Bryozoa: an introductory overview

2005·1.6 MB·English

by  Ryland, John S

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Bryozoa: an introductory overview
J.S. RYLAND
Introduction REED (1991) and, comprehensively, by
MUKAl et al. (1997), whose survey should be
Bryozoa, sometimes called Ectoprocta,
consulted by those seeking information on
constitute a phylum in which there are
zooidal soft-part morphology, cells, and ul-
probably more than 8,000 extant species
trastructure in extant bryozoans of all class-
(the much quoted figure of 4000, made
es. The account also includes much original
nearly 50 years ago (HYMAN 1959), is cer-
description based on the phylactolaemate
tainly a serious underestimate). Some au-
Asajirella (formerly Pectinatella) gelaanosa.
thorities regard the phylum Entoprocta to
This work essentially provides the English
be related to Bryozoa, but the evidence is
language update for HYMAN's (1959) classic
conflicting and opinion divided (see
text, incorporating a thorough review of 40
NIELSEN 2000). The bryozoans are a widely
years' significant research. The linkage of
distributed, aquatic, invertebrate group of
the Bryozoa with Brachiopoda and Phoroni-
animals whose members form colonies com-
da, as 'lophophorates' (following HYMAN
posed of numerous units known as zooids.
1959) may no longer be tenable; certainly,
Until the mid 18th century, and particular-
the feeding mechanisms may not be as simi-
ly the publication of John ELLIS' "Natural
lar as once supposed (NIELSEN & RllSGÄRD
History of the Corallines" (1755), bry-
1998, and references therein).
ozoans, like corals and hydroids, were re-
garded as plants. This is reflected both in
the name of the phylum, which translates as General features: size range
'moss animals' and in the term 'zoophyte' and diversity of structure
which was used by LINNAEUS (1758) to em-
Bryozoan colonies vary in size. Among
brace both bryozoans and hydroids. Seven-
Gymnolaemata, colonies of Monobryozoon,
ty-five years after ELLIS, bryozoan zooids
which live between particles of marine sand,
were distinguished from those of cnidarians
consist of little more than a single feeding
by possessing both a mouth and an anus (DE
zooid less than one millimetre in height.
BLAINVILLE 1820) and, a decade later,
Colonies of the coralline genus Pentapcrra of
EHRENBERG (1831) formalized the distinc-
European seas, however, can reach 1 m or
tion by introducing the names Bryozoa and
more in circumference; in the warm-water
Anthozoa. Bryozoans are separated into
Zoobotryon, which hangs from harbour pil-
three classes: Phylactolaemata (freshwater
ings, and the phylactolaemate Pectinateüa,
dwelling); Stenolaemata (exclusively ma-
massive colonies may exceed 0.5 m in diam-
rine); and Gymnolaemata (mostly marine).
eter. Colonies that form crusts generally cov-
The order Cheilostomatida (class Gymno-
er only a few square centimetres; erect
laemata), containing over 600 genera, is the
colonies may rise only 2-5 cm, though the ge-
most successful bryozoan group. General ac-
latinous Akyonidium (Fig. 1), washed ashore
counts of the phylum have been provided by
during autumn gales, may exceed 15 cm.
CORI (1941), HYMAN (1959), BRIEN (1960),
RYLAND (1970), and WOOLLACOTT & ZIM- The texture of colonies is variable. Some,
MER (1977), though all are now, in certain especially in fresh water and on seashores, are
respects, outdated. More specific but impor- gelatinous or membranous; others are tufted,
tant reviews have been provided by RYLAND with flat, leaf-like fronds or whorls of slender Denisia 16, zugleich Kataloge
(1976a), MCKINNEY & JACKSON (1989), branches, whose horny texture results from der OÖ. Landesmuseen
Neue Serie 28 (2005), 9-20
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opens into a digestive tract that is divided
into several regions and terminates at an
anus, which is outside (but near) the tenta-
cles (hence the name alternative name Ec-
toprocta, meaning 'outside anus'). If zooids
are disturbed, they withdraw their tentacles
inside the body cavity. Only if the zooids
have transparent walls, as in the gymnolae-
mate genera Eowerbanlaa and Membranipo-
ra, is the digestive tract visible. The internal
living parts of each zooid - i.e., the nervous
and muscular systems, the tentacles, and the
digestive tract - are called the polypide,
while the surrounding walls and their asso-
ciated tissues constitute the cystid (Fig. 2).
