Tunicates [Chordata]

Tunicates are one of the lowest animals in the phylum Chordata. All chordates share the four traits listed below. They either have a brain (vertebrates), or lack a brain (non-vertebrates).

They all start out with a dorsal hollow nerve cord, a hollow tube in the upper (dorsal) part of the body, housing a nerve cord. Some non-vertebrates maintain this nerve cord through their lives; others resorb it as they approach adulthood. In vertebrates, it becomes the brain and spinal cord.

Just below this, a flexible rod called the notochord provides support for their bodies. Again, some non-vertebrates keep it, while others resorb it as they approach adulthood. In vertebrates, it becomes the spine.

Chordates also have gill slits, which lead to a place between the mouth and the esophagus, called the pharynx. The pharyngeal gill slits remain in the non-vertebrates, as well as two classes of vertebrates: fish and amphibians. The gill slits of all other vertebrates are present only in the embryos, and then they close.

All chordates have a tail.

Tunicates
[Urochordata]
Tunicates fall into the no-brain category of chordates. Much like sponges they are basically water filters, and seem nothing like other chordates. It is only in their free-swimming larval stage that they have the characteristics of a chordate — the notochord, the dorsal hollow nerve chord and the tail. Once settled on hard surfaces they lose these parts and begin a sessile lifestyle.

A tunicate secretes a tough cellulose sac, called tunicin, in which to live. It draws water into its body through an incurrent siphon. Once the water passes the gill slits, it enters the pharynx where mucus collects detritus and plankton. The mucus is then passed to the esophagus. Waste is expelled from an excurrent siphon.

A tunicate grows either by itself or in a colony.

Solitary types typically appear as grape- or vase-shaped blobs with incurrent and excurrent siphons on top. Look for them in the lower intertidal zone clinging to hard surfaces like rocks, dock pilings and eelgrass.

Greenish, hairy sea grapes look like blobs about an inch and one-third high, and are often covered with mud.

Sea vases are clear with a yellow frill ringing each siphon spout, and grow to two and a half inches high. Both may squirt water when touched.

Colonial tunicates are made up of tiny, individual animals called zooids, which grow in a common crust or rubbery mass and share an excurrent siphon.

See if you can spot a sheet of golden star tunicate (Botryllus schlosseri) on the back of subtidal seaweed. The zooids are arranged in star-shaped clusters. Their colors may include gold, purple or dark brown.

The zooids of orange sheath tunicate (Botrylloides diegense) also grow within a thin sheet of rubbery matrix, or as rubbery blobs floating along in the current. They may not sound appealing, but the bright orange color and swirling arrangements of these zooids make this tunicate an interesting find. Thin layers of colonies can sometimes be found on underside of seaweed blades. The colony pictured at left was reeled in by a fisherman at Hilton Park in Dover.

amphibians, reptiles, fish, birds, mammals
Vertebrates
The notochord of these animals develops into either cartilaginous or bony backbones. The dorsal hollow nerve chord develops into a spinal chord and brain.

Amphibians
None in marine habitats.

Reptiles
Sea snakes, sea turtles. None in Great Bay Estuary.

Copyright © 2006 Barbara Driscoll.