This month's column is about bryozoans,
animals that seem to be effectively impossible for hobbyists
to keep, even though they are frequently imported on live
rock. Bryozoans are colonial animals that look superficially
like corals, but which have a morphology that is significantly
more complicated. While many of them form colonies that are
simply crusts, some of them also form colonies of an exceptionally
delicate beauty that would be welcome in any marine reef aquarium.
Figure 1. Triphyllozoon inornatum, a tropical
lace bryozoan, found in high current areas.
was about four inches long. This species often bears small
over its surface (visible here as white dots
extending from the left side of the specimen).
may be symbiotic with the bryozoan, getting food brought to
them in the
currents generated by the bryozoan, while at the
same time the bryozoan benefits by getting
from the nematocysts in the hydroids.
Bryozoans are often overlooked by casual
observers, such as beachcombers or amateur naturalists, and
probably because of this, I don't know of any of them that
has a useful common name. The group is known taxonomically
as the Phylum Bryozoa or the Phylum Ectoprocta. Both of these
terms refer exactly to the same group, and neither name is
probably "better" than the other. I prefer to use
Bryozoa when referring to the group, but that is simply a
matter of personal choice to avoid the term Ectoprocta, which
is similar to the name of another phylum called the Entoprocta.
I have found that while I am teaching, if I use the name Ectoprocta,
then invariably I will misspeak or sometimes the students
get confused. It turns out best, simply, to stick with Bryozoa
as the term for these animals.
The group is not one of the larger animal
phyla as there are only about 4000 extant species, compared
to some 45,000 species for chordates (fish, birds, mammals)
or 150,000 for mollusks (clams, snails, squids). On the other
hand, it is a much more species-rich group than such phyla
as the Phoronids, which have less than 25 worldwide species.
Because many of the bryozoans have a hard colonial exoskeleton,
they have a good fossil record, with over 15,000 fossil species
having been described. Interestingly, they are the last phylum
to appear in the fossil record. Their moderate diversity notwithstanding,
they are quite abundant in most marine and fresh water habitats.
Bryozoans are almost entirely colonial.
One genus, Monobryozoan, contains species that may
be either colonial or solitary, depending upon conditions.
Although the colonies may be of moderate size, up to several
inches in diameter, the animals or zooids that comprise the
colony are often quite tiny.
The small size of the zooids that comprise
the colony belies a significant complexity of structure. Even
though the colonies look superficially like hydrozoans or
small corals, the individuals in the colony are at a level
of complexity more on the order of feather duster worms. All
of them have a ring of tentacles called the lophophore surrounding
the mouth, and this also makes the zooids appear to be similar
to hydrozoans; however, these tentacles lack the nematocysts
or stinging capsules characteristic of hydrozoans. Instead
of capturing their food by stinging it to death, the tentacles
of the bryozoans are lined with microscopic cellular extensions
called cilia. These beat and move water through the tentacles,
and in dong so, they capture food particles by entrapping
them in a small invisible stream of water that carries them
to the mouth. Unlike a lot of animals that use cilia to catch
food, bryozoans do not collect the food in a mucus-lined food
groove. Instead, they simply create a small discreet water
stream that moves minute suspended particulate organic material
such as bacteria into their mouth. When viewed microscopically,
the pattern of this ciliary feeding is both beautiful and
unique. The cilia on each tentacle beat in what are called
metachronal waves; this beating gives a motion similar to
what is seen in the wind-generated waving of a field of deep
grass or grain. These waves of beating cilia beat "up"
the left side of each tentacle, continue over the tentacle
tip, and beat down the right side. These ciliary waves generate
the water currents that bring the small particles into the
tentacle crown. For you fans of obscure terminology, this
particular type of ciliary beating pattern is called "laeoplectic
ciliation." When a food particle impacts a tentacle,
the ciliary beat is momentarily reversed and the particles
are "knocked" down to the mouth.
Figure 2. Membranipora membrancea, a temperate
incrusting bryozoan. Note the ring of
the central mouth at the end of each zooid or polypide. The
are the calcified walls of the cystid. Each animal
here is about the size of a pinpoint.
