Variability in the Abundance of the Rock CrabGrapsus adscensionis(Decapoda: Grapsidae) in the Canary Islands (Eastern Atlantic)

Abstract

The rock crab Grapsus adscensionis (Osbeck, 1765) is a common intertidal species in the eastern Atlantic. Little is known, however, about the status of its population because of its relatively recent taxonomic separation from Grapsus grapsus. Adult abundance of G. adscensionis was sampled throughout the Canary Islands (24 sites in total) in March and October 2003 and 2004. Abundance varied significantly at intermediate scales (sites spaced by tens of kilometers) but not at larger scales (islands separated by tens to hundreds of kilometers). Habitat complexity (measured as difficulty in walking and terrain variance) was the main source of spatial variability in abundance. In general, G. adscensionis was more abundant in sites of high complexity, mainly in cliffs. Abundance was greater in October (spring/summer) than in March in both sampling years. The frequency of black and red adults varied in time. Red individuals, which were larger than black ones, were very scarce.