Blue crab zoea are filter feeders that consume both phytoplankton and zooplankton. Megalopae have well developed chelae which are used to capture food in a manner similar to that of adults. Megalopae feed on other planktonic organisms while inhabiting the water column but become opportunistic benthic omnivores after settling to the bottom. The juvenile and adult blue crab has been described as a opportunistic benthic detritivore, omnivore, primary carnivore, cannibal, and general scavenger. Food habit studies have shown that predominant food items vary greatly, depending upon availability. However, in general major food items of larger juveniles and adults include bivalve molluscs (clams, oysters, mussels, etc.), fish, other crustaceans, organic debris, and vegetation, and benthic infauna.
The blue crab is well known for its cannibalistic habits. Blue crabs make up as much as 13% of the diet. Blue crabs in poor health, missing important appendages, heavily fouled with other organisms, and those within or immediately following ecdysis are more likely to be cannibalized. Papershell crabs regularly consume their own molted exoskeletons.
Blue crabs feed in three different ways. Raptorial feeding involves feeding on large prey organisms; interface feeding involves feeding from the surface of objects and on sediment surfaces; and plankton feeding involves consuming small suspended material.
Blue crabs are a crucial factor in the estuarine food web because of their opportunistic feeding habits, high abundance levels, and their great tolerance of salinity. Blue crabs also dig into sediments to both forage and hide, thus killing more organisms than they consume. Given its high “feeding adaptiveness”, blue crabs play an important role in determining the abundance and behavior of its prey. Blue crabs may have detrimental effects on commercial invertebrate species such as oysters, hard clams, and soft clams and have been documented to influence the distribution and abundance of other invertebrate species.
Predation intensity on blue crabs varies with characteristics of the predator (species, size,life history stage, physical characteristics, and feeding habits). Larval blue crabs are fed upon by other plankters, small fish, filter feeding fish, jellyfish, and comb jellies. At least 60 species of fish have been identified as blue crab predators. Blue crabs also serve as prey for other vertebrate species.