Nitotacarcinus lutarius was a small true crab living on the London Clay sea floor. Its compact form suggests an animal that moved among soft sediment and seabed debris, probably feeding opportunistically on small invertebrates and scavenged organic material.

Small crabs were an important part of the Eocene marine food web even though they are less spectacular than large lobsters. They acted both as predators of tiny animals and as prey for fish and larger crustaceans, helping transfer energy through the ecosystem.