Bryozoans, often called moss animals, are a phylum of small, aquatic invertebrates that live in colonies. Each colony is composed of thousands of genetically identical individuals, known as zooids, which are typically less than a millimeter in length. These organisms are sessile, meaning they are permanently attached to a substrate like rocks, shells, or kelp, and thrive in both marine and freshwater environments worldwide. Bryozoans play a significant ecological role as effective filter feeders, using a crown of tentacles called a lophophore to capture microscopic plankton and algae from the water column. While they contribute to water filtration, bryozoan colonies also represent a common food source for a wide array of aquatic life.
Marine Invertebrate Predators
The most specialized consumers of bryozoans belong to the marine invertebrate phyla, which have evolved unique feeding mechanisms to overcome the colonial structure. Nudibranchs, or sea slugs, stand out as dedicated predators, with many species feeding exclusively on specific bryozoan colonies. These mollusks utilize a rasping organ called a radula to scrape away or pierce the outer coverings of the zooids to consume the soft tissue within. The specialized diet of certain nudibranchs, such as those in the genus Polycera, allows them to sequester the defensive compounds produced by their bryozoan prey, using those toxins for their own protection.
Other invertebrate grazers also target bryozoan colonies as part of a broader diet of sessile organisms. Sea urchins and certain gastropods use their strong mouthparts to scrape entire patches of the colony’s surface, consuming the zooids, their hard skeletons, and the surrounding substrate. Small crustaceans, such as amphipods, and sea spiders (pycnogonids) are also known to prey on bryozoans, often employing piercing and sucking mouthparts to extract the individual zooids. This specialized consumption indicates that for many small invertebrates, bryozoans are a primary and deliberate food source.
Vertebrate Consumers
Vertebrates, including fish, turtles, and waterfowl, also consume bryozoans, though their feeding tends to be less selective than that of invertebrates. In marine environments, fish species often consume bryozoan colonies incidentally while grazing on the hard substrates they encrust. Certain bottom-feeding fish and angelfish, for example, scrape algae and other attached organisms from rocks and coral, inadvertently ingesting the bryozoan zooids as part of the biological film. The large body size and crushing mouthparts of these generalist fish mean they are less deterred by the small, hard skeletons of the zooids.
Juvenile green sea turtles are documented to consume bryozoans as part of their omnivorous diet, which includes various small invertebrates and algae. Freshwater bryozoan colonies, especially the large, gelatinous masses formed by species like Pectinatella magnifica, are consumed by certain freshwater turtles and waterfowl. Ducks and other diving birds may ingest entire colonies while foraging for aquatic vegetation, or they may consume the dormant survival structures called statoblasts. In these cases, the bryozoans are consumed as part of a bulk grazing strategy on a substrate.
Bryozoan Defenses Against Being Eaten
Bryozoans have evolved several mechanisms to reduce the risk of being consumed by their numerous predators. A primary physical defense is the rigid, calcified exoskeleton, or zooecium, which surrounds the individual zooid and makes the colony difficult to chew or rasp. Many species also possess an operculum, a small, chitinous lid that quickly snaps shut to seal the opening when the feeding lophophore is retracted upon sensing a threat. Some marine bryozoans, such as Membranipora membranacea, can rapidly grow spines on the colony surface when exposed to chemical cues from nearby predators, physically deterring grazers like nudibranchs.
Beyond physical barriers, many bryozoan species produce potent secondary metabolites that act as chemical deterrents. These compounds can be highly distasteful or toxic, effectively discouraging less specialized consumers like fish and crustaceans. For instance, the compound Bryostatin-1, isolated from the bryozoan Bugula neritina, is a powerful chemical defense that helps protect the colony’s larvae and young zooids. Specialized zooids, such as the pincer-like avicularia, also serve a defensive role by snapping at small invertebrates that attempt to settle or graze on the colony surface.