Urochordata

Urochordates are small marine animals with larvae that swim freely and adults that attach themselves to the ocean floor. The 1,300 species of urochordates, like all members of the phylum Chordata, possess four characteristic anatomical structures as embryos: a flexible body-length rod (the notochord) that provides resistance against muscular contractions and allows for more efficient movement; a dorsal, hollow, nerve cord that forms the central nervous system; slits in the beginning of the digestive tract (the pharynx) that allow filter feeding and gas exchange; and a postanal tail. Urochordates, commonly known as tunicates, differ from other chordate subphyla (Cephalochordata and Vertebrata) in that the adult form has no notochord, nerve cord, or tail. In fact, an adult tunicate is an immobile, filter-feeding marine animal that in some ways looks more like a sponge or a mollusk than a chordate.

Tunicate larvae look much more like other chordates than adult tunicates do. Larvae swim using their notochord as structural support for strong, wavelike body movements. However, a larval tunicate cannot eat because both ends of its digestive tract are covered by a skinlike tissue called the tunic. The sole purpose of the larva is to find a place to attach its tail to the ocean floor, where it can metamorphose into an adult and begin sexual reproduction. Metamorphosis involves the loss of the notochord, nerve cord, and tail, and a twisting of the body so that the mouth and the anus both point away from the attachment. The adult can then start filter feeding. It does this by sucking water into its mouth through a pharynx with slits that captures food and spitting the water back out a different opening called the atriopore.

Tunicates might seem like an evolutionary off-shoot, given that the adult form is so different from other chordates. However, based on thesimilarities between larval tunicates and embryonic vertebrates, evolutionary biologists suspect that the first vertebrate chordates were very much like modern-day tunicates. They propose an evolutionary mechanism called paedomorphosis, in which the larval form evolved the ability to reproduce before metamorphosis. No longer requiring a suitable ocean floor to reproduce, these protovertebrates would have been free to exploit and adapt to new niches, eventually giving rise to the vertebrate skeleton.

To say that our vertebrate ancestors resembled tunicates is not to say that those ancestors were tunicates. Modern-day tunicates have had 500 million years to evolve since the days of the ancestors they share with vertebrates. Conceivably, then, protovertebrates did not have an adult form like that of the tunicate, and this adult form evolved later. Without fossil evidence of the earliest vertebrates, it may never be known whether or not they resembled tunicate adults.

Phylogenetic Relationships of Major Groups.

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