The mangrove tree crab Aratus pisonii in a tropical estuarine coastal lagoon

Abstract

The population dynamics and life history of the grapsid crab Aratus pisonii were studied during a 2 year span in mangroves of the Laguna de Tacarigua National Park, a Venezuelan estuarine coastal lagoon. Abundances of adult A. pisonii estimated by means of a catch-effort sampling scheme, were found similar for the three study sites, but varied through time apparently in relation to salinity. Females were significantly more abundant than males in a ratio of 1·61 to 1·00. Ovigerous females were continuously present during the study, fluctuating from 23·2% to 82·7% of the monthly capture. The percentage of ovigerous females was significantly higher during the fourth trimesters. The number of eggs per female increased as a linear function of carapace width, with a mean of 16 379 and extremes of 7 952 and 34 057. Juvenile crabs were present through the period of study, although their abundance was very low. Males grow bigger than females, which concentrate in intermediate size classes, possibly as a consequence of energy diversion towards reproduction. Carapace width (CW) and length (CL) showed an isometric relation for both sexes, with a high correlation ( r = 0·9914 for females, r = 0·9957 for males). Size (CW) frequency distribution was found to be more symmetrical for females than for males, and thus is appeared strongly skewed to the left. The estuarine population dynamics of A. pisonii is mainly determined by the rainy season, which brings about an increment of nutrients and productivity in the lagoon.