Ascidians, or sea squirts, are well-known to most of us as the cunjevoi that squirts at you as you try to exit a dive over rock platforms. I recall being fascinated by these animals as a kid (but of course I never trod on them to see how high the squirt went… hmmm!). Ascidians are also easy to spot on dives as the big sea tulips that waft in the currents; each one is a house on the end of a stalk, with a single animal inside. The animal spends it’s life drawing sea water in, filtering it, and expelling the cleaned water. Interestingly, the purple, yellow and blue colours of these sea tulips come from the encrusting sponges that live on them rather than the ascidians themselves.
Many ascidians, however, are colonial rather than individual. This means that they live in a colony of hundreds or thousands of animals, sometimes called “zooids”, which share a common home and sometimes even share nutrients between animals. Some colonial ascidians share an exhalant syphon too; each animals draws its own water in but the filtered water is pooled and expelled by a larger common syphon.
Ascidians all live their lives attached to the substrate, or “sessile”, yet they are not invertebrates like mussels or oysters. In fact, they have a rudimentary backbone so they are classified alongside fishes in the chordate phylum. Imagine that – a fish that never goes swimming! Not strictly true though, as they do have a larval stage which is free-swimming and looks a bit like a tadpole. But once the larvae settle, they attach and spend the rest of their lives in the one spot, happy to live off the tiny particles that they can glean from the passing seawater.