Plumatella nitens, a new species of freshwater bryozoan from North America (Ectoprocta: Phylactolaemata), previously misidentified

Abstract

Plumatella nitens is a newly recognized species of phylactolaemate bryozoan in North America. It has previously been combined with either P. repens or P. fungosa, from which it differs primarily by the floatoblast. In P. nitens the ventral floatoblast annulus is uniformly narrow with no significant widening at the poles. The dorsal floatoblast fenestra is conspicuously larger than in either P. repens or P. fungosa. Reticulated ridges on the dorsal fenestra are weak along the margins, becoming unusually prominent in the center. The sessoblast is densely covered with uneven papillae. Spotty sampling so far shows a distribution only in the northern half of North America, including Massachusetts, Michigan, Minnesota, Wisconsin, Ontario, and the northern regions of Illinois, Indiana, and Ohio.

Full text

Hydrobiologia 328: 147-153, 1996.
147
© 1996 Kluwer Academic Publishers. Printed in Belgium.
Plumatella nitens, a new species
of
freshwater bryozoan from North America
(Ectoprocta: Phylactolaemata), previously misidentified
Timothy S. Wood
Department
of
Biological Sciences, Wright State University, Dayton, OH 45435,
USA
Tel:
(513) 873-2542; Fax: (513) 873-3301; E-mail: twood@desire.wright.edu
Received 20 June 1995;
in
revised form
18
October 1995; accepted 29 November 1995
Key words: Ectoprocta, bryozoa, Phylactolaemata, Plumatella, taxonomy
Abstract
Plumatella nitens is a newly recognized species
of
phylactolaemate bryozoan in North America.
It
has previously
been combined with either
P.
rep ens or
P.
fungosa, from which it differs primarily by the floatoblast. In
P.
nitens
the ventral floatoblast annulus
is
uniformly narrow with no significant widening at the poles. The dorsal floatoblast
fenestra is conspicuously larger than in either
P.
repens or
P.
fungosa. Reticulated ridges on the dorsal fenestra
are weak along the margins, becoming unusually prominent
in
the center. The sessoblast is densely covered with
uneven papillae. Spotty sampling
so
far shows a distribution only
in
the northern half
of
North America, including
Massachusetts, Michigan, Minnesota, Wisconsin, Ontario, and the northern regions
of
Illinois, Indiana, and Ohio.
Introduction
The number
of
reported species
of
phylactolaemate
bryozoans
in
North American currently totals twenty
(Wood, 1991). Recent additions to the list include Plu-
matella reticulata Wood 1988 and Plumatella recluse
Smith 1992. Both are identified mainly
by
the mor-
phology
of
their statoblasts, which are the sclero-
tized dormant buds formed
by
all phylactolaemate bry-
ozoans. This paper describes a new Plumatella species,
P.
nitens, identified by a distinctive floatoblast and
glassy ectocyst.
In
recent years there has been a change in the char-
acters used to identify species
of
freshwater bryozoans.
For Plumatella species, early workers concentrated
largely on colony morphology: whether the colony was
compact or repent, branches free or attached, zooids
fused or separated, and so forth. The laboratory rearing
studies
of
Toriumi (e.g. 1971) showed many
of
these
features to be easily influenced by environmental con-
ditions and therefore unreliable
as
species characters.
In general, many species
of
phylactolaemate bry-
ozoans are distinctive enough to be easily identified at
a glance. Significant features include overall colony
growth form (such
as
tubular or globular) and the
shape and size
of
the statoblasts. However, among the
Plumatellidae positive species identification is often
impossible without statoblasts. Especially difficult is
the cluster
of
species resembling Plumatella repens,
including
P.
fungosa,
P.
coralloides,
P.
orbisperma,
P.
javanica,
P.
philippinensis,
P.
recluse,
P.
vaihiriae,
P.
vorstmani, and possibly the species
of
Stolella. All
are composed
of
branching tubules with a relatively
clear body wall, reduced keel, similar statoblasts, and
variable colony morphology. True genetic differences
among several
of
these species have been demonstrated
by enzyme electrophoresis (Mundy & Thorpe, 1979,
1980), but the technique is not refined to the point
of
identifying unknown species.
Instead, attention has recently focused on the sur-
face morphology
of
statoblasts for taxonomically sig-
nificant features. The hard outer statoblast shell
is
adorned with a variety
of
tiny bumps, pits, ridges,
grooves, and other features. Evidence so far indicates
a high consistency
of
this microstructure within certain
Plumatella species (Reynolds, 1995). Floating stato-
blasts (floatoblasts) have a peripheral ring (annulus)
of
gas-filled cells, which is generally widest on the dor-
sal, or cystigenic side. The annulus surrounds a clear,
central region, known as a fenestra. A suture around

