Intertidal habitat complexity influences the density of the non-native crabHemigrapsus sanguineus

Abstract

Habitat structural complexity can provide protection from predators, potentially affecting population density of native and non-native prey. The invasive Asian shore crab, Hemigrapsus sanguineus, occurs in variable densities in the rocky intertidal zone of eastern North America and northern Europe, often in densities greater than in its native range. The present study examined the influence of habitat complexity on the density of H. sanguineus. Artificial shelters of concrete pavers with stones arranged in increasing complexity were deployed in the intertidal zone along a rocky shore in southeastern Massachusetts, USA, for 21 consecutive weekly intervals in 2020. Crabs consistently reached the highest densities in the most complex shelters despite their lower internal surface area. In addition, crabs exhibited shelter selectivity based on body size, with large crabs occupying artificial shelters in greater numbers than adjacent natural substrate. In a subsequent lab study, crab activity over 1 h was observed in the presence of the same artificial shelters, under simulated tidal conditions. Shelter complexity had little influence on the number of crabs under the pavers although crabs were more active when submerged in water than exposed to air. These results show that crab density increases as habitat complexity increases, and complexity may serve as a predictor of H. sanguineus density but not short-term behavior.

Temporal and spatial refugia modify predation risk for non-native crabs in rocky intertidal habitats

“…Field tethering experiments were conducted in the lower intertidal zone of Clark’s Cove, New Bedford, MA (41°35′40.33″N, 70°54′37.45″W) June 28–August 26, 2020 ( Table 1 ) at +0.29 m above mean low water (average tidal range for the location is 1.1 m). Previous investigations at the study site showed crabs were most abundant at that tidal elevation ( Towne, Judge & O’Connor, 2023 ).…”

“…Both male and female H. sanguineus are highly mobile and can travel up to 16 m per day ( Brousseau et al, 2002 ). Thought to be most active at high tide, laboratory experiments conducted during daytime found crabs were more likely to move under and out from under shelter when submerged in water ( Towne, Judge & O’Connor, 2023 ). Activity at high tide may expose crabs to visual predators, like T. onitis and T. adspersus , which use a scan-and-pick foraging strategy to feed in the intertidal zone ( Dew, 1976 ; Deacutis, 1982 ) where the crabs occur ( Brousseau et al, 2002 ; Epifanio, 2013 ; Towne, Judge & O’Connor, 2023 ).…”