Wednesday, November 26, 2008

hermit crab home improvement

Among the favorite characters in the touch-tank in the museum at College of the Atlantic are the Acadian hermit crabs. Hermit crabs typically live in empty snail shells, which offer protection for their soft, slightly curved abdomens. There are some exceptions to this pattern: a few hermit crabs have straight abdomens and live in worm tubes; others, like the giant coconut crabs, don't use extra coverings at all during adulthood, relying solely on their chitinous exoskeleton for protection. The large Acadian hermit crabs we see here usually inhabit the old shells of moon snails (Lunatia heros) or ten-ridged whelks (Neptunea decemcostata). An odd thing about these shells is that they are often missing chunks along the margin of the shell opening. Odder still, the hermit crabs hack away these chunks themselves, once they've taken up residence in the shell. Diver Ed has captured this behavior on film, and he has routinely seen them doing it during his tens of thousands of hours underwater. Maybe they're getting the size of their shell JUST right. If they left it any bigger, a larger hermit crab might want it and could easily wrestle it away from the smaller resident. Hermit crabs are certainly known to compete for shells. But if customizing the shell is a way to limit competition from larger hermit crabs, why do we see this phenomenon only among the largest individuals? Do bigger hermit crabs have disproportionately larger and stronger claws, capable of pinching off pieces of shell margin, or do the largest snails have disproportionately thinner shells, making modification possible only for the hermit crabs that inhabit the largest shells? I suspect some size-related pattern, but of course I would suspect that. I'm rather obsessed with size-related patterns; my favorite mathematical expression is the allometric equation.

Wednesday, November 19, 2008

growing clams

Here is some additional information about Sarah D's clam monitoring project, that the invertebrate zoology class helped out with last month. The main part of her project involves measuring recruitment rates for soft-shelled clams (Mya arenaria, also known as "steamers" on local menus) as well as understanding what kinds of factors may enhance recruitment. A definition is probably needed here: when a larval clam, which has spent a couple weeks swimming and feeding in the plankton, encounters a suitable site, it will metamorphose from a larva into a juvenile clam, burrow into the sediment and take up residence, thus recruiting into the local population. Clearly, the larval supply will affect recruitment rate, but larval choice also plays a role, and marked preferences for a variety of factors including substrate texture, flow regime, presence of conspecifics, presence of prey, and absence of competitors or predators, have been demonstrated among a wide range of larvae of marine invertebrates. They may be tiny, but those larvae can exercise some sophisticated decision-making. For one of Sarah's treatments, she added adult clams to her site to measure the effects of adult conspecifics on recruitment rate. These adults were not enhancing the local supply of larvae; any offspring they produced would be widely dispersed during their weeks-long larval life in the plankton. But the adults do provide settling larvae with the information that the site can support clams from settlement to adulthood, and might be a good choice. As long as Sarah and her project supervisor, Chris Petersen, were moving clams (they added hundreds of clams to several large treatment plots), they decided to also measure growth rates of the clams. They marked the outer margin of the shell with permanent marker in the spring, when they added the clams to their site, with the help of students from MDI High School. Then, this month, they measured the marked clams that they found while they were monitoring clam density and recruitment rate. I was surprised that the marks persisted for 6 months, even though the method was suggested by Brian Beal, from U Maine Machias, who definitely knows his way around clam flat research. The marking technique works, and growth rates were quite variable, even among clams in the same plot, with some showing obvious, substantial shell growth, and others not changing size at all since spring. There are clearly plenty of future projects there waiting on the clam flat for some curious and energetic students. Climbing into a pair of waders and splooshing around on the mudflat offers its own unique rewards: fingers numbed from sorting through samples in November, the aroma of anaerobic sediment (think rotten eggs), gooey mud covering everything from tools to clothes to data sheets. What's not to love?

Friday, November 7, 2008

isopods...oh so pretty

Isopods are not generally described as pretty. No worries. Beyond their less-than-obvious beauty, there is plenty to admire about them. This order of crustacean arthropods contains over 4000 described species including the pill bugs; they are the most successful terrestrial crustaceans. However, most isopods are marine, and can be found in habitats from tidepools to the deep sea. They're only cockroach-sized in the tidepools, but the deep sea species are bigger than guinea pigs. Even creepier than the giant deep sea isopods are the isopods that make their living as external parasites of fish. Some will enter the fish's mouth and nibble at the tongue, eventually replacing it altogether. As alarming as this sounds, significant effects on the host fish seem to be minimal. To see more photos of parasitic isopods, and read what Richard Brusca, one of the world's experts on this group has to say about them, go here. If you decide to advertise your newfound excitement about isopods and want a shirt like the one I wore in class today, I ordered it from Questionable Content. It was Miriam at The Oyster's Garter who initially led me there via her post on giant isopods, which is definitely worth checking out; the video is nightmare-inducing.

Thursday, November 6, 2008

more soon

I've been gone too long! I was hit by the double whammy of grading a thick stack of midterms and then faculty retreat, which disrupted the fragile momentum I was developing in posting to this blog. The grading was time-intensive although gratifying (the students did well on the exam and are enthusiastic and articulate about what they're learning), and the faculty retreat was productive. Just when I was thinking about an arthropod post, we had the elections, which left me happily contemplating things other than exoskeletons and appendages for a while. I cheerfully ignored invertebrates for another day, threw caution to the wind, and rode a very pleasant wave of euphoria. Now I'm back.