If you turn over a large rock near the low tide line, you might find a fish instead of crabs.
Unlike other intertidal fish that use tidepools or hide under kelp when the tide goes out, the plainfin midshipman takes refuge in the mud, under rocks. A common species, it seldom is seen because of its habit of feeding at night while burrowing in mud during the day. In addition, the males guard eggs under intertidal rocks in spring and early summer.
The plainfin midshipman is a member of the toadfish family, a mostly tropical group that occurs in shallow water and produces croaking sounds with its modified swim bladder (usually by males to attract a mate). It also has venomous spines on its gill covers.
If you find one, take a look at the underside. There you will find a series of lines made up of white dots called photophores. These dots reminded an old marine biologist of the rows of buttons on a British Navy midshipman, hence the name. The purpose of the photophores is debated, but they probably are used in mating displays, and possibly to startle predators.
This individual is about 10 inches long and can grow to 15 inches. Ranging from southeast Alaska to the Gulf of California, it can be found from the intertidal zone to depths of 1,200 feet, where it feeds on fish and crustaceans. It is reported to be important prey for seals and sea lions.
David W. Jamison is a marine biologist and Boston Harbor resident.