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

Abstract

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.

Introduction

The American lobster (Homarus americanus) and rock crab (Cancer irroratus) are two decapod species coexisting throughout the Gulf of Maine (Richards et al., 1983, Steneck et al., 2002). Both species occupy similar habitats and have relatively like diets (Richards, 1992, Moody and Steneck, 1993); however, previous studies have shown that lobsters are the dominant competitor (Cobb et al., 1986, Hudon and Lamarche, 1989). As a result of this negative interaction, rock crabs are often driven to use alternative habitats (Wang, 1982, Cobb et al., 1986) and preyed upon by lobsters (Hudon and Lamarche, 1989, Gendron et al., 2001, Sainte-Marie and Chabot, 2002). Coexistence of lobsters and other decapod species, such as the Jonah crab (Cancer borealis), has been investigated by others in the northwest Atlantic (Richards and Cobb, 1986, Richards, 1992). Specifically, Richards and Cobb (1986) found that Jonah crabs also utilize alternative shelters rather than compete directly with lobsters. As such, subordinate species (i.e. crabs) must adopt methods for niche segregation in order to coexist in areas occupied by lobsters.

Rock crabs have managed to use a suite of mechanisms to reduce the potential risk of predation. Palma et al. (1999) found that rock crabs settle indiscriminately (like many other brachyurans (i.e. Palma et al., 2006, Pardo et al., 2007)) among various habitats and suffer high post-settlement mortality compared to lobsters that preferentially settle in rocky, cobble bottoms. As such, early post-settled stages of rock crabs display color polymorphism rendering them inconspicuous to visual predators (Palma and Steneck, 2001). Further, coexistence of both rock crabs and lobsters is possible due to the differential utilization of food resources and substrate type, particularly due to the flexibility of alternative resources utilized by the rock crab (Hudon and Lamarche, 1989). Behavioral modifications of crabs may also minimize predation by and competition with lobsters, such as daytime activity of Jonah crabs and large rock crabs in contrast with peak nocturnal activity periods of lobsters (Novak, 2004). An alternative, and possibly overlooked mechanism, is the use of unconventional substrate types such as kelp fronds by rock crabs.

Shallow subtidal areas in the Gulf of Maine consist of rock, pebble, and sand habitats with areas of high kelp abundance. Two genera, Laminaria spp. and Agarum spp., are commonly found along subtidal areas of Maine (Steneck et al., 2002) and offer an additional form of structural complexity to the ecosystem. Because both lobsters and rock crabs have been shown to prefer areas with structural complexity (Palma et al., 1998, Palma et al., 1999), the kelp canopy may represent a mechanism by which both species can co-occupy a given unit of area. Hudon and Lamarche (1989) found high densities of both lobsters and rock crabs over boulders colonized by macroalgae and suggested that this complex habitat may result in a high carrying capacity. Further, intertidal and shallow subtidal areas of New England consisting of rocky ledges covered in algae of Chondrus crispus and Mastocarpus stellatus harbored large numbers of both decapods (Jones and Shulman, 2008). Similar findings were observed in soft-bottom habitats where lobsters occupied crevices or rocks while crabs burrowed in the sediment (Cobb, 1971, Aiken, 1980, Hudon, 1987). These alternative habitats (i.e. macroalgae, sediment burrows) may function as microhabitats and play an important role to the survival of subordinate species, such as the rock crab.

The objective of this study was to investigate three-dimensional resource partitioning of lobsters and rock crabs in subtidal kelp beds in the Gulf of Maine by considering the hypothesis that kelp serves as an alternative habitat for the smaller and subordinate rock crab in the presence of the competitively dominant American lobster. We tested this hypothesis through field surveys of rock crabs on or under kelp fronds when lobsters were present and absent. In addition, we performed laboratory-based experiments with and without lobsters using both live and artificial kelp to evaluate whether field based observations could be confirmed in the laboratory. Ultimately, our goal was to examine whether kelp may serve as an alternative habitat that rock crabs may utilize in the presence of lobsters, thereby providing a likely mechanism facilitating the coexistence of these abundant decapod species.