If you turn over a mass of cordgrass stems washed up on the beach, bunches of flea-sized bugs will start hopping madly about. Poke around a rock-filled bulkhead at the shore, and you might see what appear to be giant, dark gray roaches scurrying away. These are examples of two types of coastal crustaceans, each with seven pairs of legs. Marsh scuds and beach fleas are amphipods; their bodies are flattened from side to side. Sea roaches are isopods, with bodies flattened from top to bottom.
Photo of a freshwater amphipod, Gammarus roeselii, common in Europe, a cousin to the southeastern US marsh scud. "Gammarus roeselii" by Michal Maňas (User:snek01) - Own work. Licensed under CC BY 2.5 via Wikimedia Commons -
Amphipods are generally more obvious than isopods in coastal habitats, because of their overwhelming abundance. Marsh scuds, Gammarus palustris and Gammarus mucronatus, usually no more than half an inch long and translucent green to sandy brown in color, live in great numbers in masses of dead plant stems and leaves in the salt marsh.
A mass of beach amphipods common in Europe, similar to the species found on Georgia beaches. "Orchestia gammarellus IMG 1470" by Auguste Le Roux - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons -
The beach fleas, Orchestia grillus and Americorchestia longicornis, hang out in wrack deposits on the upper beach. Orchestia grillus can also be found underneath dead plant litter in the salt marsh. Beach fleas are most active at night, when the sand is cool and they are less likely to be spotted by birds looking for a meal. Many amphipods hop about on their tails and rear legs. The especially athletic beach fleas can leap many times their body length, just as insect fleas do.
Watch the European amphipods scurry around in beach wrack in this video:
A northern sea roach. "Ligia oceanica Flickr" by me'nthedogs - http://www.flickr.com/photos/66176388@N00/2904455895/. Licensed under CC BY 2.0 via Wikimedia Commons -
Sea roaches are the best-known marine isopods; they infest docks, seawalls, and rocky shores all over the world. The southeastern species is Ligia exotica, the exotic sea roach. North of Cape Cod, a different species, Ligia oceanica, occurs. These large isopods have an uncanny resemblance to cockroaches in size, color, and behavior. Sea roaches are just as quick and secretive as their terrestrial namesakes, zipping into tight crannies at the least disturbance. Sea roaches can be spotted wherever rock seawalls have been placed to protect shorelines - for example, along the beach by the fishing pier at the southern tip of St. Simons Island or along stretches of beach on Jekyll Island where erosion has been a problem.