Members of the phylum Bryozoa (also called Polyzoa or Ectoprocta), are incredible invertebrates that inhabit a variety of environments. There are over 4,000 living species of which most are marine. Order Cheilostomata (class Gymnolaemata), containing 600 genera, is the most prodigious bryozoan group (McKinney & Jackson 1991) and the group for which we had representatives in the lab. Clumps of bryozoan may be large, but since each individual animal is tiny, I had to use the dissecting scope to better observe these “moss animals”. Each individual or “zooid” lives inside a rectangular compartment made of calcium carbonate and chitin (see Fig. 1). The tiny, sessile organism inside will grow no greater than one millimeter in length. Yet these individual zooids are characteristically part of a unified, vast colony that shares nutrients and may grow up to over a meter in diameter. Colonies present themselves in a variety of ways from bush-like masses to encrusting species that resemble small corals, and extraordinarily, gelatinous globules (Pechenik 2015). While examining the bryozoans in lab, I wondered how individual zooids function and how the colony defends itself against predators and possibly rival colonies.
Figure 1. Colonial bryozoan, Watersipora subtorquata, under the small crab. Photo credit: Megan Hill.
Colonies begin growing from a single zooid and consist of clones of the ancestrula (the founding member of the colony) produced via budding (asexual reproduction). Some colonies have an array of different zooids working together to survive diverse oceanic conditions. Cooperative living allows for morphological changes of individuals within the specialized colony because feeding individuals called autozooids can supply energy to other zooids with specialized functions (Pechenik 2015). Some of these modified “heterozooids” include:
- Kenozooids: serve as the zooecium; hold the colony to the substrate.
- Avicularia: serve as the protectors of the colony; have bird-like beak structure called “avicularium” that snaps at invaders and bites predators (e.g., nudibranchs) moving above the surface of the colony.
- Vibracula: modified operculum into a long thin bristle, which serves to clean the surface of the colony of settling detritus and sedentary animals.
- Ooecia/Ovicells: serve as the gonads of the colony; storing, producing, and holding the eggs until ready to hatch.
So, how does the colony decide when to make another kind of zooid? And how does that work? The morphological change in the heterozooids is actually determined by chemical signals sent from the parent colony, from predators or from rival colonies. It was observed that many bryozoan colonies respond to attack with production of various inducible defenses. Once such response is spine growth which occurs amazingly rapidly. In controlled laboratory experiments performed by Harvell (1984), spines grew in 48 hours following the exposure of the colony to two species of slow-feeding nudibranch. It is energetically expensive for the colony itself to stop feeding and produce spines (Harvell 1984), but one can imagine the alternative! Spines grow from existing buds and at the corner of each little house. In Harvell’s (1984) experiments, spines developed along the entire perimeter of the colony exposed to the molluscan predators. This signified that communication through the entire colony about a predator attack is possible in bryozoans. A predator in one portion of the colony can trigger a response for peripheral spine development elsewhere.
Figure 2. Ring of tentacles around mouth on lophophore of a bryozoan. Photo credit: Megan Hill.
Though heterozooids are incredibly interesting, I have to say that autozooids hold the most critical role for the health of the colony. Each little feeding animal has a ring of ciliated tentacles surrounding the mouth that can be circular or U-shaped (Pechenik 2015, see Fig. 2). The tentacles are covered with unidirectional cilia that move food particles into the mouth. When lophophores are visible, we can see their distinct filter-feeding strategy called “tentacle flicking” (Riisgård & Manríquez 1997, see Fig. 3). In lab, I recorded three different species and two different types (encrusting and arborescent) of bryozoans flicking particles into their tiny mouths through a microscope. As I alluded to earlier, bryozoan colonies will stop feeding and retreat into their cystids or zooecium to avoid predation (see Fig. 4). With the reduction of flow in the fingerbowls we kept them in, the occasional harsh lighting, and the general disturbance of being in the laboratory, it seems lucky that we got to see autozooids feeding at all. Perhaps this is a reflection of their lack of interest in the chemical signaling provided by unlikely human predators, or perhaps these guys were just in the mood to show off their amazing biology.
Videos:
Figure 3. Video of tentacle flicking of bryozoans (lophophores outside of cystid). Video credit: Megan Hill.
Figure 4. Video of Candy-striped Worm (Dorvillea moniloceras) walking across alive (lophophore under operculum inside cystid) and dead Lyrula hippocrepis or Reginella mucronata. Video credit: Megan Hill.
References:
Bullivant JS (1968) Attachment and growth of the stoloniferous ctenostome bryozoan, Zoobotryon verticillatum. Bulletin of the Southern California Academy of Sciences, 67(3), 199-202.
Harvell C (1984) Predator-Induced Defense in a Marine Bryozoan. Science, 224(4655), 1357-1359. http://dx.doi.org/10.1126/science.224.4655.1357
McKinney FK & Jackson JB (1991) Bryozoans as modular machines. pp. 1–30. Bryozoan Evolution. First edition. Chicago, IL: University of Chicago Press.
Pechenik JA (2015) Biology of the Invertebrates. Seventh edition. New York, NY: McGraw-Hill.
Winston JE (1995) Ectoproct diversity of the Indian River Coastal Lagoon. Bulletin of Marine Science, 57(1), 84-93.
Riisgård H & Manríquez P (1997) Filter-feeding in fifteen marine ectoprocts (Bryozoa): particle capture and water pumping. Marine Ecology Progress Series, 154, 223-239.
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