.09 68
TW 99 0.27 103 0.17 1450 0.48 2010 0.36 2806 0.67 2393 0.68 3570 0.23 3613
DR 2.2 0.57 0.37 0.55 0.50 0.68 0.16
D1 2.4 0.09 2.4 0.17 6.3 0.17 6.7 0.18 4.0 0.27 3.7 0.28 5.5 0.17 5.8
D2 2.1 0.14 2.4 0.14 3.4 0.35 3.5 0.16 3.2 0.29 2.9 0.29 4.4 0.22 4.6
D3 1.6 0.10 1.5 0.22 2.5 0.31 2.6 0.20 2.0 0.48 1.6 0.42 4.6 0.16 4.6
PF 3.0 0.14 3.1 0.10 5.2 0.26 5.7 0.16 5.3 0.22 4.9 0.25 6.6 0.11 6.5
PL 1.8 0.19 1.1 0.39 3.4 0.20 3.5 0.14 3.1 0.41 2.4 0.41 5.6 0.10 5.1
A1 1.8 0.15 1.9 0.14 3.7 0.22 4.1 0.18 3.8 0.27 3.5 0.30 4.9 0.25 4.9
A2 1.6 0.06 1.5 0.15 2.6 0.30 2.83 0.20 2.1 0.51 1.6 0.45 4.8 0.12 4.8
204 J. E. Skjæraasen et al.
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that sound production during reproduction is not
only found in the Gadidae family, but instead could
be more widespread in the order Gadiformes.
Indeed, drumming muscles have been reported for
European hake (Merluccius merluccius (Linnaeus,
1758)) (Groison et al. 2011). Many gadiforms are
demersal, a lifestyle that appears to favour sound
production (Table I).
Fin lengths
Sexual fin dimorphisms are found in many teleosts
(e.g. Ostrand et al. 2001; Park et al. 2001) and may
take on very elaborate forms (e.g. Kottelat et al. 2006;
Britz & Conway 2009). Notably, we found only the
pelvic fin to be sexually dimorphic in the gadids
examined. There was no sexual dimorphism in dorsal,
anal or pectoral fins for haddock, saithe or blue
whiting (P0.05 for all cases). In contrast, pelvic
fins were sexually dimorphic with males having longer
fins than females for saithe (F
(1,108)
9.09, PB0.01,
Figure 2) and blue whiting (F
(1,29)
17.9, PB0.001,
Figure 2), but not for haddock (F
(1,72)
0.798,
P0.38). The slopes of the fin lengthbody length
relationships did not differ between sexes for either
species (P0.05). Fin lengths were not compared
between sexes for Northeast Arctic cod, because only
2 out of the 50 sampled fish were female Table II), but
cod have previously been shown to possess sexually
dimorphic pelvic fins (Skjæraasen et al. 2006). Fish
were generally well above the size at which maturation
is expected to occur (Table II).
Our results concur with the results of Andersen &
Jakupsstova (1978), who detected sexual dimorph-
ism only in the pelvic fins of blue whiting, and Engen
& Folstad (1999), who examined the ventral and
dorsal fins of Norwegian coastal cod and found them
not to be dimorphic. Sexual dimorphism in the
pelvic fins is not restricted to gadids, but has also
been reported for various other families (e.g.
Schenck & Whiteside 1977; Barbieri et al. 1992;
Figure 1. Drumming muscle mass of male (grey) and female
(black) (A) haddock (Melanogrammus aeglefinus) and (B) cod
(Gadus morhua). The white point (A) indicates the ‘outlier’male
mentioned in the results. Note the different scales on the y-axes of
both graphs.
Figure 2. Pelvic-fin versus total length of male (grey circles) and
female (black circles) (A) saithe (Pollachius virens) and (B) blue
whiting (Micromesistius poutassou).
