Tunicates are a common subphylum of marine sac-like filter feeders found commonly in all the oceans of the world. Though usually found attached to the sea floor, the most familiar tunicates are free floating, including pyrosomes (bioluminescent tunicates), salps (which live in long linear colonies), and doliolids (tunicates with large siphons). Along with countless other organisms, tunicates make up the plankton. They are also called "sea squirts" or "sea pork" and come in a variety of colors, especially among the benthic (bottom-dwelling) species.
Though the simple barrel and sac-like bodies of tunicates make them reminiscent of simple animals like sponges or jellyfish, tunicates are actually more closely related to the vertebrates, including human beings. This is evidenced by their larvae, bilateral tadpole-like animals with a simple nerve cord. Based on this, tunicates are classified as chordates, the phylum that contains all vertebrates, in subphylum Urochordata.
Many paleontologists consider tunicates a window into what the earliest chordates may have looked like. Chordates would have branched off from our closest relatives, the acorn worms, sometime in the Late Ediacaran or Early Cambrian period (about (542 million years ago). However, modern tunicates barely resemble acorn worms, suggesting there may have been a number of intermediate stages. Because all these animals are soft-bodied, however, they have a very poor fossil record. Fossilized tunicates often leave behind only indirect signs, such as imprints left by their holdfasts, stalks of tissue they use to attach themselves to surfaces.
Tunicates are so named for their tunic, a tough fleshy shield that surrounds the barrel-shaped body of the animal. Prior to becoming adult tunicates, larval tunicates float in the water column, building filter-equipped protein "houses" (tests) for themselves that help concentrate food particles prior to their being drawn into the pharyngeal slits. As the larva grows and the filters on the test become clogged, they are discarded. The larva are the only tunicates that are able to discard their tunics in this fashion. Though these tests are very small, the number of tunicate larva in the oceans is so great that discarded tests make up a substantial portion of all carbon that descends into the ocean's deep.