Saltwater marine crabs like the white speckled hermit crab (Paguristes puncticeps) and emerald crab (Mithraculus sculptus) are opportunistic feeders and scavengers. Yet, while they are very fond of sifting through the sandy substrate, sucking the nutrients out of them as a source of calcium, similar to us human beings, they need a well-balanced diet of protein and vegetables, in between.