Boring Clams
Life is a grind
It seems odd that the words “boring/bore” have such diverse meanings (but then this is English). This essay is about boring clams, that is, about clams that bore, as in making holes in things, not as in being uninteresting or soporific. Clams belong to the molluskan Order Bivalvia (two shells) or Pelecypoda (hatchet foot).
A clam is an animal that lives between two hinged shells that are made of calcium carbonate secreted by the clam’s integumental mantle. One line of clam evolution burrows in soft substrate by means of a muscular foot. The hatchet shape of the foot allows the clam to push it into the soft substrate where it balloons laterally to anchor itself, and then to contract to pull the clam below the substrate surface. Once buried, the clam extends siphons from the mantle cavity to the surface and circulates water over its dual-function gills, filtering out small food particles while also exchanging gases.
Over evolutionary time, some clams gained the ability to burrow in ever harder substrates, with the results that there are now clams that can burrow in rocks. Admittedly, the rock is not as hard as granite or gneiss, but relatively soft sedimentary rock. The clam’s mantle secretes shells studded with ridges that, when moved back and forth, or up and down, grind away at the rock on which the baby clam has settled. This forms a hole that becomes the clam’s residence. Different species of boring clams have different ways of anchoring their bodies inside the hole so that they can move the grinding shells--- some have evolved a suction-cup-like foot to hold on to the wall, while others merely press their bodies against the opposite wall of their burrows. As the clam grows, it grinds deeper into the rock making a cavity that is ever wider at the bottom but remains narrow at the top. There is no escaping from this rock prison.
But occasionally, the sea erodes and breaks the rock that is home to these clams, fragmenting it further into clam-perforated pieces of a range of sizes. I picked up several such pieces on the beach in La Jolla, California and added them to my Found Objects collection. On this sandy beach with only occasional rounded pebbles, these holey rocks appear strange, the last remnants of what was once home to scores of happy boring clams, but now more closely resembling a sculpture by Henry Moore.
My other Found Object drifted ashore in St. Joe Bay in north Florida. It is not the kind of object beach combers are likely to pick up, probably because it looks like mere debris. But interest is in the eye of the beholder, or so I am told, and this debris had a story to tell me, the story of a second line of boring clams, the shipworms, that bore in floating wood and pilings. They make borrows that may be many centimeters long, and their body is so elongated and modified that it resembles worms more than clams (hence the name).
As in all bivalves, the body is enclosed within the mantle, that magical tissue that secretes the limestone (calcium carbonate) shell in other clams. The shell is reduced to two small, ridged shells at the animal’s anterior end, and the foot is reduced to a small structure that anchors the animal in its burrow while boring. The shell is reduced to a rasping organ that grinds wood into sawdust at the head end of the burrow, extending it. The shipworm ingests this sawdust as food, and with the help of microorganisms, digests it as its staple nourishment. Because wood is so low in nitrogen, some shipworm species have evolved a mutualism with nitrogen-fixing bacteria. What a remarkable transition--- from filter feeding on suspended “stuff” in seawater to eating wood!
In the shipworm, aside from creating the small rasping shells, the mantle deposits limestone as a calcareous lining of the burrow (the white lining below). The individual animals seem to be able to sense their neighbors and mostly avoid breaking into their burrows. Like other clams, the shipworm extends its siphons from the opening, circulating water over the very long, narrow gills in the mantle cavity mostly in order to exchange gases rather than filter feed.
What I like about creatures like shipworms is that they show the amazing changes that can evolve from a basic body plan like the quahog clam: reduce the shell to a small rasp at the head end; elongate the body extremely; reduce the iconic clam foot to a stubby hold-fast; give up filter feeding; grind, eat and digest sawdust; convert the mantle to mainly keeping the walls of the burrow smooth and neat; rely on the wood for skeletal support. This new creature can now exploit an abundant and specialized resource (floating wood) and make wooden ship owners tear their hair out.
This is a wonderful piece. So many intimacies to marvel at in the bivalve world.