Aspectos ecológicos do caranguejo-fantasma ocypode quadrata fabricius, 1787 (brachyura-ocypodidae) em praias arenosas do litoral cearense

Abstract

Based on the ecological importance of the O. quadrata crab for Brazilian sandy beaches and its use as an indicator of human impacts, this study aimed to evaluate the spatial distribution of this specie along beaches on the Ceará coast, to investigate aspects of their population biology and to check a possible seasonal age structure and density patterns on two beaches with different intensities of human disturbance. The burrows density and diameter were studied in 17 sandy beaches in Ceará between July 2012 and March 2013. The Icaraí beach had the highest density of burrows of O. quadrata, which may be related to a higher concentration of individuals in beach range, due to habitat reduction. The most densely occupied area was the debris zone, probably because the greater availability of food. Due to the reduced ability to dig and the greater susceptibility to desiccation of juveniles, smaller burrow diameters were found on Lagoinha beach and Flecheiras beach, as well as in the areas closest to the waterline. The ghost crab population biology study at the Prainha do Canto Verde beach showed a higher proportion of males (F 1: 2.48 M), while the proportion between the right and left side of the larger chelate was 1: 1.02 ( L: R). The average size of the males was greater than ovigerous and non-ovigerous females, although differences have not been detected. A unimodal size frequency distribution was observed, and the morphometric data showed a positive allometry between the carapace width and every dimension analyzed. The results of the O. quadrata’s burrows density temporal evaluation showed higher values at the Prainha do Canto Verde beach (0.0295 burrows / m²) than at the Futuro beach (0.015 burrows / m² ), a fact that persisted in almost every month studied. The intertidal and debris areas has a higher densities in both beaches, while Prainha do Canto verde beach had higher density comparing each zone of both beaches. In several studied months, the burrows diameter was higher at Praia do Futuro. The average holes diameter was lower in the intertidal zone and higher on the dune area at Prainha do Canto Verde beach and Futuro’s beach supralittoral. The average burrows diameter was higher at Futuro beach, especially in supralittoral. The distribution of O. quadrata’s burrows diameter frequency indicated a bi-modal distribution in both beaches with small diameters observed since June 2013, probably related to incoming recruits on the two populations. The recruitment peak happened between August and December 2013, months of low rainfall. Thus, although variations in the burrows density and diameter occur over time because of changes in population characteristics, low density and the largest diameter of burrows indicate that anthropogenic disturbances affect the ghost crab population at Futuro beach.