Tunicate

On such surfaces as marine dock pilings, rocks, ships, offshore oil rigs, and coral reefs, one can often find humble blobs of jelly among the sponges and hydroids. Some are as small as sesame seeds and some as big as potatoes. Some are solitary and others live in dense clusters. Some are transparent and nearly invisible, while others are as brilliantly colored as a flower gar­den. These are tunicates, named for the cloaklike tunic that supports their saclike bodies. The tunic ranges from gelatinous to stiff. It often contains cellulose, a product normally associated with plants.

Tunicates, like humans, are in the animal phylum Chordata. Their swimming, tadpolelike larvae have a notochord and dorsal nerve cord as hu­mans and other vertebrates do, but these are absent from the adults. Tunicates are classified in the subphylum Urochordata, signifying that the notochord is present only in the tail of the larva (uro means “tail”).

Adult tunicates are filter-feeders, meaning they feed by pumping water through their bodies and straining plankton from it. Most of them are also sessile, or fixed in one place. Some, however, are swimming members of the marine plankton community. Adult tunicates have two body openings called siphons. They suck water into one opening, strain plankton from it with a filter called the branchial sac, and expel the water through the other siphon. When taken from the water, tunicates may expel a jet of water from this exit siphon, earning them the alternative name sea squirts.

References

Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.

Rupert E. E., and R. D. Barnes. Invertebrate Zoology, 6th ed. Philadelphia, PA: W. B. Saunders, 1994.