Bryozoans are aquatic invertebrates commonly called “moss animals” due to their sessile, plant-like appearance, which often leads to confusion about their biological classification. They are found in both marine and freshwater environments across the globe, forming diverse encrusting or upright structures. This article clarifies the specific biological role of these organisms within the food web to determine how they obtain the energy necessary for life.
Identifying Producers and Consumers
The foundation of any biological food web rests on the distinction between producers and consumers, which describes how organisms acquire energy. Producers, also known as autotrophs, are organisms capable of generating their own food source, typically using light or chemical energy. The most common example is photosynthetic plants and algae, which convert sunlight and carbon dioxide into sugars to sustain themselves.
Consumers, or heterotrophs, cannot create their own food and must instead ingest other organisms or organic matter to gain energy. This category includes all animals, fungi, and many types of microorganisms that rely on external sources for their nutritional requirements. All living things are placed into one of these two categories based solely on their method of energy acquisition.
The Bryozoan Lifestyle: Filter Feeding
Bryozoans are classified as consumers because they are heterotrophs that must ingest outside food sources. They lack the necessary photosynthetic pigments, such as chlorophyll, required to convert light energy into chemical energy. Instead, each individual member of a colony, called a zooid, is an active feeder that captures tiny particles suspended in the water column.
Their specialized feeding apparatus is a crown of tentacles known as the lophophore, which extends from the body when feeding. The lophophore is lined with microscopic, hair-like structures called cilia that beat rhythmically in the water. This coordinated movement creates a current that draws water and suspended food particles toward the zooid’s mouth.
The primary diet of a bryozoan consists of microscopic plankton, including diatoms, small protozoans, and bacterial cells. Once captured by the tentacles, the food is directed down the lophophore to the mouth, where it is then processed within the digestive tract. Because they actively ingest other living or dead organic matter, their classification as primary consumers is firmly established.
The confusion surrounding their classification often arises because they are sessile and appear stationary, much like corals or certain types of algae. However, unlike true producers, bryozoans are complex invertebrates with a complete digestive system, confirming their identity as animals. Their existence is sustained by the continuous, active filtration of their surrounding aquatic environment.
Bryozoans in the Ecosystem
Bryozoans perform a significant ecological function through their collective filtering activity. A large, established colony can process substantial volumes of water daily, effectively removing excess particulate matter and plankton from the water column. This constant filtration contributes to water clarity and helps regulate the concentration of suspended organisms, benefiting the overall health of the aquatic ecosystem.
Bryozoans serve as a direct food source for specialized predators, such as certain species of nudibranchs (sea slugs) and sea spiders. These predators often have adaptations that allow them to consume the colonies directly, integrating the “moss animals” into the higher levels of the food web.
The rigid, skeletal structures that many bryozoan colonies construct also provide valuable habitat and substrate for other small invertebrates. These encrusting mats and branching structures create micro-shelters that increase local biodiversity and offer protection for juvenile organisms. Their presence influences the structure of the benthic community where they reside.