Maine – Bristle Worm
Continuing with our trip to Maine in 2018, here is a Bristle Worm that we found in the sand on the beach on July 8.
These are Polychaete worms, and are in the same phylum as segmented earthworms and leeches. While they look a bit similar to centipedes, they are in fact not closely related to them at all. In fact, I think they are no more closely related to arthropods like centipetes than they are to mammals.
The time tree of life site agrees with me on this. They say the last common ancestor of polychaete worms and centipedes was 1.6 billion years ago, and that the last common ancestor of polychaetes and mammals was also 1.6 billion years ago. And that last common ancestor had not yet developed segments, appendages, or even a complete digestive system with both a mouth and an anus.
So while these worms have segments and legs kind of like arthropods do, they apparently evolved them independently. Their bristles don’t seem to be jointed very much, and are more like oars, paddles, or stilts. This one was still immersed in water, and is using its bristles to sort of crawl and sort of swim.
Incidentally, I don’t think that third picture is the same species as in the first two pictures. I haven’t found the number of species that live in Bar Harbor, but a bit further south in Chesapeake Bay they are reported to have at least 110 species of bristle worms.
These don’t seem to have much in the way of mouthparts. They mostly feed on plankton and algae, so they most likely pretty much inhale their food. The ones we found, at least, certainly don’t bite in any way that a human would be concerned about.