Thaliacea

Thaliacea is a class of marine, pelagic tunicates (subphylum Tunicata) that includes the groups commonly known as salps, doliolids, and pyrosomes. Like other tunicates, they possess a tunic made of cellulose-like material, but in contrast to sessile sea squirts, thaliaceans are free-swimming and planktonic. They occur in oceans worldwide, from the surface to deeper waters, and range from solitary individuals to extensive colonial projections.

Thaliaceans have simple, gelatinous bodies often elongate or barrel-shaped. Most possess two siphons: an oral siphon for water intake and an excurrent (atrial) siphon for water outflow. They move by muscular contractions that pump water through their bodies, producing jet-like propulsion. Their bodies are typically transparent, which aids in camouflage in the open ocean.

Feeding occurs as they filter feed on microscopic particles. Water is drawn in through the oral siphon, and food is captured by pharyngeal slits and mucus nets produced by the endostyle. The captured material is then transported to the digestive system for processing. Feeding is continuous in many species, supporting rapid growth in favorable conditions.

Life cycles in thaliaceans often involve alternation of generations. Some species are solitary but reproduce sexually, while others form colonial bodies composed of numerous zooids that function together as a single organism. Colonial forms include pyrosomes, which can form long, hollow tubes.

Ecologically, thaliaceans are important components of open-ocean ecosystems. They feed on phytoplankton and small particles and can contribute significantly to carbon export through fast-sinking fecal pellets and discarded bodies. They also serve as prey for a range of larger zooplankton and fish.