Form and function: zooids
Fig. 1: Erect, gelatinous colonies of lightly calcified zooid walls. Still other Although zooid appearance and struc-
Alcyonidium diaphanum (Ctenostomatida). colonies are hard and have strongly calcified ture vary considerably from class to class,
Many zooids in the colonies contain
skeletons. Such colonies may form rough-sur- and even between orders, all conform to a
clusters of developing oocytes and/or
larvae in zooidal brood chambers derived faced patches or may rise in slender branch- common basic plan (Fig. 2). The predomi-
from the tentacle sheath. Scale bar 5 x 1 ing twigs (such as those that form a network nant, feeding zooid in a bryozoan colony is
cm. Origin: west coast of Scotland, Junde in the 'lace corals', e.g. Reteporelia). the autozooid. Autozooids are rarely longer
1998.
than one millimetre and the primitive shape
The colonies, diverse and complex in
appears to have been cylindrical (as e.g. in
structure, are composed of individual mod-
Fig. 6). The skeleton is external, ranging
ules, or zooids, each of which has consider-
from a thin cuticle to a thick, calcified lay-
able individuality. A bryozoan colony usual-
er. The tentacles, collectively termed the
ly has many zooids, which may be of one
lophophore, can be raised above the zooid
type or of types that differ both functionally
on a slender extension of the body wall (the
and structurally. Neighbouring zooids are
introvert or tentacle sheath clearly apparent
usually firmly joined and communicate via
in Fig. 6) and outspread for feeding. If dis-
tiny pores in the dividing walls. Zooids ca-
turbed, the tentacles can be withdrawn into
Fig. 2: A stained zooid of Membranipora pable of feeding have a protrusible ring of
membranacea, the polypide brown and slender tentacles at their distal end, on the body cavity in a movement that in-
the cystid walls pale. Mural pore chambers volves inrolling the tentacle sheath, with
which are found cilia that propel tiny parti-
can be seen (Preparation made by Dr.
the mouth and tentacles being pulled down
cles of food toward the mouth. The mouth
Genevieve Lutaud).
within it by the action of paired retractor
muscles. Eversion of the withdrawn tenta-
cles is effected by raising the hydrostatic
pressure of the body fluid, so compressing
the tentacle sheath; but the precise mecha-
nism differs from class to class. Phylactolae-
mates have a muscular and contractile body
wall to achieve this function; in gymnolae-
mates the wall is non-muscular but in whole
or part flexible, so that it can be pulled in-
ward by the body musculature associated
with it (parietal muscles). In most extant
gymnolaemates the zooids are not cylindri-
cal but flat, with rigid side walls as seen in
Fig. 9. The upward facing or frontal wall ei-
ther remains flexible or has concealed below
its calcified surface a membranous cavity,
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the ascus (sac), which can be inflated with to the autozooids, as in Schizoporelia. In
sea water, thereby compressing the body flu- Bugula the avicularia are movable on short
id. At the free end of a cylindrical zooid or stalks and closely resemble miniature birds'
near the distal end of a flat zooid is an open- heads; hence the name avicularium (cp.
ing known as the orifice, through which the contribution of WÖSS, this volume). An-
tentacle sheath and tentacles emerge; in other specialized form of zooid is the vibrac-
cheilostome gymnolaemates the orifice has ulum, in which the operculum has become a
a closable lid, the operculum. Stenolaemate whip-like seta. The functions of avicularia
zooids are different, and the walls form a and vibracula are not clearly known, but
slender calcareous tube, no part of which both types of zooids may help to keep the
can be inflected to evert the tentacles; in- colony free from particles and epizoites (i.e.,
stead, body fluid is forced from one part of organisms that attach to the surface of the
the zooid to another by muscles (NIELSEN & colony but do not parasitize it). A recent
PEDERSEN 1979). summary may be found in McKlNNEY &
JACKSON (1989).
The alimentary canal forms a deep loop
(Fig. 2); the pharynx, ciliated distally, de-
scends to the stomach, the anterior part of Form and function: colonies
which is termed the cardia or forms a gizzard
Despite their ill-defined shape, colonies,
in a few genera, such as the gymnolaemate
at least in extant bryozoans, are not just ag-
Bowerbankia (MARKHAM & RYLAND 1987);
gregations of zooids but whole organisms
the main stomach section is the caecum,
having a physiology and behaviour that ap-
and the slender posterior region, which is
pear to be coordinated to some extent. Inte-
ciliated, the pylorus; the rectum continues
gration is made possible by a system of in-
from the pylorus; and the anus is situated
terzooidal pores (Fig. 2) and the cells that
just outside the lophophore. A detailed and
traverse them (see BOBIN 1977 and MUKAI
fully illustrated account of the lophophore
et al. 1997 for details). Most conspicuous are
and gut has been provided by MUKAI et al.
