Recruitment of Homalaspis plana in intertidal habitats of central Chile and implications for the current use of Management and Marine Protected Areas

Abstract

Information about the life history of the stone crab Homalaspis plana was necessary to investigate the possible reasons for the failure of Marine Protected Area (MPA) and Management and Exploitation Area (MEA) in increasing stone crab abundance, in contrast with other exploited benthic species. In this study, we analyzed some aspects of the early life history of the stone crab, focusing especially on (1) recruitment patterns among habitats and between years in intertidal zones, (2) habi- tat preferences, including some intra- and interspecific interactions that may affect habitat use, and (3) intraspecific interactions that may affect survival. We also evaluated the percent cover of suitable habitats for juvenile stone crabs in a MPA and a MEA. Sheltered habitats showed higher densities of juvenile H. plana than exposed areas, and sand with boulders showed the highest crab densities, fol- lowed by shell hash with boulders. Differences in mean densities were detected for all benthic stages among sheltered substrates, except for megalopae. Bare substrates (without boulders) showed the lowest density. Throughout the settlement period, abundance of smaller instars ( J4 were twice as high in 1996 than in 1995. Neither intra- nor interspecific space competition seems to explain the distribution of juvenile stone crabs in the field. However, cannibalism among juveniles may have an important effect on survival. Canni- balism among juveniles is density-dependent, and may have greater effects as the abundance of larger conspecifics increases since mutual interference does not affect proportional prey mortality per predator. We suggest that losses are high in low quality habitats (probably through emigration, predation or habitat disturbance due to wave impact), and cannibalism is an important source of mor- tality in high quality habitats (where crab density is high). Although substrate type and post-settle- ment processes can help explain the distribution of juvenile stone crabs, wave action seems to be the major determinant of juvenile distribution. Neither sheltered habitats nor the most suitable substrate are common in the MPA and MEA studied. These factors are not currently being considered in the assignation of MEAs or planning of MPAs, although they could explain the lack of effect of both pro- tection strategies in increasing the abundance of the stone crabs in Chile in comparison to other exploited benthic species.