148
the statoblast periphery marks the joint between the
two valves, which separate upon germination.
This paper describes a new species originally clas-
sified
as
Plumatella repens or
P.
fungosa, but with con-
sistent features that set it clearly apart from all other
described species.
Materials and methods
Colonies in Illinois, Indiana, Ohio, Michigan, and Wis-
consin were collected in shallow water along with a
thin layer
of
substrate on which they were growing.
The colonies were narcotized with menthol, fixed
in
10% formalin, and stored
in
70% ethanol with 3%
glycerol. For laboratory rearing studies, living sta-
toblasts were stored in water for at least six weeks
at 4°
C.
, then brought to 23°C where they germinat-
ed within one week. Colonies were reared for eight
weeks using methods described by Wood (1989). I
also examined preserved specimens from collections
in
Massachusetts, Michigan, and Minnesota.
I measured all statoblasts with an electronic video
digitizer (Fink, 1987), using CODA software by Julian
Humphries. Statoblast measurements from
17
sites
were combined for statistical analysis by ANOVA
(parametric), with confidence limits calculated at 95%.
For examining individual valves, I immersed fioa-
toblasts for one minute in a hot, saturated KOH solu-
tion, then mounted them
in
distilled water and teased
apart the valves with needles.
For electron microscopy statoblasts were dehydrat-
ed
in
an
ethanol series, dried
in
Peldri n® (Ted Pella,
Inc.), sputter coated
in
palladium alloy, and examined
with a Philips 500 scanning electron microscope.
Description
PLUMATELLA NITENS, new species
Plumatella repens (Bushnell, 1965)
Plumatella repens (Wood, 1989)
Plumatellafungosa (Smith, 1989)
Plumatellafungosa (in part, Wood, 1991)
The colony
is
tubular and branching, widely repent
on unrestricted surfaces, becoming crowded and com-
pact with contiguous recumbent or upright zooids. The
zooids are capable
of
fusing into a stiffly solid mass.
Rope-like strands
of
fused zooids may extend freely
into the water for several centimeters.The young ecto-
cyst
is
soft, transparent, and undistinctive; becoming
relatively stiff, smooth, and shiny; sometimes slightly
reddish or amber
in
color. A keel is usually absent,
or else only barely discernible. The rare colonies with
a light frosting
of
particles on the ectocyst display a
ragged, transparent, V-shaped region around the zooid
tip, with the point
of
the V drawn out
as
a transparent
band, or furrow, along the zooecial tube. Internally nar-
rowed regions often appear abruptly at intervals along
the tubular zooecia; some
of
these are further constrict-
ed by small, incomplete septa. In most specimens such
septa are rare or absent, although in laboratory reared
colonies they occur between most zooids. The number
of
tentacles ranges from 38
to
51, with a mean
of
42
(53 counts).
Overall the fioatoblast
is
broadly oval to near-
ly
round, and the annulus relatively narrow on both
valves. The dorsal fenestra
is
large and truncated at the
poles by annular encroachment (Figures
la
and
lc)
.
This contrasts sharply with fioatoblasts
of
Plumatella
rep
ens (Figure
lb)
or
P.
fungosa, in which the dorsal
fenestra
is
relatively small and the annulus
is
distinctly
wider at the poles. Tubercles at the fenestra margin
in
P.
nitens become fiatter towards the center, often
disappearing entirely (Figures
lc
and ld). Reticulation
is
faint at the fenestra margin, becoming increasingly
elevated and prominent at the center (Figures
lc
and
Id). Polar grooves
in
the annulus at either end
of
the
fenestra are deep and conspicuous, their length vary-
ing between 0.2 and 0.3 times the fioatoblast width
(Figures
lc
and
ld)
. The fioatoblast dorsal annulus has
tubercles present only on the inner margin where the
annulus folds over
to
meet the fenestra; distal to this
margin the tubercles become abruptly lower, blending
into the smooth, slightly
lumpy
~
surface
of
the annulus
(Figure
If)
. An extremely faint dorsal reticulation
is
sometimes seen in scanning micrographs. On the fioa-
toblast ventral fenestra high tubercles at the margin
become lower toward the center, but seldom disappear
entirely; reticulation
is
generally uniform throughout,
the enclosed areas becoming smaller towards the cen-
ter. There
is
usually a raised point at the center
of
the fenestra. The fioatoblast ventral fenestra is nar-
row and nearly uniform in width, but otherwise much
like the dorsal annulus. A continuous cord marks the
suture between the valves, and short lateral ribs appear
at close intervals with either an opposite or alternate
arrangement (Figure 1
f)
. The sessoblast frontal valve
is
densely covered with tall, rounded papillae
of
varying
height; the lamella bears the raised, reticulated lines