those of the funiculus, which in gymnolae-
(1997). Respiratory, circulatory, and excre-
mates becomes a colonial network capable
tory systems are absent in bryozoans. The re-
of distributing nutrients to nonfeeding
productive organs (ovary, testes) are situat-
zooids (such as ovicells or gonozooids) or ar-
ed on the lining of the body wall or on the
eas (such as the growing edge). The zooidal
funiculus, a cord of tissue that links the
nervous system of bryozoans consists of a
stomach to the lining of the body wall (and
small ganglion positioned between the
often also links zooids throughout the
mouth and the anus that supplies nerves to
colony, see below). The polypide degener-
the various organs. In some bryozoans a
ates periodically during the lifetime of a
colonial network, that unites the zooids
zooid, and a compact mass, called a brown
through the interzooidal pores, has been
body, is either expelled or remains alongside
demonstrated. A stimulus that causes the
the new polypide which soon differentiates
lophophore to withdraw in a zooid of the
from living cells of the cystid (GORDON
gymnolaemate Membranipora almost instan-
1977).
taneously evokes the same response nearby;
and nerve impulses can at that time be
Zooid polymorphism exists among
recorded. Nevertheless, to a large extent the
cheilostomate colonies in particular, having
colony is not individualistic; for example, it
given rise to several types of heterozooid
usually has no definite shape, can grow in
(SlLEN 1977). The operculum seems to have
any direction, and can be partially destroyed
been significant in the evolution of some
without harm to the rest. It may live a few
types of specialized zooids in this order. The
months or a couple of years, or it may be
avicularium type of zooid has a small body
theoretically immortal, its life of continual
and a rudimentary polypide; the operculum,
budding terminated only by some catastro-
however, is proportionally larger, has strong
phe. The high species diversity of bryozoans
adductor muscles, and has become, in effect,
and the remarkable adaptive radiation of
a jaw. Avicularia are found among normal
the Cheilostomatida during the Cretaceous
zooids, but usually are smaller and attached
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reproduction, topics that have been ex-
plored by MCKINNEY & JACKSON in their
book "Bryozoan Evolution" (1989).
Reproduction and life cycles
Bryozoans, like other modular animals,
have a life cycle that incorporates phases of
asexual and sexual reproduction (Fig. 4).
Asexual reproduction in larger invertebrate
animals (e.g., sea anemones) may, by fission,
produce genetically identical progeny,
which separate to form clones. In bryozoans,
as in hydroids and corals, the progeny (the
zooids) are produced by asexual budding and
almost invariably remain in intimate con-
tact, giving rise to a colony. Some bryozoan
Fig. 3: Erect, foliose colony of Flustra and Tertiary, have been attributed by RY- species, however, also propagate colonies
foliacea (Cheilostomatida) supporting LAND (1979) to the unique combination of asexually. The cheilostome Discoporella lives
colonies of Crisia (Cyclostomatida: white)
zooidal individuality combined with a so- as small, non-attached, saucer-like colonies
and Scrupocellaria (Cheilostomatida:
brown). No source information; frame size phisticated system for metabolic integration on the sea floor. Groups of zooids at the
approximately 9 x 6.5 cm. (provided by the funiculus-interzooidal pore colony rim detach at special fracture zones
system), which have facilitated the evolu- and grow into new colonies. The statoblasts
tion of heterozooids and replicated patterns (dormant buds) of freshwater bryozoans
of heterozooids - which may be interzooidal constitute another means of asexual repro-
duction, and probably evolved initially as a
or variously aggregated on a bearer auto-
means of surviving adverse conditions (e.g.
zooid to form cormidia BEKLEMISHEV (1969)
Fig. 4: Life cycle in Celleporella (a cheilo- - that make up a colony. The several dis- winter or drying out) in situ rather than for
stomate). Reading clockwise from the left, dispersal. Asexual reproduction typically,
tinctive basic patterns evolved by colonies
the diagram shows an ancestrula, the
whether leading to a clone, a colony, or a
pattern of early budding, a selection of of Bryozoa (encrusting crusts, erect fronds,
clone of colonies, is a means of perpetuating
fully developed zooids (autozooids, small spirals, free-living disks, and so on (Fig. 1,3)
males, and ovicellate females), and the and locally increasing a successful genetic
are related to the habitats they occupy, and
coronate larva (Various sources, from constitution (genotype) rather than one for
RYLAND 1989). to the requirements of obtaining food and of
colonising new territories (WILLIAMS 1975).
Budding. A bryozoan colony originates
from either a primary zooid (ancestrula) or a
statoblast. The ancestrula is formed by the
metamorphosis of a sexually produced larva
(ZlMMER & WOOLLACOTT 1977). The ances-
trula is frequently smaller and/or morpholog-
ically different from the budded zooids (Fig.
