Isopods are small crustaceans that have flat bodies and 8 legs all about the same size and shape. They eat detritus and marine plants.
The genus Spheroma has white blotches and grow up to 3/8" (9mm). They are found in the intertidal zone among seaweed and under stones. They roll up into a ball when disturbed.
The genus Idotea are often green or brown but their color and pattern vary greatly. They may have stripes or bands and can have blotches of color. The species Idotea balthica and Idotea phosphorea live in New England and are found in eelgrass, seaweed or rocks in the lower intertidal zone or shallow water. They can grow up to 1.5" (38mm) long.
The genus Erichsonell is commonly found on eelgrass and algae. They grow up to 5/8" (15mm) long and may be green or brown.
The species Limnoria lignorum is a wood boring isopod also called a gribble. They are found in pilings and on driftwood. They can do considerable damage to untreated wood structures in the water. Gribbles are gray and grow up to 3/16" (5mm) long.