This amphipod lives among and consumes marsh detritus.
This amphipod lives among and consumes marsh detritus.
Courtesy: NOAA

Amphipods and Isopods
Amphipods (Gammarus spp., Talorchestia spp., Jassa spp.)
Isopods (Sphaeroma spp., Idotea spp., Erichsonella spp., Limnoria spp.)

Alternate common names: Beach Fleas, Sea Pill Bugs, Gribbles.
Color: Amphipods range from grey to sand color to brownish green, and can be mottled with red or orange. Isopods are a dull grey to yellow brown.
Size: Amphipods are 1/4 inch to 1 inch long; flattened laterally; and swim on their sides. Isopods are 1/4 inch to 2 inches long; flattened ventrally.
Habitat: Sandy beaches, seaweed, tide pools, tidal marshes above the high-tide line, eelgrass beds and meadows.
Seasonal appearance: All year.


Most of the small bug-like creatures seen in tide-pools, under rocks, burrowing in the sand, and scuttling around seaweed on the beach belong to two groups of crustaceans called amphipods and isopods. Both amphipods and isopods are Arthropods, which includes animals with jointed appendages and hard exoskeletons, such as crabs, lobsters, insects, and spiders. The exoskeleton consists of armor-like, overlapping plates.

There are many different species of amphipods found in Rhode Island waters. The name "amphipod" means double, or two kinds of legs. Of their eight pairs of legs, the first five are used for walking and the last three pairs, in the tail region, are modified for swimming. Some amphipods also have modified tail appendages used for jumping. Most have several pairs of antennae, and some have an appendage used for grasping. Amphipods generally swim on their sides. The bodies of amphipods are flattened side-ways, and they have highly arched backs.

In contrast, isopods are flattened ventrally, are larger in size, and are bottom dwellers. They resemble land dwelling insects and are commonly found crawling and swimming among weeds, eelgrass, tide pools, dock pilings, and rocks.

Life History and Behavior

Both amphipods and isopods eat dead and decaying algae and seaweed and other plants and animals. Bacteria and smaller organisms that consume the waste of amphipods and isopods continue the decomposition process. Both amphipods and isopods include parasitic species that live on other aquatic animals. Parasitic amphipods can be found on jellyfish, while parasitic isopods can live in the gills and fins of fish.

Special Notes

Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.