Niche segregation between American lobster Homarus americanus and rock crab Cancer irroratus

Niche segregation of American lobster Homarus amerjcanus and rock crab Cancer irroratus was investigated in the southern Gulf of St Lawrence using density, biomass, size structure and diet of these 2 species captured in 4 habitats.Rocky bottoms colonized with macroalgae supported the highest density and biomass of both species, but they also coexisted in lesser numbers on bare rocky bottoms.Low densities of crabs of all sizes and of large lobsters were found on eelgrass beds, and crabs only on bare sand.The proportions of size groups of lobsters and crabs differed significantly among substrate types.Small lobsters and large crabs predominated on bare rocky bottoms, while large lobsters and small crabs were mostly found between boulders with macroalgae.Examination of stomach contents of lobsters and crabs coexisting on rocky bottoms with and without algae showed the lobsters' diet to remain similar at both sites, while that of crabs differed between sites.Diets of small lobsters and small crabs were more slmilar than those of larger ~ndividuals, possibly because small individuals of both species remain hidden In sediment and crevices between stones.Lobsters fed principally on horse mussels, rock crabs, lobsters, gastropods and ectoprocts, while rock crabs fed on horse mussels, ectoprocts and polychaetes.Large crabs occasionally preyed upon small lobsters.At both rocky sites and for all size groups, crab diet always exhibited a lower average calorific value than that of lobster.In the bare stone habitat, supporting low densities of large lobsters, the average calorific value for large rock crab diet was consistently higher than in the large rocks with algae habitat, supporting a hlgh density of large lobsters.Whlle lobster diet remained similar between sltes, rock crab diet was poorest on grounds where large lobsters occurred, possibly reflecting increased risk due to predation while foraging for food.The coexistence of American lobster and rock crab is made possible by their different use of the substrate and food resources with increasing size, and by the flexibility of the rock crab.in low densities on bare sand (Cobb 1971, Scarratt & Lowe 1972, Shotton 1973, Winget et al. 1974).Habitat use also appears to change ontogenically: contrarily to the adults, which are commonly found in the open, small lobsters ( c 2 5 mm carapace length, CL) and small crabs (<25 mm carapace width, CW) tend to remain hidden in gravel, sand, and softer sediments between boulders (Scarratt & Lowe 1972, Pottle & Elner 1982, Hudon 1987).However, regardless of size, crabs seldom appear to excavate or maintain well-defined burrows as lobsters do (Atema & Cobb 1980).Lobsters and crabs exhibit different behaviours and substrate use when confronted with a predator.When they are caught outside their burrow, small (<25 mm CL) lobsters often attempt to escape predators by swimming away using rapid abdomen flexing, whereas