Bryozoans - Reefkeeping.com

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.
This specimen was about four inches long. This species often bears small hydroids living
over its surface (visible here as white dots extending from the left side of the specimen).
These hydroids 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
some protection 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
tentacles surrounding the central mouth at the end of each zooid or polypide. The white lines
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.
The individual at the left is drawn showing the animal extending from the house; the individual on the right
is drawn to show internal structures.

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 in
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.

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
background colony, 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 bryozoans.

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.

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….

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
specimen 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.

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.