The population structure of two sympatric hermit-crab species on a subtidal rocky shore of an island in southeastern Brazil.

Daniel J.M Lima,Mariana A.B De Aquino,Adilson Fransozo,Valter J Cobo

doi:10.1590/0001-3765201420130472

Abstract

The main goal of this investigation was to characterize the population structure and shell occupancy of two sympatric hermit-crab species, Pagurus brevidactylus and Paguristes tortugae. The study was undertaken at Couves Island on the southeastern coast of Brazil, from March 2010 through February 2011, on subtidal rocky bottoms. Specimens were collected by SCUBA diving sessions. A total of 195 individuals of P. brevidactylus and 132 of P. tortugae were examined. Both populations showed unimodal size-frequency distributions, which were non-normal for P. brevidactylus and normal for P. tortugae. The median size of P. brevidactylus was significantly smaller than P. tortugae; in both species, males were significantly larger than females. For both, juveniles and ovigerous females were recorded in all size classes and in almost the entire sampling period. No significant departures from the 1:1 sex ratio were detected, although some size classes were skewed. Overlaps in shell occupation were recorded. Pagurus brevidactylus and P. tortugae showed similar population features; they reached sexual maturity at small sizes, and the nearly year-round presence of young and ovigerous females suggests continuous reproduction. These inter-specific interactions involving resource partitioning suggest a regulatory process that is probably part of the equilibrium strategy of these populations.

Highlights

The hermit crabs Pagurus brevidactylus and Paguristes tortugae are widely distributed along the southwestern Atlantic coast, where they are dominant and sympatric on rocky bottoms (Mantelatto and Garcia 2002)
The crabs were distributed in eight size classes with 0.35 mm cephalothoracic shield length (CSL) intervals, ranging from 0.9-3.7 mm CSL for P. brevidactylus; and into nine size classes with 0.70 mm CSL intervals, ranging from 1.17.4 mm CSL for P. tortugae
For P. brevidactylus, juveniles were recorded in the size classes of 0.90—]1.25 and 1.25—]1.60 mm CSL

Summary

Introduction

The hermit crabs Pagurus brevidactylus and Paguristes tortugae are widely distributed along the southwestern Atlantic coast, where they are dominant and sympatric on rocky bottoms (Mantelatto and Garcia 2002). Mantelatto and Sousa (2000) and Mantelatto et al (2005) noted that the pagurid P. brevidactylus and the diogenid P. tortugae are small species compared to other hermit crabs in the same area (e.g., Dardanus insignis, Paguristes erythrops, Petrochirus diogenes). Both species are common in the western Atlantic, distributed from North Carolina and Florida, USA, for P. tortugae and P. brevidactylus respectively, to the state of Santa Catarina, Brazil. DE AQUINO and ADILSON FRANSOZO and sandy bottoms and in algae beds, from shallow water to approximately 50 m deep (Melo 1999, Nucci and Melo 2007)

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Conclusion