149
a
Figure J. Statoblast details. (a) Separated valves
of
P nitens from Shabbona Lake showing the large dorsal fenestra (left) and the uniform
ventral annular width (right), x
100
; (b) Separated valves
of
Plumutellu repens from Lake Wildwood Haven, Fulton County,
IL,
showing the
relatively small dorsal fenestra (left) and a distinct widening
of
the polar annulus on both valves, 100
x;
(c) SEM micrograph showing dorsal
side
of
P nitens ftoatoblasts from the Hoosic River, Massachusetts, with deep polar grooves (arrows) and the characteristic central reticulation,
230
x;
(d) SEM micrograph
of
P nitens ftoatoblast from Shabbona Lake, showing an oblique view
of
the dorsal surface, 460 x ; (e) Sessoblast
of
P nitens from Shabbona Lake, 120
x;
(f) Floatoblast
of
P nitens from Shabbona Lake showing the suture zone between valves marked by a
raised cord and an opposite arrangement
of
late
ral
rib
s,
1250
x.

150
typical
of
the genus (Figure Ie).
Type
Material: The holotype was collected June 30,
1990 at Shabbona Lake, DeKalb Couny, Illinois; lati-
tude
41
0
44' 19"N, longitude
88
° 51' 9"W, deposited
at the National Museum
of
Natural History (Smith-
sonian), in Washington, D.C., USNM No. 16061. A
paratype from the same collection
is
deposited at the
Illinois Natural History Survey, Springfield, Illinois.
Etymology. The name nitens
is
the present participle
of
the Latin verb meaning 'to shine',
in
reference to the
glassy ectocyst.
Dimensions: Critical statoblast measurements are giv-
en
in
Table 1. For comparison, corresponding mea-
surements for Plumatella repens appear
in
Table
2.
Distribution. Plumatella nitens
is
currently known
in
North America from Massachusetts west into Illinois
and Minnesota, and north
as
far as Georgian Bay and
the upper peninsula
of
Michigan (Figure 2). All but
two
of
the known occurrences are above the
41
st north
parallel.
Specimens.
We
have examined material from the fol-
lowing locations: In ILLINOIS - Cook Co., Maple
Lake; DeKalb Co., Shabbona Lake; Kankakee Co.,
Bird Park Quarry; Lake Co., Deep Lake; Stephenson
Co., Lake Le-Aqua-Na; Will Co., Lake Renwick (all
collected by
T.
Marsh and
T.
Wood). In INDIANA -
Noble Co., Bear Lake and Crooked Lake (all collect-
ed by
T.
Wood). In OHIO - Columbiana Co., High-
landtown Wildlife Ponds; Defiance Co., Oxbow Lake;
Lake Co., Holden Arboretum Blueberry Pond; Mont-
gomery Co., Unnamed pond at Harshman Road and
Brandt Pike; Ottawa Co.; Lake Erie (Lost Ballast and
Starve Islands) (all collected by
T.
Wood). In MICHI-
GAN - Alger Co., Kingston Lake (colI.
T.
W.
Porter);
Barry Co., Crooked Lake (colI.
T.
Marsh and
T.
Wood);
Chippewa Co., Bass Lake; Isabelle Co., Hall Lake;
Kalamazoo Co., Gull Lake; Livingston Co., Indian
Lake (all colI. by
J.
Bushnell). In MASSACHUSETTS
