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Tunicates

The tunicates or sea squirts are an entirely marine group of animals, and are unfamiliar to many people. Despite their unimpressive appearance, they are chordates, and—technically—are more closely related to human beings than to any of the invertebrates listed above. They have a notochord, a primitive backbone, only in their larval form. Once they settle out of the plankton and become sessile filter-feeders, the backbone is unceremoniously shed. (So much for the vaunted evolutionary superiority of "higher order" forms.)

The tunicates seen on Indonesian reefs are all in the class Ascidiacea, a name derived from the ancient Greek word for leather bottle. They are rather like little bottles, with (usually) two openings rather than just one. Water is drawn in through the uppermost of these siphons, filtered through a basket-like arrangement internally, and then passed out through the lower siphon. Peer into the opening of a large tunicate and you may be able to make out the fine sieving apparatus within. Many tunicates have stout spikes projecting from the inner wall of their siphons, to thwart small fish or other unwanted intruders.