Bryozoans, often called “moss animals,” are a diverse phylum of aquatic invertebrate animals that form colonies. Classified as Bryozoa or Ectoprocta, these organisms are globally distributed but frequently go unnoticed due to their small size and stationary nature. They are found in nearly every aquatic environment, from the deep ocean floor to freshwater lakes and streams, comprising around 6,000 living species. Bryozoans are a major component of many marine and freshwater ecosystems.
Fundamental Characteristics and Anatomy
Bryozoans are characterized by their modular, colonial structure, known as a zoarium. The colony is made up of numerous individual units called zooids, typically about 0.5 millimeters long. These zooids are physiologically interconnected clones, functioning as a single cooperative organism.
Each colony exhibits a division of labor through specialized zooids. The most common type is the autozooid, responsible for feeding and supplying nutrients. Other zooids, known as heterozooids, take on specialized roles such as reproduction, defense, or anchoring the colony. For example, some marine species possess avicularia, specialized zooids resembling tiny bird heads used for defense and cleaning.
The defining anatomical feature of the feeding zooid is the lophophore, a crown of slender, ciliated tentacles surrounding the mouth. This structure is extended to capture food particles from the water column. The lophophore is often circular in marine species, while freshwater species typically have a horseshoe or U-shaped lophophore.
The zooid’s body is encased in an outer chamber called the cystid, which provides structural support and protection. This outer layer, or exoskeleton, can be composed of flexible chitin, a proteinaceous matrix, or rigid calcium carbonate. The material dictates the colony’s appearance, which can range from delicate, branching forms to hard, flat, encrusting sheets that resemble small corals or lace.
Life Processes and Reproduction
Bryozoans are primarily suspension feeders, relying on the lophophore to filter microscopic food particles. Cilia, tiny hair-like projections on the tentacles, beat rhythmically to create a current that pulls water toward the mouth. Food particles, such as diatoms and phytoplankton, are channeled into the mouth, while waste is expelled through an anus located outside the lophophore crown.
Colony expansion is achieved through asexual reproduction via budding. New zooids develop directly from existing ones, allowing the colony to rapidly increase its size and colonize new areas. If a fragment breaks off, it can sometimes regenerate and grow into a new, independent colony, particularly in high-energy marine environments.
Sexual reproduction involves the production of gametes, as most species are hermaphrodites. Sperm are released into the water and captured by neighboring lophophores to fertilize eggs internally. The resulting embryos are often brooded within specialized chambers before being released as planktonic larvae. These larvae, such as the triangular cyphonautes, eventually settle onto a substrate to start a new colony.
Freshwater bryozoans (class Phylactolaemata) have evolved a unique survival mechanism called statoblasts. These are tiny, disk-shaped, dormant reproductive structures encapsulated in a protective chitinous shell. Statoblasts are highly resistant to freezing, desiccation, and harsh environmental conditions. This allows the colony’s lineage to survive when the parent colony dies off during winter or drought.
Diverse Habitats and Distribution
The vast majority of bryozoan species, approximately 6,000 extant forms, are found in marine environments across the globe. They inhabit diverse areas from shallow intertidal zones to the deep abyssal plains. Marine bryozoans are particularly diverse in tropical waters and on the Antarctic shelf.
A smaller group, totaling only around 100 species globally, is adapted to freshwater habitats. These forms are found in streams, rivers, and stagnant ponds, often forming large, gelatinous masses attached to submerged logs or aquatic plants. Their presence in inland waters is maintained by the dispersal of statoblasts.
Bryozoans are sessile organisms, meaning they live permanently attached to a solid surface. They prefer hard substrates, often colonizing rocks, shells, kelp blades, and artificial structures like dock pilings and boat hulls. Their growth forms reflect their environment and skeletal composition, appearing as flat, crust-like mats, delicate, tree-like structures, or soft, gelatinous blobs.
Ecological and Human Relevance
Bryozoans play a significant ecological role as primary suspension feeders and natural water purifiers. By filtering large volumes of water to capture phytoplankton and microscopic particles, they help clarify the water column. Bryozoans themselves serve as a food source for numerous small invertebrates, including sea slugs (nudibranchs), sea urchins, and various fish species.
The extensive skeletal remains of bryozoans contribute significantly to the formation of marine sediments in carbonate deposits. Bryozoans have an exceptionally long fossil record, dating back to the Ordovician period over 480 million years ago. Scientists use their fossilized colonies to reconstruct ancient marine environments and ecological conditions.
The ability of bryozoans to rapidly colonize hard surfaces directly impacts human maritime activities. They are major contributors to biofouling, the undesirable accumulation of organisms on submerged man-made structures. Bryozoan colonies can foul ship hulls, water intake pipes, and aquaculture equipment, necessitating costly preventative and removal measures.
Bryozoans are also a source of interest in biomedical research, despite their role in biofouling. Certain species, such as Bugula neritina, host symbiotic bacteria that produce potent chemical compounds. These substances are being investigated for their potential as novel pharmaceuticals, including promising anti-cancer agents.