Many species of tunicates live a solitary adult life; they do not form colonies. The tunicate on the left (most likely Styela clava) is an example. It's 5 - 15 cm long, anchored to a solid substrate by a tough stalk several cm long (not seen in this picture). It has a brown, leather-like tunic, which is often encrusted with other organisms, as seen here (I think there is a colony of Botryllus near the bottom of the Styela). When I picked it from the side of a dock float I thought it was just a piece of wood and I was only interested in the organisms attached to it. It was several hours later in the lab that I discovered that the "piece of wood" moved when I touched it, and when undisturbed for 10-30 seconds extended the two shiphons seen at the top of the animal. The intake siphon is the larger one in the forground, and the pattern of four pairs of dark brown stripes on both siphons is characteristic of the species.
It is native to the Western Pacific and was most likely brought to the Eastern Pacifid (the West coast of the United States) on the bottom of ships. It is a prized food in Korea (mideuduck). According to ref 2, there are two other native species and two exotic species which can be confused with S. Clava.
This solitary tunicate is typically smaller than S. clava. The "tadpole" of the adult is smaller than the Botryllus "tadpole", containing only 2,500 cells.
Its surface is free of hitch-hiking organisms and the body is translucent; you can see internal organs. In the image to the right the animal (6 cm tall) is attached to the shell of a mussel, while the animal seen below has been removed from its attachment.
There are six spots of pigments around the two siphon openings. The intake siphon (right) is the larger, water flows exits from the smaller (left).
Ciona intestinalis, a closely related animal, (easily confused with C. savignyi), has been studied for many years by biologists interested in basic mechanisms of development. This motivated the determination of the nucleotide sequence of the genome of C. intestinalis, described in Science 298:2157(2002). The total genome contains approximately 160 million base pairs (1/20 the size of the human genome). There appear to be about 16,000 genes that code for proteins, which is half the number in the human genome.
A common mechanism for the creation of new genes is the duplication of a gene followed by mutation of one of the copies to produce a gene with a new function (the other version of the gene continues to carry out its original function). The entire genome of vertebrates appears to have been duplicated sometime in the distant past (100 million years ago), producing pairs of genes which have then drifted apart in function. However, the genome of C. intestinalis does not contain this duplication. Thus, the tunicates evolved from the common ancestor of tunicate and vertebrates before this duplication.
In the image to the left, the stomach is the pink-brown object at the bottom. Dark-brown fecal material is seen moving up the tube along the left edge of the exit siphon.
In the image below the intake siphon is seen at higher magnification. The rectilinear nerve net can be clearly seen.