The body of these animals is often smaller
than the hydroids that they superficially resemble, but it
is far more complicated. They have a complete digestive tract,
surrounded by a relatively spacious internal body cavity.
This cavity is fluid filled and serves the role of a circulatory
system. The gut is divided into specific regions or organs,
and includes an organ which in some species is lined with
a hard cuticle. This area probably functions as a gizzard,
and grinds the food. Grinding of the food is only necessary
if the food contains hard parts, and so it's possible that
the bryozoans with functional gizzards are eating diatoms
or perhaps some other minute algae with shells such as coccolithophores.
The anus is found on a papilla that is located outside the
crown of tentacles.
Food nutrients digested in the gut diffuse
out of the back of the cells lining the gut into the body
cavity fluid. Movement of the animals sloshes this liquid
around and distributes food to all the cells and tissues of
the body. In a similar manner the fluid accumulates dissolved
wastes and moves them to the body surfaces where these wastes,
mostly ammonia and carbon dioxide, diffuse across the surface
epithelium to water surrounding the animal.
Figure 3. A diagram showing some of the body parts
of an encrusting bryozoan such as Membranipora.
individual at the left is drawn showing the animal extending
from the house; the individual on the right
is drawn to show
Bryozoans are unique among animals in that
many of them have "disposable bodies." The body
of the animal, called the "polypide" is only a part
of the whole organism, as the body lives in a structure called
the "cystid." This rather odd structure is comprised
of the animal's outer body plus the non-living secreted physical
structure of the house. The cystid can be visualized as the
shell or house the animal lives in, which is lined by a thin
tissue layer. This tissue layer can regenerate the body or
polypide. This is important as every so often, the whole body
of the organism retracts to form a small ball, and these body
structures then degenerate to form a mass of non-living debris.
This mass is often brownish and the mass of tissue remnants
and debris is called a "brown body." A short time
after this process, the cystid regenerates another body or
polypide around the brown body, which is often incorporated
into it. In some cases the brown body is incorporated into
the gut and expelled from the new individual, but in other
cases, it seems to be incorporated permanently into the new
polypide. Generally, as well, the cystid is the site of egg
and sperm formation, so in effect, it governs both sexual
and asexual reproduction and growth, but paradoxically, has
little in the way of defined structure itself.
Asexual reproduction in bryozoans is common,
about as much so as it is in soft corals and the hard corals
that they sometimes resemble. The colonies tend to spread
and grow by budding and fragmentation, although the diversity
of asexually reproduced body "parts" and structures
is significantly greater than is seen in the soft corals,
however. There are a lot of changes in shape of the individuals
that constitute the colony, depending upon where they are
in the colony, and there are several different types of specialized
types of individuals.
Figure 4. An individual of Caulibugula, a common
bryozoan genus; species of this uncalcified
bryozoan are found
in all the world's oceans. A zooid is extended to the left.
It is the only intact
polypide visible in this view. Several
cystids bearing "brown bodies" are visible elsewhere
the image. These cystids will regenerate new polypides
in the future.
In addition to the basic individual, made
of the polypide and cystid or, more familiarly, made of the
body and the house, there may be specific modified individuals
called "ovicells" which have their house structure
modified to protect and contain developing embryos, while
they develop to a swimming larval stage. Many species also
have specialized non-feeding, zooids, or individuals, which
are highly modified to pinch animals that encroach on the
colony surface. These pinchers look like small biting birds'
heads, complete with beaks. This resemblance has given them
the name of "avicularlia," a word derived from "avis,"
the Latin word for bird. Another type of modified individual
is called a "vibraculum." These structures look
like long straight pins or very thick hairs. Their nervous
systems appear to be connected throughout the colony. Touching
some colonies that have vibraculae, causes all of them throughout
the colony to bend and point toward the point of disturbance.
They are thought to deter predation.
Here are a couple of nice images of different
types of Bryozoan larvae, a planula-like larvae,
and a specialized larvae called a cyphonautes.