4). New zooids bud from the ancestrula to
produce colonies of definite or indefinite
shape and characteristic growth habit (ZlM-
MER & WOOLLACOTT 1977). The process of
colony formation and differentiation is
known as astogeny (in contrast to the mor-
male
phogenesis and ageing of a zooid, which is
ontogeny). Early astogeny often progresses
through a gradation of metric or morpholog-
feeang ZOOKS ical changes until the typical (ephebic, see
RYLAND 1970) zooid form is achieved
(BOARDMAN & CHEETHAM 1969, 1973). In
phylactolaemates, the primitive zooids are
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cylindrical in form, and astogeny results in a hermaphroditic (i.e., both male and female
branched colony. In more highly evolved reproductive organs occur in the same
phylactolaemates, colonies are more com- zooid); small gonads are attached in clusters
pact, and discrete zooids can be recognized to the membrane that lines the body wall or
only with difficulty. New polypides, which the polypide. In a few species the individual
originate by ingrowth of the superficial cell zooids are of one sex only. In these circum-
layer, or epithelium, remain suspended with- stances, female zooids are usually larger
in a common colonial coelom, or body cavi- (e.g., in the cheilostomate Repmdeoneüa),
ty (generally regarded as a coelom). Among male zooids may be simpler or just have few-
living members of the primitive (and mainly er tentacles in the lophophore (e.g., in the
fossil) marine stenolaemates, the long and ctenostomate Alcyonidium nodosum and the
slender zooids have calcified tubular skele- cheilostomate Hippoporidra (RYLAND 2001),
tons. A larva metamorphoses into a hemi- or female and male reproductive zooids each
spherical primary disk (pro-ancestrula). A may be distinguishable from ordinary feed-
cylindrical extension grows from the pro-an- ing zooids (e.g., in the cheilostomate Cdie-
cestrula, and the matrix of the colony then is poretia, Fig. 4). Sperm are discharged
built up by repeated divisions of the zooidal through the terminal pore found in all ten-
walls. Internal walls of the colony are called tacles, or just through the dorso-medial pair
septa. The growth and budding zones of the (between and below which the nerve gan-
colony are found at its outer edges. Cells glion is situated), and must be dispersed
from the surface epithelium push inward to through the enveloping water before enter-
produce the polypide, and the septa create a ing another zooid (SlLEN 1972). Experimen-
chamber (cystid) around it. In the gymno- tal evidence is accumulating that cross, or
laemates, in which the zooids frequently are between colony, fertilization is the norm for
flattened, budding occurs as transverse septa bryozoans, irrespective of the mode of em-
form and cut off parts of the primary zooid bryonic development (RYLAND & BlSHOP
(or any other parent zooid). As each bud en- • 1993; TEMKIN 1996; HUGHES et al. 2002).
larges to become a zooid, a polypide forms Among living stenolaemates most zooids
inside. In the order Cheilostomatida, bud- contain only testes. The few female zooids
ding usually produces rows of identical enlarge to form spacious brood chambers,
zooids that radiate from the primary zooid. which are called gonozooids. During devel-
The rows divide periodically to keep pace opment, a remarkable form of asexual bud-
with the increasing circumference of the ding (polyembryony) takes place: young em-
colony and to maintain 'hexagonal close bryos squeeze off groups of cells that form
packing' organisation that maximizes the secondary embryos; these in turn may form
number of feeding lophophores per unit area tertiary embryos. In this way, many larvae
(THORPE & RYLAND 1987). Successive can develop in a single brood chamber. The
zooids in a row are separated by transverse unique features of reproduction in stenolae-
septa, but adjoining rows are separated by mates may be related to the usually small
double walls. Interzooidal pores are present size of both zooids and colonies, resulting in
both in the walls and in the septa (Fig. 2). scarcity of sperm (RYLAND 1996, 2000).
