Article by: Brian Marais
Upon first glance at a tunicate (Sea Squirt), one would assume that these creatures are primitive, even alien. This couldn't be further from the truth, as they are highly evolved organisms and fall in the phylum Chordata, which is the same phylum as we humans.
Tunicates are creatures that siphon water through an inhalant siphon, catching food particles through an organ called a pharynx. The excess water filters through the pharynx and enters an atrium where it is pumped out via the exhalent siphon. The pharynx, by means of gill-like structures called stigmata, separates the food particles and pushes them down towards the stomach, where the food is digested and excreted out the anus into the atrium and out the exhalent siphon. It does this by means of an organ called an endostyle that produces mucus that catches the particulates in the water passing through. Some colonial species of tunicate share a common exhalent siphon amongh several individuals.
A small heart is responsible for pumping blood through tiny blood vessels around the pharynx to collect oxygen from the water. The interesting thing about the heart is that it pumps blood in one direction for a few minutes and then, for a few more minutes, stops beating completely before finally restarting and pumping blood in the opposite direction.
Tunicates also have gonads, which contain both the testes and ovary in order to reproduce. Individual species reproduce sexually by releasing eggs and sperm into the atrium and out the exhalent siphon. Colonial species, however, reproduce asexually by splitting themselves.
Once the eggs are fertilized, they form free-swimming organisms not unlike tadpoles that join the ranks of zooplankton in the ocean for fish and other planktivorous species to consume. These free-swimming creatures have the same inhalant and exhalent siphons as their adult counterparts, a hollow nerve chord that extends the length of the body, and a structure called a notochord that is responsible for providing some rigidity to the creature as well as protect the delicate nerve chord. This is similar to a spine, except that it does not encase the nerve chord with vertebrae.
Above: Bluebell Tunicates
It will latch onto a surface by using its stolons in order to start its metamorphose stage in which it will become the adult tunicate that we can see. Once the creature has completed its metamorphose stage, all that remains of the nerve chord and notochord is a small structure next to the pharynx called a ganglion. Tunicates, despite having a hollow nerve chord, have no brain. It is believed that the ganglion serves as a brain but only to provide the most basic life-support functions.
Tunicates are difficult to keep in aquariums. Many people, including myself, have tried to keep them only to have them die inexplicably a few months later. The reasons for this are as yet unknown in the hobby. Speculation regarding the subject is still an on-going debate as to whether these creatures need live phytoplankton on which to feed, but I think that, while this may be true, they would need a constant supply of it. One would assume they wouldn't need live phythodplankton, seeing as we get away with feeding other suspension-feeders with what is essentially dead phytoplankton, but there is so much speculation regarding saltwater aquariums still that it becomes difficult for us to separate fact from fiction. Hopefully, we at Dorry Pets help figure out these unknowns for you, but even we get it wrong from time to time.
This is not to say that you won't have success with tunicates at all. There are hobbyists keeping sea squirts alive in their tanks, but they are few and far between and with varying degrees of success. If you ever buy this delicate creature and get it to survive in your aquarium then let us know about it, so that we can help other hobbyists to succeed. The pictures in this article represent only a few of the +- 2600 species of tunicates found all over the world. Judging by their wide array of colours alone, it comes as no surprise that these creatures are highly sort after in the trade. But, as mentioned before, caution is needed as they are very delicate and even the most advanced aquarists suffer losses.
Above: Lightbulb Sea Squirt