Even though there is obviously coordination
and control throughout the colony, and throughout all the
zooids, in some species, at least, how this occurs and is
affected is poorly understood. These animals appear to have
a very rudimentary nervous system, consisting of a small brain,
and a ring of nerves around the mouth. Other nerves are small
and hard to demonstrate, and seem to be arranged in a nerve
net, similar to that found in the Cnidarians, or animals such
as corals and sea anemones. In examining the scientific literature,
it appears that there has been no modern neurobiology done
on the group.
Figure 5. An encrusting bryozoan showing ovicells containing
developing orange embryos.
They have a well developed, albeit miniaturized
muscular system, which consists primarily of muscles that
move the tentacular crown. The lophophore is retracted by
the use of longitudinal muscles running through the body,
and the speed or retraction is truly amazing. Literally as
you watch them, the tentacles retract so fast that they simply
appear to vanish. When the animal resumes feeding, the lophophore
is extended by a relatively slow contraction of the musculature
surrounding the body wall.
Figure 6. An avicularium of the bryozoan, Caulibugula.
The upper beak is clearly visible above the
but the color of the background somewhat obscures the lower
beak extending out
from the body of the zooid. The lower beak
can snap shut crushing microscopic animals.
Bryozoans, like most invertebrates, pass
through a developing dispersal stage called a larva. However,
there is no simple, single larval type found in the phylum.
This is probably not surprising given the ancient and diverse
lineages found in the group. Typically, the larva is small
and ciliated. If it feeds, as a few do, the cilia bring food
to the animal as well as move it through the water. Many of
the larvae do not feed and appear to be quite similar, in
gross structure, to the planula larvae of corals and sea anemones.
In all cases the larva undergoes a drastic metamorphosis when
it settles out of the plankton to become the first sessile
member of the new bryozoan colony.
Taxonomists generally agree that there
are three major subdivisions, called classes, in the phylum
Bryozoa. These are the Class Phylactolaemata, whose members
are found only in fresh water, and two Classes, the Stenolaemata
and the Gymnolaemata whose members are wholly marine. Bryozoans
are predominantly marine organisms, and only four genera and
a few dozen species are found in fresh water. That having
been said, these animals are widely dispersed in fresh waters,
being found in virtually all bodies of fresh water. Additionally,
some fresh water colonies are huge, several tens of yards
in diameter. Such large colonies are never seen in marine
Images of fist-sized colonies of fresh
water bryozoans growing on a twig will be found by following
this link: http://terrence.marsh.faculty.noctrl.edu/PFUN1.JPG
The fresh water bryozoans, in the Class
Phylactolaemata, differ from their marine counterparts in
a number of ways. First, the crown of ciliated tentacles is
horse-shoe or "U" shaped instead of being circular.
Second, they are never calcified. The colony may be imbedded
in a gelatinous matrix, but it never has calcified components.
Most marine forms are calcified, at least to some extent.
Third, there is no polymorphism in the colony, all of the
individuals look and act alike; no avicularia or vibraculae
are found in the fresh water species. Finally, the fresh water
forms often produce a small dispersal stage called a statoblast.
Statoblasts are "packets" of generative cells contained
within highly resistant proteinaceous coat. They are produced
when the environmental conditions surrounding the colony take
a turn for the worse, such as when the pond starts to dry
up (and hence all dissolved ions become more concentrated)
or when the temperature starts to drop in autumn. Statoblasts
are dust-sized and can blow for great distances in the wind.
Some of them also have hooks all over their surface, and are
thought to disperse by becoming passively attached and subsequently
detached from the feather of aquatic waterfowl.
Images of statoblasts may be found here:
Statoblast hooks are very evident in this
The fresh water bryozoans have one very
neat "claim to fame" as well. One genus of them,
Cristatella, forms small mobile colonies that slowly
creep over the substrate at the galloping rate of a few inches
per day. Nevertheless, these are truly mobile colonies, and
they look something like a cross between a slug and a leather
coral. Quite kewl
For some images of these mobile moss animals
follow these links:
The marine bryozoans are represented in
all seas by individuals from two classes, Stenolaemata and
the Gymnolaemata. The polypides of all of these animals are
quite similar, and they differ primarily in shape and structure
of the colonies and the microstructure of the cystids, or
houses. All bryozoans classified into the Class Stenolaemata
have a calcified tubular house, without a trapdoor, or plug,
to close the aperture. Although this group was very diverse
in the past, it went into a decline about the middle of the
Cretaceous period, well before the end of the dinosaurs, and
has continued to decline ever since. Nonetheless, there still
are several hundred species of them living today.