Secondary sexual characteristics in codfishes (Gadidae) 205
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Oliveira & Almada 1995; Kottelat et al. 2006; Britz
& Conway 2009; Arbour et al. 2010). Yamanoue
et al. (2010) proposed that the pelvic fin may be
more readily modified by sexual selection than other
fins, given their limited use for propulsion during
swimming.
In our complementary study the coefficient of
variation (CV) was generally lower for fin lengths
than for body weight or drumming muscle mass
(Table II). We found no indication that males had a
larger CV than females in the sexually dimorphic
traits (Table II). These findings match the results of
Skjæraasen et al. (2006); potential explanations for
why this occurs are outlined there and therefore not
reiterated here.
Sexual dimorphisms in relation to habitat use,
sound production and social behaviour
Sexual dimorphisms can give insights into mating
systems. For example, in Paedocypris progenetica
(Kottelat et al., 2006) the males possess modified
pelvic fins with hypertrophied muscles and a kerati-
nized pad in front of the pelvic girdle (Kottelat et al.
2006). This is thought to function as a clasping or
holding device used during reproduction to either
facilitate internal fertilization, secure the male’s
position on a spawning site, or give males the
possibility to manipulate eggs.
Compiling the limited drumming muscle data and
general life-history information of the different gadid
species, some patterns emerge. First, drumming
muscles, and presumably sound production appear
common, but occur predominantly in demersal
species (Table I). As in other teleosts, gadids use
sound production for social communication (Ladich
& Fine 2006). The main contexts in which sound
production occurs are male mating calls and aggres-
sive vocal displays towards other males during the
reproductive period and in food and territorial
contests (Hawkins 1993). The latter may thus
involve both juveniles and adults throughout the
year. Arguably, territorial contests are more likely to
occur at the seafloor where potential landmarks may
make resources defendable (Brawn 1961c). Tenta-
tively supporting this, some of the world’s most
highly vocal fish are both demersal and highly
territorial such as Lusitanian toadfish Halobatrachus
didactylus (Bloch & Schneider, 1801) and plainfin
midshipman Porichthys notatus Girard, 1854 (e.g.
Bass et al. 2008; Amorim et al. 2010). Saithe are
interesting as they possess drumming muscles as
juveniles when occupying the demersal, benthic
habitat, but lack these muscles in the adults that
are pelagic (Hawkins & Rasmussen 1978). For
saithe, the primary function of sound production
may thus be to support interference competition for
food or shelter or social aggregation formation
during the juvenile phase.
Drumming muscles are absent in most pelagic/
semi-pelagic gadids examined to date (Table I). The
only exceptions were the bentho-pelagic cod and
walleye pollock and the pelagic/bentho-pelagic pol-
lack. Interestingly, the pelagic whiting (Merlangius
merlangus (Linnaeus, 1758) exhibit similar repro-
ductive behaviour to cod and haddock, but drum-
ming muscles are absent and no sounds are
produced during reproduction (Hawkins & Rasmus-
sen 1978). Hence, although sound production is
associated with courtship and aggression in cod and
haddock, it is not an obligatory feature of this
particular gadid reproductive behaviour.
It has been suggested that sound production in
male haddock may be important in attracting distant
females to male-biased spawning aggregations
(Hawkins & Amorim 2000). This is known as
acoustic chorusing, and has been well studied in
other taxa such as insects and lekking anurans (e.g.
Ryan et al. 1981; Castellano et al. 2009). Fishing
targeted at northeast Atlantic haddock spawning
shoals produce catches dominated by males, clearly
indicating that haddock do indeed form such sex-
biased shoals (Knut Korsbrekke, Institute of Marine
Research, Bergen, Norway, pers. comm.).
