Apr 22 2011 -- Jun 22 2011 Crab molts and dead crabs on the beach
Photo of Molted Crab by Bonnie Henderson Dead crab, photo by Cynthia Trowbridge
by Bonnie Henderson
Finding lots of crab shells on the beach? The crabs may have died, or they may have simply molted. The species whose shell is most commonly found on Oregon beaches is the Dungeness crab (Metacarcinus magister), followed by the red rock crab (Cancer productus, smaller than the Dungeness, similar in color but redder, often striped, with black claw tips). Crabs molt when they outgrow their shell; adults at least once a year; juveniles several times. Typically that happens when there’s plenty to eat, such as in spring and summer. Females tend to molt in the spring, right before mating, and males in the fall, though it varies.
A molting crab backs out of its shell, leaving the shell almost entirely intact, right down to the gills; on protected beaches you could find whole molted shells, though often they’re broken up by the time the waves deposit them on the beach. Here are some clues to help determine whether the shells you’re seeing indicate dead crab or crab molt:
1. The eyes. Hold the shell so you’re looking at the crab’s “face”: in the center, above the mouth parts, is the rostrum (the crab’s version of a snout), on either side of it are little antennules, and on either side of the antennules are bulbous eyes, set at the end of eyestalks. Molting crabs leave behind transparent sheathes over the eyes; dead crabs’ eyes are black or brown.
2. The shell. A molting crab’s carapace breaks open at the rear along what’s known as the suture line, separating the top and bottom of the carapace, allowing the crab to back out of its old shell. A dead crab’s carapace wouldn’t have split, unless by wave action.
3. The mouth. The mandibles, or mouth parts, often break off in the molting process.
Dead crabs are most common during episodes of oxygen depletion offshore (dead zones), usually in August and early September. Unusually low summer wave action can starve crabs and other crustaceans of oxygen as well.
Contact: Phillip Johnson, Executive Director, (503) 238-4450, or EMAIL