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What is a Bryozoan?
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| Bryozoans, sometimes referred to as moss animals
or ectoprocts are tiny, colonial organisms. Their development does not
follow either a true protostome or true deuterostome pattern. They,
along with the Phoronids (worm-like animals) and the Brachiopods
(bivalve-like animals sometimes referred to as lampshells) are thus
classified based on the presence of a specialized feeding structure
called a lophophore, an extension of the body wall into a
tentacled structure that surrounds the mouth and is either
horseshoe-shaped or circular. Bryozoan colonies can be encrusting,
arborescent (branching, and tree-like), or even free living. Individuals
within colonies may be referred to as either zooids, or polypides.
The term polypide refers to the contents of each zooid (gut, lophophore,
muscles, etc.) within the body wall
(Barnes 1980).
Zooids of most species are enclosed in a protective
tunic made from either chitin (a tough protein also found in insect exoskeletons) or calcium
carbonate. This exoskeleton has an orifice, or opening, through which
the lophophore is extended into the water column for feeding. In
some species, the orifice is covered by an operculum.
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Sketch of 3 types of individuals in
a colony of Bowerbankia, an arborescent type of bryozoan.
Individuals in arborescent colonies live attached to a common stolon and
have chitinous body walls. A. Individual zooid with its
lophophore extended for feeding. B. Retracted
individual. C. A degrading individual that is brooding an
embryo. Redrawn from Barnes, 1980.
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| Sketch of a colony of Electra,
an encrusting type of bryozoan. In encrusting bryozoans, an individual
zooid is housed in a calcified structure called a zooecium.
Redrawn from Barnes, 1980. |
Bryozoan Biodiversity:
Bryozoan diversity (as measured by
number of species) within the Indian
River Lagoon is approximately 1/3 that of bryozoa in coastal and
offshore habitats (Winston 1995). Approximately 36 species of bryozoa
are known to inhabit the Indian River Lagoon, Florida. The highest level of diversity among ectoprocts occurs in areas where salinities
are above 30 ‰. Only 12 species have been found in the less saline
areas of the lagoon. Winston (1995) reported that bryozoan species
composition within the lagoon has remained fairly stable for the last 20
years, however, there are often large seasonal or year-to-year changes in
population structure. Further, many of the less common species have
been observed in only one location within the lagoon. This observation
has important ecosystem management implications because, if many species
do, in fact, exist within the lagoon at only one to a few locations,
then any degradation in water quality in these critical habitat zones,
could quickly reduce ectoproct diversity in the IRL (Winston 1995).
Balanced against this finding is the fact that approximately 97% of
species in the IRL also occur at other sites: estuarine, coastal and
offshore (Winston 1995). Thus, most species could be repopulated
following restoration of the habitat.
Reproduction:
All freshwater and most marine
bryozoans are hermaphroditic (Barnes 1980) with some species being simultaneous hermaphrodites,
(producing both sperm and
eggs at the same time), and others being protandric hermaphrodites.
In dioecious
species, the entire
colony may consist of same-sex zooids, or both male and female
individuals can be present. In hermaphroditic species, the ovaries are
typically located distally, while the testes are located basally.
Commonly, one to a few ovaries and many testes are active. These
structures usually consist of masses of eggs or sperm covered by
peritoneum. The masses eventually bulge into the coelom and the
peritoneum ruptures. Some species release eggs and sperm directly to the
water column, while others brood their eggs either within the coelom or
externally in the cavity of the tentacular sheath or in the atrial wall.
Brooded eggs are generally large, few in number, and are heavily yolked
(Barnes 1980).
Embryology:
Following fertilization, larvae are
produced which show wide variation in body form from species to species.
The larvae of non-brooding bryozoans feed during the larval stage, while
the larvae of brooding bryozoans do not, since these larvae tend to settle soon
after release. The most common larval type in bryozoans is the
cyphonautes larva which is somewhat triangular in shape and has an
apical tuft of cilia. Upon settling, larvae attach via adhesive sacs and
undergo metamorphosis to the adult form. The first zooid in a colony is
called the ancestrula. It is from this individual that the rest of the
colony will grow asexually from budding.
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The cyphonautes larva, the most common larval form in the bryozoa.
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Another type of bryozoan larva. This one
is from Bugula, an arborescent type. |
Temperature:
On a local scale, temperature controls all aspects of bryozoan life. In
spring, rising water temperatures and increased intensity of sunlight
stimulate phytoplankton growth, which initiates active budding in
bryozoans, and, to some degree, sexual reproduction.
Salinity:
Three IRL species (Victorella pavida, Conopeum
seurati, and
C. tenuissimum) are truly estuarine and found almost entirely in
brackish water. The remaining 33 species are warm water, euryhaline
species which inhabit harbors, bays, river mouths, and coastal areas
where salinity is somewhat variable, yet generally above 30‰ (Winston
1995).
Trophic mode:
All bryozoans are suspension feeders.
Each individual zooid in a colony has ciliated tentacles which are
extended to filter phytoplankton less than 0.045 mm in size (about
1/1800 of an inch) from the water column. Bullivant (1967, 1968) showed
that the average individual zooid in a colony can clear 8.8 ml of water
per day.
Habitats:
Seagrasses as well as floating
macroalgae, mangrove roots, shells, docks, pilings and other structures
provide support for bryozoan colonies. In turn, bryozoans provide
habitat for many species of juvenile fishes and their invertebrate prey
such as polychaete worms, amphipods and copepods. (Winston 1995).
Importance in the Indian River Lagoon:
Bryozoans are ecologically important in
the Indian River Lagoon due to their feeding method. As suspension
feeders, they act as living filters in the marine environment. Using
Bullivant's (1967, 1968) calculation that individual zooids may filter
an average of 8.8 ml of water per day, Winston (1995) reported that
colonies of Zoobotryon verticillatum located in 1 square meter
of seagrass bed could potentially filter and recirculate an average of
48,600 gallons of seawater per day. |
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