Male cod also aggregate in reproductive shoals
that resemble leks and produce a loud chorus that
can be detected several kilometers away (Nordeide &
Kjellsby 1999; Nordeide & Folstad 2000). Forma-
tion of similar sex-biased shoals has also been noted
for walleye pollock (Baird & Olla 1991 and refer-
ences therein), but has hitherto not been examined
in the sound-producing bentho-pelagic/pelagic pol-
lack. Interestingly, the bentho-pelagic gadoid Eur-
opean hake possess drumming muscles (Groison
et al. 2011), and the closely related Argentinean
hake (Merluccius hubbsi Marini, 1933) form sex-
biased shoals off the Patagonian coast (Martin
Ehrlich, INIDEP, Buenos Aires, Argentina, pers.
comm.).
Previously it has been suggested that sound
production in gadids may be linked to fish size in
relation to predation pressure, i.e. larger gadids are
safer from predators and have much lower risk when
producing sound (Hawkins & Rasmussen 1978),
and, second, that it is mostly absent in schooling fish
(Hawkins 1993). While our comparative analysis
does not dismiss such explanations (Table I), we
suggest that there is clearly merit in examining
whether sound production is also linked to the
formation of sex-biased spawning shoals whenever
present in semi-pelagic gadids. Obvious candidates
for a comparative study are the sound-producing
206 J. E. Skjæraasen et al.
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bentho-pelagic/pelagic pollack and the ‘silent’pela-
gic saithe (Table I).
The male pelvic fin likely has a special significance
during reproduction in gadids. It has been shown to
be used prominently in both courtships towards
females and during antagonistic interactions be-
tween males for cod and haddock (Brawn 1961a,
b; Hawkins & Amorim 2000). Similar reproductive
behaviour has also been observed for walleye pollock
(Baird & Olla 1991; Park et al. 1994) and whiting
(Hawkins & Rasmussen 1978), with the latter also
showing the same fin dimorphism (Table I).
Given the observed dimorphism in blue whiting
and saithe (Figure 2), similar courtship and antag-
onistic displays may be present in these species as
well. It is curious that the pelvic fins were not
sexually dimorphic in haddock, despite their docu-
mented use in haddock reproductive behaviour
(Hawkins & Amorim 2000) and in contrast to the
dimorphisms exhibited by our other study species.
We can only hypothesize as to the causes, but it may
be that their large investment in drumming muscle
size (Figure 1) and the associated, complex (Haw-
kins & Amorim 2000), energetically costly sound
production (e.g. Amorim et al. 2002) has hindered
the development of sexually dimorphic pelvic fins.
Concluding remarks
Our review of previously published accounts indi-
cates that drumming muscles, and as a consequence,
sound production, is common in gadids, and seems
to be associated primarily with the benthic habitat.
Close to the bottom, sound production probably has
a function during both contests for food and
territories, and for mate attraction and agonistic
encounters between males, mainly during the repro-
ductive season. For pelagic/bentho-pelagic gadids,
the presence of drumming muscles may be linked to
the formation of sex-biased spawning shoals during
spawning, but more research is needed to further
investigate this assumed function.
The sexually dimorphic pelvic fins are likely to
play an important role during reproduction in some
North Atlantic gadids, potentially in support of male
courtship and aggressive displays. Sexually di-
morphic traits and/or courtship behaviour have
been studied only in few gadid species so far.
Complex mating systems and non-random mate
choice may be widespread and hence we encourage
morphological studies to shed light into the repro-
ductive biology of these fishes, which includes
several heavily harvested species. Such studies
should preferably also be designed in a way that
makes it possible to further disentangle inter-,
intrasexual, and natural selection and their differ-
ential influences on dimorphic characters (e.g.
Lailvaux & Irschick 2006; Bonduriansky 2007;
Clutton-Brock 2009).
Acknowledgements
We thank Marius Moe for his invaluable contribu-
tion in the laboratory analyses and the scientists and
crew aboard the IMR research vessels for their help
in collecting the samples. A special thanks in this
regard goes to Asgeir Aglen and Erik Berg at IMR.
We also thank J. Nilsson and M. Ehrlich for sharing
unpublished results. The study was supported by the
Research Council of Norway projects ‘172649’and
‘190228’and by the Bergen Research Foundation.
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