DENSITIES, DIEL ACTIVITY, BURROW SHAPE, AND HABITAT CHARACTERISTICS OF GECARCINUS (JOHNGARTHIA) PLANATUS STIMPSON, 1860 (, BRACHYURA, ) AT SOCORRO ISLAND, REVILLAGIGEDO, MEXICO

BY

A. PÉREZ-CHI1,2) 1) Departamento de Zoología, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala s/n, C. P. 11340, Mexico, D. F., Mexico

ABSTRACT Gecarcinus planatus is a terrestrial that constitutes an important part of the diet of endemic birds at Socorro Island. Spatial and temporal variations in the crab’s population density, diel activity, and habitat characteristics were investigated between March 1992 and May 1994. Night censuses were done at twelve sampling sites measuring 400 m2 each. G. planatus’ maximum density was − recorded in summer (335 · 400 m 2 at 800 m a.s.l.). There were two diel periods of maximum activity in the coastal environment but only one in the inland zone. Shallow, J- and W-shaped burrows prevailed at the coast, while deeper J- and U-shaped burrows were most abundant in the − − inland habitat. Burrow density ranged from 0.05 m 2 on the rocky substrate, to 4.4 m 2 in the clayey, coastal substrate. G. planatus might be regarded as a generalist, being a good colonizer, and among the most “terrestrial” species of gecarcinid crabs, characterized by remarkable behavioural and ecological plasticity.

RÉSUMÉ Gecarcinus planatus est un crabe terrestre qui constitue une part importante du régime alimentaire d’oiseaux endémiques de l’île de Socorro. Les variations spatio-temporelles de sa densité de population, le rythme de son activité quotidienne et les caractéristiques de son habitat ont été étudiés de mars 1992 à mai 1994. Des recensements nocturnes ont été effectués sur douze sites d’échantillonnage de 400 m2 chacun. La densité maximale de G. planatus a été enregistrée en été (335 crabes sur 400 m2 à 800 m au-dessus de la mer). Deux périodes d’activité quotidienne maximale dans l’environnement côtier ont été observées et une seule dans le milieu terrestre. Des terriers peu profonds, en forme de J et de W prédominaient dans l’environnement côtier tandis que des terriers plus profonds, en forme de J et de U, étaient les plus abondants en milieu terrestre. La densité de − − terriers variait de 0,05 m 2 en substrat rocheux à 4,4 m 2 dans les sols argileux côtiers. G. planatus peut être considéré comme généraliste, bon colonisateur, et entre les plus “terrestriens” des crabes Gecarcinidae, caractérisé par une remarquable plasticité écologique et comportementale.

2) e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2005 Crustaceana 78 (3): 255-272 Also available online: www.brill.nl 256 A. PÉREZ-CHI

INTRODUCTION

Gecarcinus (Johngarthia) planatus Stimpson, 1860, locally known as the “moyo” crab, is the most conspicuous inhabitant of Socorro Island, where its distribution encompasses all terrestrial communities from sea level to an altitude of 1050 meters above sea level (a.s.l.) (Jiménez et al., 1994). This crab constitutes the main type of food for endemic birds, including Socorro’s yellow-crowned night- heron, Nyctanassa violacea gravirostris Van Rossem, 1943 (cf. Rodríguez-Estrella et al., 1996) and is also occasionally consumed by man. The geographical distribution of G. planatus ranges from Bacochibampo, Sonora, Mexico to Gorgona Island, Colombia, and it also inhabits the Revil- lagigedo archipelago and Clipperton Island, off Central America (Rathbun, 1918; Manrique, 1981; Hernández et al., 1986; Hendrickx, 1995). G. planatus inhabits rocky areas, burrows excavated under roots and in soft soils above the highest tide mark, frequently associated with coastal vegetation (Bright & Hogue, 1972). Jiménez et al. (1994) described crab density and number of burrows in relation to the woodland’s structural complexity on Socorro Island. However, detailed characteristics of the burrows, such as size and shape, and their relationship with the type of substrate and vegetation in either coastal or inland environments, have not been described in detail to date. Ehrhardt (1968) assessed the crab’s population size on Clipperton Island, reporting an average of 6 crabs m−2. The Centro de Investigaciones Biológicas de Baja California Sur (CIBNOR, Southern Baja California’s Biological Research Center) investigated crab density in two woodland areas subjected to sheep overgrazing on Socorro island (Jiménez et al., 1994), and recorded an average of 0.12 crabs m−2. However, no information is available about the seasonal and spatial variations in crab density on the island. Furthermore, field observations on the crab’s diurnal activity and its relationship to plant cover (Jiménez et al., 1994) suggested the need to make detailed 24-hour recordings in contrasting habitats. Although there is information available on the biological and ecological features of G. planatus, it is worth stressing that this was obtained only for specific locations, and without analysing temporal variation. The potential existence of spatial and temporal variations in population density in relation to altitude, vegetation, and environmental heterogeneity, on the one hand, and the development of a research project in the marine community off Socorro Island in 1991 (Holguín et al., 1992), on the other, allowed to investigate the moyo crab’s population density at four different dates, as well as its diel activity pattern and habitat characteristics in relation with the island’s two major habitats where the crab is found.