It feeds on bivalves and barnacles, and is in turn eaten by predators including the Atlantic blue crab, Callinectes sapidus.

The closest relative of D. sayi is D. texanus, which lives in the Gulf of Mexico; the two species differ in subtle features of the genitalia and the last pair of walking legs.

[4] The natural range of D. sayi extends from the Baie des Chaleurs (eastern Canada) to the Florida Keys (south-eastern United States),[5] where it lives from the intertidal zone down to depths of 46 metres (151 ft).

The first sighting was in Swansea Docks, South Wales (United Kingdom) in 1960, and the scientist who reported it, E. Naylor, believed there was "no doubt" that the species had arrived through trans-Atlantic shipping.

[7] The first record from the Mediterranean Sea was made in 1993, when the species was discovered in the Venetian Lagoon (north-eastern Italy),[5] although it is thought to have been living there since the late 1970s.

[6] In its native environment, it hides among colonies of polychaetes to avoid being preyed on by the Atlantic blue crab, Callinectes sapidus.

[2] In the Adriatic Sea, it has been observed to feed on the striped venus clam, Chamelea gallina, and the introduced Asian date mussel, Musculista senhousia.

[14] In 1986, Joel W. Martin and Abele placed N. texanus and N. sayi in a separate genus, Dyspanopeus, reaffirming their close relationship.