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.

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?

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.

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.

hot nudibranchs

As my students recover from turning in their midterms, the various viruses that seem to be sweeping campus, as well as sundry surgeries, I thought this slideshow might prove therapeutic. But don't get so dazzled by those lurid colors and such that you stop thinking altogether. Remember that one of the special things about gastropods is that they exhibit torsion, a 180-degree twist of the visceral mass (including most internal organs and nervous system) relative to the foot. Among many consequences of torsion is the loss of the left post-torsional gonad and displacement of all the pipes that empty into the mantle cavity over to the right, downstream of the gills, which is a better place to dump your urine, feces, and gametes, after all. While nudibranchs and their sea-sluggy opisthobranch kin are detorted (untwisted) as adults, notice they all retain a penis that emerges from the right side. Yes, they ALL have a penis, being simultaneous hermaphrodites, and it's on the right side for all of them. A remnant of torsion past, both ancestrally and developmentally.

Mudflats

We had a great field trip at Hadley Point this week, where we helped with a student project on clams and collected all manner of worms to look at back in class. More on the worms in a later post. First, we helped Sarah census her baby clams to see if soft-shelled clams, Mya arenaria, exhibit enhanced recruitment in her two treatments (raked and brushed) compared to control plots. Raking roughens the bottom while "brushing," in which small branches of spruce trees are stuck into the mud, provides structure that affects water flow, which may increase the likelihood that clam larvae that are ready to settle out of the plankton encounter the bottom. Using sections of 6 inch PVC pipe to collect core samples, students scooped out the mud and counted all the little clams they found in the sample. I'm hoping Sarah will present her results to the class soon and will also comment here in the blog. Adequate recruitment is only the first step in maintaining a population that's able to be harvested sustainably. Subsequent predation on the baby clams before they reach harvestable size is also an important factor, and some resource managers will seed clam flats with babies from hatcheries or elsewhere, thus ensuring there are plenty of individuals there to start with, and then protect those flats with netting to exclude predators. That's an approach used here on MDI, in Southwest Harbor. Soft-shelled clams are an important component of Maine's fishing industry, although their importance is dwarfed by the commercial lobster fishery. To find out more about Maine's fisheries, including both recreational and commercial, check out the Department of Marine Resources website.

Siboglinidae

The phylum Annelida, the segmented worms, gets more interesting all the time. In addition to earthworms, leeches, and the marine polychaetes, there have been some recent additions. Creatures such as Riftia, the large, red-tentacled worms that live at hydrothermal vents, once considered to belong in a separate phylum, are now included within the annelids. Morphological and molecular evidence unambiguously supports uniting these vestimentiferans, along with the pogonophorans into a single taxon, the Siboglinidae within the annelids. Most analyses interpret the siboglinids as nesting within the Class Polychaeta, the largest annelidan class.
Even more interesting than their evolutionary relationships is the unusual approach to nutrition employed by some members of this group of worms. Although Riftia is a very large worm, significantly taller than I am, it doesn't eat or have a gut; instead it relies on symbiotic bacteria to fix carbon and generate food from the oxidation of inorganic molecules. The details of the biochemical pathways used in this process are being revealed in spite of the fact that these endosymbiotic bacteria cannot be grown in culture in the lab.