Sexual reproduction. Sexual reproduc- Among the phylactolaemates, the fertil-
tion, by the production and subsequent fis- ized egg develops in an internal embryo sac;
sion of gametes, generates the genetic vari- a larva, which already contains the' first
ability necessary for a species to survive in a polypide, is formed there, then liberated.
habitat that varies from place to place and Phylactolaemates also produce asexual sta-
from time to time (WILLIAMS 1975; RYLAND toblasts, which develop on the funiculus, a
1981; JACKSON 1986). As the colony con- cord of tissue that links the stomach to the
tinues its growth by budding, some zooids lining of the body wall. As it grows, each
become sexually mature, producing eggs and statoblast is surrounded by a hard protective
spermatozoa. Fertilized eggs develop into case that may also include an air-filled float
larvae (Fig. 4), which disperse and found and slender, hooked spines. Statoblasts usu-
new colonies. Mature gymnolaemate and ally develop in late summer and are liberat-
phylactolaemate zooids are most commonly ed as the colony disintegrates with the ap-
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zooid (OSTROVSKY & SCHÄFER 2003) - in
which the embryo develops (Fig. 5). The
egg at transfer generally has sufficient yolk
to nourish its embryonic development; but
in the cheilostomes Bugula and Celleporella
the egg, which is small at transfer, establish-
es a pseudo-placenta with tissues of the
mother zooid and receives nourishment as
the embryo develops (general review by
STROM 1977). The ciliated larvae, sub-
spherical and often about 0.25 mm in diam-
eter, are liberated when fully developed and
may swim first toward the light and thus
away from the parent colony; later, howev-
er, the larvae avoid light as they seek a place
in which to settle (RYLAND 1976b, 1977).
Metamorphosis of larvae to adults occurs
within a few hours after larvae are liberated
Fig. 5: Encrusting colony of Schizoporella proach of winter. Statoblasts survive dry and (metamorphosis and early astogeny re-
unicornis (Cheilostomatida). Many zooids in freezing conditions and can initiate a new viewed by ZIMMER & WOOLLACOTT 1977).
the central band support ovicells, a
colony when favourable climatic conditions
majority containing pinkish-red embryos. In certain genera (e.g., Membrarüpora and
Frame approximately 18x12 mm. Origin: recur. some species of Alcyonidium) of the class
west coast of Scotland, May 1996.
In most gymnolaemates one oocyte at a Gymnolaemata, each zooid sheds many tiny,
time enlarges and bursts from the ovary into fertilized eggs directly into the sea. These de-
velop into oval or triangular, bivalved larvae,
the coelom, probably then being fertilized
known as cyphonautes, which for several
by exogenous sperm already present. The
weeks live among, and feed on, plankton.
oocyte grows and is transferred to a brood
Larvae from brood chambers (termed 'coro-
chamber. This may be an undifferentiated
nate') and cyphonautes settle in a similar
part of a zooid; in some ctenostomates, for
way; i.e., both locate a suitable surface and
example, one or more late oocytes pass
explore it with sensory cilia (R.YLAND 1976b).
through the supraneural pore into a space
Attachment is achieved by flattening a sticky
Fig. 6: Cylindrical zooids of Bowerbankia created by the tentacle sheath (as in Fig. 1).
holdfast, which pulls the larva down on top
imbricata (Ctenostomatida) with expanded Usually among cheilostomates, however,
of it. As metamorphosis proceeds, larval or-
lophophores. Scale not recorded but each oocyte passes into a special globular or
expanded zooids are about 0.25 mm ganization degenerates and the first polypide
hooded ovicell (or ooecium) - usually, if not
diameter. Origin: west coast of Scotland, develops - with reversed polarity - inside the
May 1985. invariably, produced from the next distal primary zooid (RYLAND 1976b, 1977; ZlMMER
& WOOLLACOTT 1977).
Food and feeding
Bryozoans feed on minute planktonic
particles that are captured by the (8-30) ten-
tacles, which, in marine species, spread as a
funnel with the mouth at its vertex (Fig. 6,
7, 9). The beating of long lateral cilia draws
water into the top of the funnel and propels
it out between the tentacles. Particles are
projected toward the mouth; it was suggested
by STRATHMANN (1973) that those leaving
the funnel between the tentacles would be
flicked back into it by reversed beat of the
lateral cilia, though some of STRATHMANN'S
interpretations were unsupported by the
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work of BEST & THORPE (1983). Recent
work suggests, moreover, that the lateral cil-
ia function more as passive sieves (RllSGÄRD
& MANRJQL'EZ 1997; NIELSEN &. RIISGÄRD
1998; LARSEN & RllSGÄRD 2002). More
study on mechanisms is needed. Shorter cil-
ia on the inner face of the tentacles may as-
sist in the transport of food particles toward
the mouth without the involvement of mu-
cus; from there they are sucked into the
pharynx. Diatom shell valves are separated
or broken in the gizzard, when present
(MARKHAM & RYLAND 1987). Digestion and
absorption occur in the stomach, and indi-
gestible remains are compacted by rotation
and expelled as faecal pellets. Freshwater
bryozoans generally have more tentacles, dis-
posed in a crescentic lophophore, the ends of
which project behind the mouth. VSKY (2002). In a further development, tor Fig. 7: Encrusting colony of Flustrellidra
hispida (Ctenostomatida) with expanded
example in species of Hippoporidra and in the
The bryozoan method of feeding, espe- lophophores. Coast of Wales but no further
ctenostomate Alcyonidium nodosum, the hy-
details recorded; the expanded lopho-
cially in extensively spreading colonies of
drodynamic system evolved to promote feed- phores have a diameter of about 1 mm to
optimally packed zooids, imposes colonial ing efficiency has been utilized in reproduc- the point of out-curving (bright in the
organization as well as appropriate zooidal photograph).
