Walking Gustavus beaches

predator leavings, big snails, and boring clams

A recent walk with two friends on some of the great sandy beaches of Gustavus provided several observations of interest. The four-footed friend probably had the advantage of us mere humans, because she could sniff many messages that were beyond our ken. Nevertheless, the curious-naturalist humans found much to see and discuss.

A line of wolf tracks followed the upper edge of the sand, steadily headed…somewhere. One huge wolf scat held remains of a murre, probably scavenged from a carcass, and another was made up mostly of clay, with a few feathers. Do wolves self-medicate with clay, as some birds do (to counter toxins in their food)?

There was evidence that predatory birds had feasted on murres, mallards, and a loon. Owls and eagles undoubtedly accounted for some of these avian remains. But also, perched on a log within distant binocular range was a slim, gray bird that we thought might be a peregrine falcon. Some owl pellets held the bones of voles, including skulls with teeth, which made identification of the prey relatively easy. A set of vole molars looks, on the grinding surface, like a row of tightly packed triangles; this is quite different from the cusped molars of deer mice, for example. Perhaps I needn’t have bothered to look closely, though: I was interested to learn, from a well-known naturalist in Gustavus, that deer mice are scarce over there, for reasons undetermined.

Scattered along the sand were several strongly ridged, giant snail shells, the biggest whelks I’ve ever seen. These specimens were four or five inches long, but they are said to reach a length of seven inches or so. They belong to the genus Neptunea, but the species name is still undetermined, thanks to some taxonomic confusions. They hang out in the sediments but emerge to travel, feed, and lay their eggs. Neptuneas make their living by drilling (with their file-like radulas) into the shells of other molluscs and slurping out the contents, eating polychaete worms, and by scavenging dead and dying critters. Females produce masses of egg capsules that are spread over rocks and in rocky crevices. Each capsule contains about two thousand eggs, but many of these are not fated to become juvenile snails, because they are eaten by their siblings. After developing inside the protective capsule for many months, well-fed young snails emerge.

Clam shells were everywhere, mostly horse clams. But on one beach we found deeply arched clam shells, each with a pronounced internal projection, for muscle attachment, near the hinge. This beast was entirely new to me, so my learning curve took a jump. These clams are called piddocks (Zirfaea pilsbryi). Piddocks and some other bivalve molluscs burrow into the substrate using their shell as augers; piddocks make their tunnels in clay, sand, or even rock (!). The sharp, jagged teeth on the front part of the shell slowly rasps away, back and forth, as the piddock rotates, eventually making a full circle, only to start over on the next round. Their tunnels can be over a foot long, so their siphons (or the so-called neck: the paired tubes, one of which is used for breathing and drawing in food particles, and the other for excreting wastes) are substantial. If the piddock is eating well and grows as it slowly burrows, the first part of the tunnel becomes too small for the clam to back out, and it can only go forward.

piddock-kmh
Piddock (Zirfaea pilsbryi). Photo by K. Hocker

I am not a marine biologist of any sort, but I love finding out more about this unfamiliar world.

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