Transparent Sea Squirt         Corella willmeriana

From late summer to winter you may find a surprising animal, about 2 inches in size, under floats, on boat hulls, or at the base of piling exposed during lowest tides.  It looks like an ice cube just waiting for you to scoop it up and plop it into a tall glass of soda.  In reality it is a very complex animal called a sea squirt (because it squirts out water when squeezed).  Sea squirts are attached animals that, along with several free swimming groups, are called tunicates (due to their tunic-like covering).  

Sea squirts are divided into three major groups: the compound form in which individuals are fused together into a single entity; a social form where the individuals are separate but closely associated; and a solitary form like the Transparent sea squirt.  Some introduced species can grow to a large size fouling boat hulls.

Looking through the transparent body wall, the mesh bag that filters out plankton can be seen in the middle of the body with a brown, food filled digestive tube coming from it.  Plankton are drawn into the body through an incurrent siphon and expelled through an excurrent siphon, much like a clam.  However tunicates are not related to clams, but to vertebrates.  The evolutionary relationship can be seen in the sea squirt larva (looking like a frog tadpole), which has a quasi-backbone called a notochord, a post-anal tail and gill slits, all of which are characteristic of a primitive vertebrate.

The transparent sea squirt can be found on hard surfaces from the low intertidal into the shallow subtidal zones ranging from southern Alaska to southern California.