tion. In both bryozoans the zooids are single
behaviour. Filtered water accumulates below
sex rather than hermaphroditic, and small Fig. 8: Colony of Membranipora
the expanded lophophores, which are held male zooids are clustered on the summits of membranacea (Cheilostomatida) on a kelp
above the colonial surface. Hydrodynamics (Laminaria) frond. The direction of growth
the mammillae, so that sperm are discharged
requires that this water, very viscous at the is to the right, bounded by a wide
in the strong outflow (RYLAND 2001).
marginal budding zone (rapid growth
scale of individual zooids and often far from
rate). In the older part the regularly spaced
the colony margin, be removed without di- darker spots indicate the positions of
luting the food-bearing inflow (GRÜNBAUM exhalant chimneys (see text). Actual size
not recorded. Locality: Shetland, June 2001.
1995; LARSEN & RIISGÄRD 2001). The solu-
tion has been the formation of regularly
spaced 'exhalant chimneys' first described in
the flat spreads of Membranipom (BANTA et
al. 1974; LARSEN & RIISGÄRD 2001; Fig. 8,
9). Each 'chimney' is essentially a space,
sometimes centred on a non-functional
zooid (Fig. 9), enlarged by the divergent pos-
ture of the surrounding lophophores. Exha-
lant water enters the base of the 'chimney'
and outflows to well above the height of the
surrounding lophophores. The efficiency of
the system is often enhanced, as in the
cheilostomate genus Hippoporidra, by the
colony surface being regularly mammillated,
each summit coinciding with and elevating
the 'chimney'. The colony-wide adaptations
to feeding have been comprehensively re-
viewed by MCKINNEY & JACKSON (1989),
though further important contributions to
our understanding of and the need for and
functioning of, chimneys have been made by
ECKMAN & OKAMURA (1998), LARSEN & RI-
ISGÄRD (2001), and SHUNATOVA & OsTRO-
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Ecology
Freshwater bryozoans. Freshwater bry-
ozoans live mainly on leaves, stems, and tree
roots in shallow water. Before drinking wa-
ter was filtered, they regularly polluted wa-
ter supply pipes. Though not uncommon,
freshwater bryozoans are often inconspicu-
ous in pools, lakes, or gently flowing rivers,
especially in slightly alkaline water.
Marine bryozoans. The most familiar
marine bryozoans are those that inhabit
shores, though they occur in greater num-
bers below tidemarks. Dredge hauls of stones
and shells yield colonies in abundance.
Colonies also occur on the ocean bed, even
at great depths, but the frequently muddy
bottom of the oceanic abyss is an un-
Fig. 9: ZOOIÜS ui iviemuranipora Distribution and abundance favourable habitat. A few species tolerate
membranacea with expanded lophophores.
hypersaline or brackish waters. The predom-
At the bottom centre is an "empty" zooid Bryozoan colonies are found in both
inantly marine Gymnolaemata have a few
which will become the focus of a chimney.
fresh and salt waters, most commonly as
Zooids measure about 0.4 mm length. freshwater representatives, e.g. Paludicelia.
Locality: west of Scotland. growths or crusts on other objects (Fig. 3,
Shallow, sheltered channels that have
10). Freshwater bryozoans live among vege-
currents but are protected from severe waves
tation in clear, quiet, or slowly flowing wa-
are typical bryozoan habitats. Open coast-
ter. Marine species range from the shore to
lines support fewer species, but non-calcare-
the ocean depths but are most plentiful in
ous species (e.g. of Akyonidium, Bower-
the shallow waters of the continental shelf.
