Tuesday, November 22, 2011

ABC Wednesday: S is for Sea Squirt

Sea squirts are also knows as ascidians (from their scientific name Ascidiacea). They are invertebrates, tubeshaped animals that attach themselves to rocks and feed by straining water through their body. In the photo, you can see the two siphons they use for that.

What makes them especially remarkable is their development from egg to adult animal. The larvae are free-swimming, they have a tail, a primitive eye, a notochord and a nerve cord with a small brain - they look strikingly like vertebrate embryos. With vertebrates, the notochord is eventually replaced by the backbone. Sea squirts have no need for all this once they have chosen a place to settle down and so they lose the tail with the notochord, almost all sensory organs and their brain.

an illustration from Ernst Haeckel's wonderful book "Kunstformen der Natur"

Sea squirts either live alone or in colonies and they almost all are hermaphrodites, they have both male and female sexual organs. Those living in colonies are often able to reproduce asexually as well, growing little buds that turn into new sea squirts.
Despite being invertebrates, they are counted among the Chordata,like mammals, birds, fishes ect. It's very possible that animals just like them were the link between invertebrates and vertebrates. You can see photos of a sea squirt larva and its development here

See what else S stands for with ABC Wednesday.

Sources and further reading:
Hannover University

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