by Amanda Kahn
Sea stars are intense invertebrate predators. Like, eat-everything-in-sight voracious. It might be hard to imagine at first. Some sea stars are soft and look cuddly, like the Pteraster pictured below, which always makes me think of the stars from Super Mario Brothers games.
The Pteraster pictured above harbors a more sinister side. Namely, the oral side, where there are five mouth plates with mouth spines on each (that’s right, pentaradial symmetry means they don’t just have TWO jaws like we’re used to, but 5 chompers!).
But sea stars are slow-moving, aren’t they? They couldn’t possibly be effective predators against other invertebrates that can move…right? Well first of all, they’re not that slow. Those little tube feet can stick and unstick quite quickly, easily overtaking other benthic invertebrates. Crabs are quick, and usually can get away…but a race between a snail and a sea star isn’t really a race at all.
But as seen with the sea cucumber above, animals that normally move slowly have special escape strategies that, while energetically quite expensive, can work to free them from a sea star’s inexorable reach. Snails do a similar trick, torquing their shells around and causing them to tumble away downhill and, hopefully, down away from the sea stars. Scallops clap their two valves (shells) together, allowing them to swim away from the area with the offending predator. So there are strategies for avoiding the occasional sea star encounter, at least based on typical densities of sea stars.
However, ChrisM, founder and author of the Echinoblog, pointed out a phenomenon that has been noted a few times in the waters of British Columbia, in which super dense swarms of the tube-footed predators carpet the seafloor.
Check out his post for the full story, amazing pictures from Neil McDaniel, and some hypotheses for why these swarms might occur. I’ve reposted the info here because I’m curious–have any divers noticed this happening in the Barkley Sound area? Other accounts have been off the coast of Vancouver, within the Strait of Georgia, but I can’t imagine it would be too different in Barkley Sound, and we definitely have lots of divers that go out of Barkley Sound who could have noticed something like this. Leave a message in the comments below if you have, do check out the full story of these swarms on the Echinoblog, and try to forget that image of FIVE sets of mouth spines before you go to sleep tonight!