Thanks to alert Google+ viewer Dmitry Burstein for correcting my identification of this species. Rather than being a species of sponge, these are actually a species of tunicate known as the green barrel sea squirt, Didemnum molle.
Although looking something like a sponge, tunicates are almost as far removed from sponges as you can get in the animal kingdom. Strange as it may seem, they belong to the same major group of animals, the phylum Chordata, to which humans also belong. That is because at one stage in their life cycle they possess a notochord, or a flexible rod made out of material similar to cartilage.
In the early stages of development, all animals with backbones have a notochord. Many lower vertebrates retain the notochord beyond the embryonic stage. In tunicates it is found through the development of a tadpole-like larval stage.
The only feature that these sea squirts share with sponges is that both filter their food from the passing water.
Today’s photo was taken at a location on Rainbow Reef off the island of Vanua Levu known as Freeway. The beautiful green and red colors of these sea squirts caught my eye, and are the result of photosynthetic bacteria that live within the tissues of the sea squirt.
This photo was taken with a Canon EF100 mm f/2.8 macro lens on a Canon EOS 5D Mk. III in an Ikelite underwater housing. Lighting was provided by an Ikelite DS161 strobe set to eTTL mode. The exposure was set to 1/160 sec. at f/11 and ISO 200.