Larvaceans (aka Appendicularians)

A larvacean (the small, white, peanut-shaped object) inside its mucus house, feeding on marine snow at 700m depth at the Slope Base Site. Photo Credit: NSF-OOI/UW/CSSF; Dive R1733; V14

Larvaceans are pelagic, free-swimming tunicates, which are frequently seen in the water column at coastal and Axial Base OOI sites. Although these chordates are generally small and keep their tadpole-shaped bodies as adults (unlike other tunicates that settle and metamorphose into sponge-like sessile forms), they form a large mucus house around themselves to assist in filter feeding.

The larvacean itself has a distinct trunk and flapping tail with a notochord, and can be seen in the center of the mucus house, pumping water through the complex structure. In some species, the house can grow to be a meter across, ten times larger than the animal in the center! The houses screen out particles that are too large for the larvacean to eat while concentrating the smaller particles and moving them towards the animal’s mouth. Eventually, the houses get clogged with marine snow and waste and are discarded, joining the rain of particulate material that falls to the seafloor and becomes food for benthic animals.