Tunicate - Feeding

Feeding

Most tunicates are suspension feeders, capturing suspended particles and plankton by filtering sea water through their siphons via their pharyngeal slits, but some, such as the Megalodicopia hians, are marine sit-and-wait predators.

Tunicates have two openings in their body cavity: an incurrent and an excurrent siphon. The incurrent siphon takes in water plus any food it contains, and the excurrent siphon expels wastes plus sieved water. The primary food source for most tunicates is plankton that gets entangled in mucus secreted from the endostyle. The tunicate's pharynx, or branchial sac, is lined with ciliated epithelium. The action of the cilia passes food particles and plankton down to the esophagus and also passes the stream of water from the inhalant to the exhalant siphon. The gut is U-shaped and also ciliated, and the anus opens into the dorsal or cloacal part of the peribranchial cavity near the atrial aperture, where the constant stream of water carries wastes to the exterior.

Tunicate blood is particularly interesting. It contains high concentrations of the transition metal vanadium and vanadium-associated proteins as well as higher than usual levels of lithium. Some tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. Specialized cells can concentrate heavy metals, which are then deposited in the tunic.