Geoscience Reference
In-Depth Information
A cross-section of a typical barrier island and nearby mainland showing the diversity of habitats.
The surf clam is the largest clam living along the Atlantic coast, reaching a maximum length of 23 centimeters (9
inches).
Another invertebrate in the swash zone is the diminutive mole crab. These peanut-sized crustaceans are
cryptically colored to blend in with their sandy environment, with only their eye stalks and two pairs of anten-
nae poking up like periscopes above the beach. They emerge from their burrows as waves break over them,
then deploy the feathery antennae to capture diatoms and other algae from the backwash of waves. As they are
tumbled back toward the sea, they latch onto the bottom and quickly bury themselves using a powerful pair of
paddles known as uropods, found on either side of the heavy triangular tail, or telson. The uropods scull like
tiny propellers, allowing the tiny crab to seemingly disappear into the sand between waves before beginning its
feeding cycle again. The antennae, which have as many as 150 joints, roll up and deliver the microscopic food
items to the mouth of the crab, which swallows its food whole. It follows the moving tide up and down the
beach but feeds only as the waves recede.
Mole crabs mate in spring and early summer, when the female carries bright orange-yellow egg masses ce-
mented to her legs under the abdomen. Male mole crabs are only half the size of a full-grown female, which
may grow to 2.5 centimeters (1 inch). During the mating cycle, two or three males cling to the female's back
with specialized legs equipped with suction discs. Female mole crabs live little more than a year and the males
less than that, either dying off after mating or, in some cases, undergoing a sex change into female crabs.
Mole crabs themselves are important prey, vulnerable to the probing bills and keen eyes of shorebirds skit-
tering along the surf line, and to juvenile fishes such as Atlantic silverside, spot, and white mullet that are able
to navigate the shallow waters of the swash zone. South of Cape Henlopen, Delaware, another crab emerges
from its small burrows to prey on them as well. The well-named ghost crab, or sand crab, is primarily a noc-