bankia (Fig. 6) and Flustrellidra (Fig. 7)) oc-
They cover seaweeds, form crusts on stones
cur abundantly on intertidal algae in tem-
and shells, hang from boulders, or rise from perate waters. A familiar gymnolaemate
the seabed. Bryozoans readily colonize sub- genus is Membranipora (Fig. 8), which is
merged surfaces, including ship hulls and found throughout the world and is well
the insides of water pipes. A few form free- adapted to living on kelp weeds at, and just
living or apparently non-attached popula- below, low water mark. Although the zooid
tions on sandy seabeds. walls of Membranipora colonies are calcified
(Fig. 9), they contain flexible joints, which
Fig. 10: Encrusting colony of Cryptosula
pallasiana (Cheilostomatida). This species The zooid walls, which constitute the allow the colony to bend as the alga sways in
has a porous frontal wall, a very large most permanent portion of the colony, are the waves. Membranipora, which may cover
orifice, and (unusually) lacks ovicells, the generally calcareous, giving bryozoans a fos- large areas with a million or more zooids, al-
embryos developing internally. Zooids
sil record that dates from the Ordovician ways grows predominantly toward the
approximately 0.8 x 0.4 mm. Origin: west
coast of Scotland, May 1985. onward (i.e., from about 500 My ago). youngest part of an algal frond (Fig. 8).
Overhangs, which form when soft rock
erodes along a shoreline, floating pontoons,
and the shaded pilings of jetties and piers,
are other favoured habitats. Since bryozoans
do not require light and can grow in dark
places, they can avoid competition from al-
gae that could smother them. Opistho-
branch sea slugs and pycnogonids appear to
be the principal predators of bryozoans.
Evolution and palaeontology
The Bryozoa have a long history. From
the Lower Ordovician (500-430 My ago)
16
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onward, most limestone formations, espe- The ctenostomates (class Gymnolaema-
cially those with shale alternations, are rich ta) have left a sparse fossil record. During the
in bryozoan fossils. The skeletons of calci- Late Jurassic they apparently gave rise to the
fied bryozoans are easily preserved. Steno- complex and successful cheilostomes. These
laemates are abundant fossils; and, after had encrusting flat zooids similar to some of
their appearance in the Upper Jurassic (140 their contemporary ctenostomates but with
My ago), cheilostomate fossils also are abun- side walls that were calcified. This type of or-
dant. The soft-bodied phylactolaemates, on ganization, termed anascan (meaning with-
the other hand, have left no fossil record, out an ascus), permitted inflexion of the
and fossilized ctenostomates are rare (usual- front wall to evert the lophophore but
ly in the form of moulds resulting from im- seemed to offer little protection. The as-
muration; VOIGT 1979) but long antedate cophoran (ascus bearing) organization
the cheilostomates. evolved in the Late Cretaceous by calcifying
the membranous front but preserving its hy-
The most ancient bryozoans are steno-
drostatic function by a flexible infolding (as-
laemates from the Lower Ordovician of the
United States and Russia (Arenig series, cus) below the wall. The parietal muscles at-
about 499 My old); both cystoporate and tach to the ascus and pull its lower surface in-
trepostome stenolaemates have been found. to the coelom to evert the lophophore, while
The ceramoporoids, a group belonging to the ascus itself fills with seawater.
the order Cystoporata, flourished during the
Ordovician and evidently were the progeni-
Classification
tors of a more advanced group, the fistuli-
poroids, which were successful until the end Although both colony type and zooid
of the Permian (280-225 My ago). morphology are used to classify bryozoans,
zooidal characters are more reliable. Howev-
Dominant among the early Palaeozoic
er, the cylindrical zooids of stenolaemates are
(570-225 My ago) stenolaemates, however,
of rather uniform appearance, making classi-
was the order Trepostomata, which evolved
fication difficult; wall structure and the mor-
rapidly during the Ordovician and attained
phology of the embryo chambers are impor-
its peak during the upper part of the same
tant taxonomic characters. In cheilostom-
period. The long, slender zooids of trepos-
ates the skeletal features of the zooids, par-
tomes grew together to form large, solid,
ticularly the presence, extent, and structure
colonies. As a zooid grew longer and longer,
of the frontal wall - together with shape of
diaphragms (or transverse partitions) were
the orifice, type of ooecia, and zooid poly-
deposited. The trepostomes declined in im-
morphism - provide the most important dis-
portance after the Ordovician, perhaps as a
tinguishing taxonomic criteria. A simple di-
result of competition from the cryptostomes,
vision of cheilostomates into two suborders,
and were extinct by the close of the Permi-
Anasca and Ascophora, is now recognized as
an (225 My ago).
being an over-simplification, though these
Cryptostomes evolved rapidly during
former taxa certainly represent grades of or-
the Ordovician. They were similar to the
ganization. The Ascophora, in particular, are
trepostomes but evolved freely erect, leaf-
certainly polyphyletic, the frontal wall-sub-
like, branching or lacy colonies in the ptilo-
mural ascus combination having evolved in
dictyoids, or branching in rhabdomesoids,
a number of separate ways (GORDON 2000).
and were the dominant bryozoans from the
Among ctenostomates and phylactolae-
start of the Devonian until the Permian
mates, whose zooids lack skeletal features,
(395-225 My ago). For reasons not yet clear,
colony form is more important. Statoblasts
the cryptostomes dwindled and became ex-
are of taxonomic value. Internal characters
tinct soon after the end of the Palaeozoic
have been used less but, in ctenostomates,
Era (225 My ago).
the presence or absence of a. gizzard, number
The Cyclostomatida arose in the of tentacles, and colour of developing em-
Palaeozoic, flourished during the Jurassic bryos are of taxonomic significance.