- Berkshire Co., Hoosic River below Cheshire Reser-
voir (collected
by
D. Smith). In MINNESOTA - Carv-
er Co., Lake Waconia (collected
by
G.
Proulx). In
WISCONSIN - Gogebic Co., Paul Lake (collected
by
L. Wood).
Habitat. Plumatella nitens occurs exclusively
in
lentic
habitats where it grows mostly on old wood and veg-
etation. The Massachusetts specimens were on old
concrete. Living colonies with statoblasts have been
found from June through early October. At the time
of
collections
in
Illinois the water temperatures were
24 °
-28.5
°C, the pH was 7.9 to 8.6, and the con-
ductivity ranged from 240 to 680 mhos. Fredericella
indica
occurred with P nitens at seven out
of
six-
teen sites
in
Illinois and Ohio, which is more than
twice
as
often
as
any other bryozoan species. Oth-
er bryozoan species seen
to
occur
in
the same lake as
P nitens include Cristatella mucedo, Fredericella aus-
traliensis, Lophopodella carteri, Pectinatella magnifi-
ca,
Plumatella casmiana, P emarginata, P jruticosa,
P fungosa, P orbisperma, P repens, P reticulata, and
Pottsiella erecta. At four sites
in
Illinois P nitens was
the only bryozoan species found.
Discussion
It
is
tempting
to
dismiss the P nitens f1oatoblast as
an
environmentally induced phenotype
of
P repens.
When Toriumi (1970) reared P repens at low tempera-
tures the colonies produced f1oatoblasts similar in pro-
portions to those
of
P nitens. The absence
of
P nitens
in
more southerly (and warmer) states would be con-
sistent with this idea. However,
in
Ohio and Illinois our
collection dates ranged from June through September
with
no
change
in
f1oatoblast morphology.
We
also
reared both P repens andP nitens
in
the laboratory
in
the same container, presumably under identical condi-
tions. In each case the second generation offloatoblasts
matched the first, both
in
critical dimensions and
in
sur-
face ultrastructure. Thus the morphological evidence
for two distinct species
is
fairly strong.
Were it not for its characteristic f1oatoblast, P nitens
would be hard
to
distinguish from P repens or P fun-
gosa.
In all three species the colonies have mostly hya-
line, tubular zooecia. Septa may be frequent or entirely
absent. The arrangement
of
zooids ranges from widely
open to densely compact. There
is
little or no keel or
furrow. All three species produce large quantities
of
f1oatoblasts, and their sessoblasts are very similar.
These similarities explain how Plumatella nitens
could have been so often observed
by
others but not
recognized
as
a different species. In my 1989 mono-
graph
of
Ohio bryozoans, I failed to interpret the fea-
tures
of
P nitens as distinct from those
of
P repens.
Bushnell (1965) described P nitens as Plumatella
repens
and provided excellent photos
of
f1oatoblasts
and a septum.
It
is
likely that the natural popula-

