Invertebrate animals from the phylum Arthropoda are characterized by jointed legs, a hard external skeleton, and paired antennae on their heads. Arthropods include spiders, insects and crustaceans. Crustaceans are the most common arthropods found on the rocky shore. These animals are covered in a hard exoskeleton that is made out of chitin (ky-tin) which is a protein material, similar to fingernails, that provides a hard, protective covering. This group of invertebrates must shed their exoskeleton periodically in order to grow larger, a process known as molting.

Amphipods and isopods are two kinds of crustaceans found in the rocky intertidal. They are pictured above. Both organisms feed on algae and detritus. An example of an amphipod is on the left. Amphipods can be found under seaweed canopies and under rocks. They forage for food when the tide is high and seek refuge in dark moist environments when the tide retreats. When disturbed they often scurry away giving them the common name scud or sidewimmer. Isopods, pictured to the right, are grazers and feed on the microscopic algae living on rocks and on the shells of other animals.