All arthropods have a hard exoskeleton made of chiton, a type of protein. This shell provides protection for the animals, and gives support for the attachment of the arthropod's muscles. Although arthropods grow, their exoskeletons do not grow with them. So they must periodically shed, or "molt" their exoskeletons in favor of a new one. Arthropods ("arthro" meaning joint, and "pod" meaning leg) also have jointed appendages. Lost limbs can gradually be regenerated after successive moltings.
Amphipods have two types of legs: five pairs attached to the thorax are used for
walking, and three pairs on the abdomen are used for swimming. The most common type
of amphipods are the gammarids. You probably know them better as beach hoppers,
side-swimmers, scuds, or sand fleas. Unlike real fleas, amphipods do not suck blood.
Another type of amphipod, the skeleton shrimp, looks a lot like a praying mantis and
hangs out on seaweeds, bryozoans, and hydroids.
Crabs belong to the subphylum Crustacean, the largest group of marine arthropods,
which also includes lobster, shrimp, and krill, a shrimp-like crustacean. Crabs move
sideways, walking on four pairs of legs, and holding their two legs with claws away
from their body. If you're feeling a bit fearless, you might attempt to pick up a
crab to see if it's a male or female: Female crabs have a wide abdomen to hold eggs,
while males have a thin, pencil shaped flap. Crab eggs hatch and mature in three
stages: from egg to zoea larva to megalops larva to adult.
The most common crab along the New England shore is the green crab. It lives in crevices in the rocky shoreline, hiding in kelp, or under submerged rocks.
The spider crab features a brownish shell that grows up to four inches across, while its legs can span a full foot. Its round shell is covered with short hairs and spines, called tubercles. They camouflage themselves by snagging algae and assorted debris to these hairs. Spider crabs are harmless, lethargic creatures that eat sea worms, dead fish, detritis and algae.
Fiddler crabs are more commonly found along marshy tidal flats. Male fiddler crabs are easily recognized by their extremely large claw which they wave wave in the air and "clack" loudly to attract females during mating. Female fiddlers do not have the distinctive large claw. Fiddler clarbs often travel together. The sight of many fiddler is crabs also a sign of a healthy marsh. They are very sensitive to pollution, which makes them an "indicator species."
Hermit crabs (photo above) are perhaps the most entertaining creatures in a tide pool. They scuttle quickly about on the bottom of pools and tidal flats scavenging for food. When danger approaches, they withdraw into their shells, blocking the entrance with their thick claws. Hermit crabs are also harmless--if you pick one up and hold still for a bit, they will move out of their shell to explore your hand. (Remember to return them back to the ocean quickly!)
These crabs, who look like tiny lobsters, inhabit discarded snail shells. Their soft, twisted abdomen has been converted into a hook that reaches into an empty snail shell. It then carries the protective shell on its back.
When hermit crabs outgrow their current shell, they must find another one. A hermit crab goes through its own little ritual when it does find a better shell. First, it inspects the new shell, touching and grasping it, and turning it until it can inspect the opening with its antennae. If it likes what it finds, it quickly moves in, sometimes changing just as quickly back to their original shell.
Some hermit crabs have anemones attached to their shells. As the hermit crab eats, scraps of food float away which the anemone eats. In turn, the anemone provides protection for the hermit crab. When a crab changes shells, it may even place its anemones on the new crab. This type of relationship--where both members involved benefit--is called symbiotic.
Most arthropods are scavengers, eating just about anything and everything that settles to the ocean floor. Skeleton shrimp feed detritus, algae or animals. Crabs feed on mollusks they crack with their powerful claws. Their biggest predators are gulls.