What Makes the Small Holes in the Rock?

Small Holes in the Rock

These small holes in the reef's rocky substrate are the work of boring clams

In some areas, the soft sedimentary rock that forms the intertidal reef at Fitzgerald Marine Reserve is riddled with holes. One of the animals responsible for burrowing these holes is the rock boring clam, Penitella penita, which live out in the mid-tide and low-tide zones.

This elongated bivalve starts off life as free-floating larvae, and eventually settles onto a soft rock and takes hold with its muscular foot. Its shells have file-like ridges on the outside, and the clam drills its way into the rock by contracting its muscles to spin the shell first one direction, then the other. The clam continues to enlarge its cone-shaped burrow as it grows to its full length of about two-and-a-half inches. Often the clam’s shells are completely hidden in its burrow. During high tide it sticks its fleshy siphon out the open end of the hole, which may be just half an inch wide, to obtain food and discharge waste.

When boring clams die, their burrows become shelter for encrusting animals such as sponges and bryazoans, marine worms, isopods, small snails, tiny sea stars, and crabs. Some of these creatures may make a permanent home in the abandoned hole, while others may simply shelter briefly from heavy surf or the drying rays of the sun or a carnivore on the hunt.

Examine any rocks you find washed up on the beach for evidence of boring clams. Often a rock becomes so riddled with clam bores that it loses strength and breaks apart, revealing the long, conical profiles of the holes.

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