151
Table 1. Critical measurements
of
Plumatella nitens tloatoblasts from thirteen sites in Illinois,
Indiana, and Ohio. Dimensions are expressed in micrometers.
Number
of
Floato measurements
Maximum Minimum Mean measurements
Overall length
382
272
327
±4
152
Overall width
307
207
256
±4
1
52
Overall length/width
1.53
1.06
1.28±0
.
02
152
Dorsal fenestra length
232
145 193
± 5
82
Dorsal fenestra width
224
138
179
±4
82
Dorsal fenestra length/width
1.41
0.
91
1.
08
±0.02
82
Ventral fenestra length
307
202
255
±5
70
Ventral fenestra width
237
160
206
±4
70
Ventral fenestra length/width
1.42
l.ll
1.24
±2
70
Table 2. Critical measurements
of
Plumatella repens tloatoblasts from eighteen sites in Illinois.
Dimensions are expressed in micrometers.
Floato measurements Maximum
Overall length
418
Overall width
275
Overall length/width
1.78
Dorsal fenestra length
221
Dorsal fenestra width
181
Dorsal fenestra length/width
1.57
Ventral fenestra length
284
Ventral fenestra width
274
Ventral fenestra length/width
1.55
tion detailed
by
Bushnell (1966) was also Plumatel-
la
nitens, not P repens. In a collection
of
Michigan
bryozoans at Michigan State University, six
of
the six
specimens labelled 'Plumatella repens' had all the fea-
tures
of
P nitens. I have not yet seen specimens
of
what
I would consider Plumatella repens from Michigan. In
Massachusetts a specimen
of
P nitens was identified
by Douglas Smith
as
P fungosa based on the strong
mutual adherence
of
the zooids. However, the floato-
blast leaves no question that this is P nitens (Figure 1d).
So far there is no evidence
of
P nitens outside North
America. From the British Museum (Natural History)
we have examined British specimens
of
Plumatella
repens
(Nos. 1937.8.9.19, 1908.3.26.6, 1909.7.25.8)
P fungosa, (Nos. 1911.12.
17
.
1,
1917.1.1.2,
1971.2.16.1) and P coralloides (Nos. 1934.4.6.1,
1917.1.1.17, 1971.2.16.
2)
without finding any floa-
toblast resemblance to P nitens. The scanning elec-
tron micrographs
of
European Plumatella floatoblasts
(Mundy, 1980; Geimer
& Massard, 1986; Franz, 1992)
Number
of
Minimum Mean
measurements
290
339
±3
430
185
231
±2
430
1.25
1.47
±0.01
430
107 157
±4
245
99
141
±3
245
0.84
l.ll
±0.01
245
170
223
±3
218
134 178
±2
218
0.91
1.26±0.01
218
also lack the surface microstructure characteristic
of
P nitens. Similarly, the drawings
of
Kraepelin (1887)
and lullien (1885) illustrate floatoblasts that are rel-
atively more oval and with wider polar annulus than
occurs
in
P nitens. On the other hand, Allman (1856)
illustrates
in
Plate 5, Figure
7a
stylized floatoblast
of
'Plumatella repens' with an annulus
of
uniform width
exactly
as
it might occur in P nitens. Allman reviews
the confusing synonymy associated with P repens but
does not describe the floatoblast in any detail. Unfortu-
nately, our attempts to locate Allman's bryozoan speci-
mens have been so far unsuccessful, and the collection
may no longer exist.
Plumatella nitens brings
to
three the number
of
Plu-
matella
species known to form colonies
in
which the
zoo ids sometimes fuse together. The other two species,
P casmiana and P fungosa, both occur within part
of
the North American range
of
P nitens. However,
in
P casmiana the uniquely short branches and distinc-
tive statoblasts make this unlikely
to
be mistaken for

152
Figure
2.
Map showing the known distribution
of
Plumatella nitens
in
North America. Closed circles indicate sites where the species has been
documented; open circles indicate known collection sites at which the species was not found.
any other species (Wood, 1989). Floatoblasts
of
P.
fun-
gosa
resemble those
of
P.
repens (Figure 1b) but they
are distinctly asymmetrical
in
lateral view, with one
valve strongly convex and the other nearly flat; a
nd
both the annulus and fenestra bear a reticulated sur-
face pattern as seen with scanning electron microscopy
(Wood, 1989; Geimer & Massard, 1986, 1987).
Acknowledgments
This work benefited substantially from advice and
field assistance
of
Terrence Marsh,
of
North Central
College, Illinois. Additional field assistance was pro-
vided by Anne Seitz, Frank Weber, Kristine Wood
McDaniel, and Lisa Wood. Marc Allen Smith did the
comparative laboratory rearing
of
Plumatella repens
and
P.
nitens. Preserved specimens were generous-
Iy
provided
by
Mary Spencer Jones (British Museum
of
Natural History), Douglas G. Smith (University
of
Massachusetts at Amherst), Richard
1.
Snider (Michi-
gan State University), and Raymond Newman (Uni-
versity
of
Minnesota). Kevin Reynolds provided the
micrograph
in
Figure If. Laura Marsh reviewed an
earlier version
of
the manuscript. The study was sup-
ported
in
part
by
a grant from the Illinois Department
of
Conservation.
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