Tunicates are marine invertebrates found in oceans globally, inhabiting diverse environments from shallow coastal areas to the deep sea. These animals display a variety of forms, including sessile sea squirts that attach to surfaces like rocks or ship hulls, and free-floating species such as salps and larvaceans. Their adaptability allows them to thrive in both solitary and colonial arrangements.
Primary Food Sources
Tunicates primarily consume microscopic organisms and organic particles suspended in the surrounding water. Their diet largely consists of phytoplankton, tiny marine plants, and zooplankton, small animals and larval stages of other marine life. They also filter detritus, dead organic matter, from the water column. Some deepwater tunicate species, such as ghostfish, consume small crustaceans, nematodes, and other tiny invertebrates. Certain species also derive nutrition from symbiotic green algae or cyanobacteria that reside within their tunics.
The Filter-Feeding Mechanism
Tunicates filter feed by continuously drawing water through their bodies. Water is drawn into the tunicate through an opening known as the incurrent or buccal siphon. Inside, water passes through a large pharyngeal basket, a muscular tube lined with numerous small pores or gill slits. A mucous net, continuously secreted by a ciliated groove called the endostyle, traps food particles as water flows through the pharynx.
Cilia, tiny hair-like structures lining the gill slits, create the water current, moving it through the filter into a surrounding cavity called the atrium. The mucous net, along with the trapped food particles, is then rolled into a cord and transported to the esophagus for digestion. Filtered water and waste products are expelled from the tunicate’s body through another opening, the excurrent or atrial siphon.
Ecological Impact of Tunicate Feeding
Tunicate feeding habits broadly influence marine ecosystems. They contribute to water clarification by actively filtering large volumes of seawater, removing plankton and other organic debris. This filtration helps maintain water quality and influences planktonic communities. Their consumption and processing of organic matter also play a part in nutrient cycling within marine environments.
Tunicates occupy a position in the marine food web as primary consumers, converting microscopic particles into biomass that can be consumed by larger organisms. While some species are prey for fish, starfish, and sea turtles, certain invasive tunicate species can outcompete native filter feeders like scallops and mussels for food, and overgrow other sessile marine life. Their rapid growth and ability to form extensive colonies can alter habitats, impacting local biodiversity and food availability for other species.