Figure 7. Tubulipora, an encrusting Stenolaemate
bryozoan. The whole colony here is about the size
of a dime.
The tubular nature of the colony form is evident.
Individuals in the other marine bryozoan
class, the Gymnolaemata, may or may not be calcified, if they
are they generally do have some means of sealing off the aperture
from the surrounding environment. Colonies from species belonging
to this class are often highly polymorphic with avicularia,
vibraculae, as well as differential morphology of some of
the "regular" zooids.
While most marine bryozoans are attached
to the substrate, a recent find from the Antarctic shows that
not all colonies live this way. Floating golf-ball sized spherical
colonies of a free-swimming bryozoan, new to science, were
recently described. To find out about this truly bizarre bryozoan,
follow this link.
Figure 8. The whitish material here is a large Membranipora
membrancea colony (the zooids of a different
of this species were shown in Figure 1). It is growing along,
and largely obscuring, a large kelp blade.
The image shows
about three feet of the kelp; so bryozoan colonies need not
be small, even though the zooids
are. The orange structures
are sea cucumbers, Cucumaria miniata. These encrusting
bryozoan colonies can
adversely affect the kelp, cutting off
a lot of the light it needs for photosynthesis.
Identification, Natural History And Aquarium Care
Bryozoans are common space-occupying organisms
on shallow water rocky or hard substrata throughout the world,
and like many groups reach a great diversity of species number
in the tropics. Nonetheless, except for a few forms, they
tend to be overlooked. In this regard they are much like the
"understory" plants in a forest. While we may think
of a forest as being primarily composed of the large and evident
trees, there are often many more varieties of smaller trees
and shrubs found there. Few bryozoan colonies get large, and
they tend to be overlooked by aquarists. Nonetheless, they
are commonly found on live rock.
Unfortunately, the reef rubble that is
euphemistically sold as "live rock" has been largely
killed by the collection and distribution process. Generally,
although not always, some of the first animals to be destroyed
by the harsh treatment that the rock goes through are the
bryozoans. They are often relatively delicate organisms and
only in exceptional circumstances will colonies survive to
make it into the marine aquarium.
Some do survive, however, and may be recognized
by a few characteristics. The most common forms appear to
be "calcified crusts" growing over the rock like
a thin layer of very hard frosting. The crusts may be commonly
white, orange, red, or yellow. Crusts of other colors, such
as lavender and green are also occasionally seen. On close
examination, the surface of these crusts will be seen to be
perforated by very many small holes. These holes are typically
about the size of a pin point, and they are always arranged
with apparent geometric precision; in precise rows or radiating
lines. Some colonies form delicate filigreed structures which
would be welcome in any aquarium, but these are almost never
found intact, although they are occasionally offered for sale
in the hobby. If the bryozoans are alive, a small "fuzz"
layer of tentacles will be seen to emerge from the colony
and it will retract with lighting speed if disturbed.
Bryozoans feed on small particulate material
and would probably do well in some aquaria, particularly those
with a good sand bed which produces a lot of bacterial particulates.
They may also feed on the smaller varieties of phytoplankton
such as small Nannochloropsis species. If some make
it alive into reef aquarium systems, maintaining them should
not be difficult.
In nature, they are fed upon by grazing
predators such as nudibranchs, some grazing snails, some sea
urchins, and many crustaceans. Many types of small sea spiders
also eat them. In general, such predators are lacking in aquaria,
so predation is not likely to be a problem. Bryozoans generally
do not seem to be particularly adept competitors, and will
not typically overgrow animals such as soft corals, corals
or sponges. Nonetheless, they often do seem to be able to
persist in the intensely competitive environment often found
on marine shallow water hard substrata. Many species are "weeds"
specialized to grow rapidly, reproduce well, and then die,
but long-lived species are also common. If we could have live
rock imported with care for the specimen animals on it, we
would find that many of these small beautiful colonies would
make attractive additions to our aquaria.