Crabs are crustaceans. There are more than 44,000 species of crustaceans. They include lobsters, crayfish, shrimp, prawns, and crabs. Most are found in salt water, or brackish water, but some may be found in fresh water or on land. There are 1,146 species of native Hawaiian crustaceans and more than 60 introduced species.

Crabs have hard shells or exoskeletons and 6 legs and 2 claws. The claws may be used for fighting, catching, food or defense. More information is needed to determine what species of crabs live in Hamakua canal. The Samoan crab, Scylla serrta, is commonly found in the canal, but it is yet not known if the coot, stilt and koloa prey upon it. This crab grows up to 9 inches across and can be found throughout the Hawaiian Island in areas with shallow muddy bottoms in calm bays and brackish river mouths.