(190-136 My ago) and Lower Cretaceous,
and still survive.
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Phylum Bryozoa lophophore; zooids separated by septa; new
zooids produced by division of septa; limited
Sedentary, aquatic invertebrates that
polymorphism; marine; Ordovician to pres-
form colonies of zooids by budding; each
ent; about 550 genera.
zooid with circular or crescentic lophophore
surrounding a mouth from which slender, Order Cyclostomatida
ciliated tentacles arise; anterior part of body Orifice of zooid circular; lophophore cir-
forming an introvert within which the lo- cular; no epistome; zooids interconnected by
phophore can be withdrawn; alimentary ca- open pores; sexual reproduction involves
nal deeply looped; anus opening near mouth polyembryony, usually in special reproduc-
but outside lophophore; excretory organs tive zooids (gonozooids); all seas; Ordovi-
and a blood vascular system absent; each cian to present; about 250 genera.
zooid secretes a rigid or gelatinous wall to
support colony; at least 8,000 extant species. Order Cystoporata
Zooid skeletons long and tubular, inter-
The classification of bryozoans began in
connected by pores and containing di-
1837 when the freshwater and marine Bry-
aphragms (transverse partitions); cystopores
ozoa were separated into the classes now
(not pores but supporting structures be-
known as Phylactolaemata and Gymnolae-
tween the zooid skeletons) present; Ordovi-
mata: later a third class, Stenolaemata, was
cian to Permian; about 80 genera.
separated from the Gymnolaemata. The cy-
clostomates and the fossil trepostomes were Order Trepostomata
placed in the new class (for a recent review, Colonies generally massive, composed
see BOARDMAN 2002). In recent years, the of long tubular zooid skeletons with lamel-
cryptostomes have also been placed in the late calcification; without interzooidal
Stenolaemata. The most satisfactory system, pores; orifices polygonal; sometimes with
therefore, separates the bryozoans into three numerous diaphragms, zooid walls thin
classes, distinct since the beginning of the proximally, thicker distally; Ordovician to
fossil record. Permian; about 100 genera.
Most of the bryozoan orders were named Order Cryptostomata
many years ago (though the suffixes have Colonies mostly with foliaceous or retic-
been changed to accord with a more logical ulate fronds or with branching stems; zooid
hierarchy): Cheilostomatida; Ctenostomati- skeletons tubular, shorter than in trepos-
da, and Cyclostomatida in 1852; Trepostom- tomes; without pores; with diaphragms;
ata in 1882; and Cryptostomata in 1883. proximal portions thin walled, distal por-
Cystoporata was introduced in 1964 to in- tions funnel-like, and separated by exten-
clude the Palaeozoic ceramoporoids and fis- sive calcification; Ordovician to Triassic;
tuliporoids. about 130 genera.
Class Phylactolaemata Class Gymnolaemata
Zooids basically cylindrical, with a cres- Zooids cylindrical or squat; with a circu-
centic lophophore and an epistome (hollow lar lophophore; no epistome; body wall
flap overhanging mouth); body wall non- sometimes calcified; non-muscular; eversion
calcareous, muscular, used for averting the of lophophore dependent on deformation of
lophophore; coelom continuous between body wall by extrinsic muscles; zooids sepa-
zooids; new zooids arising by replication of rated by septa or duplex walls; pores in walls
polypides; special dormant buds (stato- plugged with tissue; new zooids produced
blasts) are produced; zooids monomorphic; behind growing points by formation of
exclusively freshwater; cosmopolitan; appar- transverse septa; polymorphic; mainly ma-
ently primitive, but with no certain fossil rine; all seas; probably Ordovician to pres-
record; about 12 genera. ent; abut 650 genera.
Class Stenolaemata Order Ctenostomatida
Fossil except for some Cyclostomatida; Zooids cylindrical (Fig. 6) to flat; walls
zooids cylindrical; body wall calcified, with- not calcified; orifice terminal or nearly so,
out muscle fibres; not used for everting the often closed by a pleated collar; no ooecia or
18