Three-dimensional resource partitioning between American lobster (Homarus americanus) and rock crab (Cancer irroratus) in a subtidal kelp forest

The American lobster (Homarus americanus) and the rock crab (Cancer irroratus) commonly coexist, and at high densities, compete for space in subtidal kelp forests in the Gulf of Maine. Competition between the two species was examined in field surveys using visual SCUBA quadrats and complementary laboratory experiments with both live and artificial kelp. Field surveys showed significantly higher crab densities (0.50/m2 ± 0.08 SE) found up the kelp versus on the ground (0.22/m2 ± 0.04 SE) when lobsters were present (P = 0.002). In contrast, crab densities were significantly higher on the ground (0.44/m2 ± 0.07 SE) as opposed to up the kelp (0.23/m2 ± 0.06 SE) when lobsters were absent (P = 0.023). However, the interaction between survey location ⁎ date was significant in five of the eight surveys, suggesting this pattern may not be ubiquitous in all subtidal kelp locations within our study region. Average size of crabs up the kelp or on the ground was not different when lobsters were present or absent (P > 0.05), and no size correlation was found between lobsters and crabs, irrespective of crab position. Experimental trials in the laboratory confirmed field surveys regardless of the order crabs and lobsters were added into the tank, or the use of live versus artificial kelp, and significant differences were observed in five of the six 5-day experimental trials. Our results suggest that when shelter availability may be limited, kelp could serve as an alternative habitat for rock crabs, perhaps to minimize competition between the two most abundant decapods in the shallow subtidal zone of New England.