Tunicates are filter feeders, a defining characteristic in marine environments worldwide. These marine invertebrates, often recognized as sea squirts, rely on filtering water for nutrients. Found in diverse marine habitats, their widespread presence underscores this effective feeding strategy.
Understanding Tunicates
Tunicates are marine invertebrates belonging to the subphylum Tunicata. They are distinguished by a sac-like body, often encased in a tough, protective outer layer called a “tunic,” composed of a cellulose-like substance. While many adult tunicates, particularly ascidians, are sessile, meaning they remain attached to surfaces, other forms like salps and larvaceans are free-swimming and pelagic. Tunicates exhibit a range of forms, existing as solitary individuals or forming complex colonies made up of many smaller units called zooids.
The Filter Feeding Mechanism
The filter-feeding process in tunicates involves a continuous flow of water through their bodies, facilitated by two distinct openings: the incurrent (or oral) siphon and the excurrent (or atrial) siphon. Water is drawn into the tunicate through the incurrent siphon by the rhythmic beating of tiny hair-like structures called cilia, which line specialized gill slits within the pharynx. This water then passes into a large, basket-like pharyngeal or branchial sac, which acts as the primary filtering apparatus.
Within this sac, a continuous sheet of mucus, secreted by a ciliated groove called the endostyle, forms a fine net. Microscopic particles like phytoplankton, zooplankton, bacteria, and detritus become trapped in this sticky mucus net as water flows through. The cilia then transport the mucus net, laden with captured food, towards the tunicate’s digestive tract. After filtration, the cleaned water is expelled from the body through the excurrent siphon, completing the cycle. Tunicates are highly efficient, with some species capable of filtering hundreds of liters of water per day, and can capture particles ranging from submicrometer size up to 1 millimeter.
Ecological Significance of Tunicate Feeding
The filter-feeding activities of tunicates have substantial implications for marine ecosystems. By constantly drawing in and expelling water, they play a considerable role in water clarification, removing suspended particles and contributing to overall water quality. This removal of particulate matter can enhance light penetration in the water column, benefiting photosynthetic organisms. Tunicates occupy a significant position in the marine food web as primary consumers, converting microscopic organisms and detritus into their own biomass. This conversion makes them a food source for other marine animals, including certain fish, crabs, and sea stars. Their feeding also contributes to nutrient cycling by processing organic matter. Furthermore, tunicates can serve as bioindicators for environmental changes, as some species accumulate heavy metals and other pollutants, reflecting the health of their surrounding waters.