Estuarine, Coastal and Shelf Science (1989) 28,639-650 The Mangrove Tree Crab Aratus pisonii in a Tropical Estuarine Coastal Lagoon JesGs Eloy Condeajb and Humberto Diaz” “Centro de Ecologia, Instituto Venezolano de Investigaciones Cientificas, Apartado 21827, Caracas 1020A, and %entro de Investigaciones Marinas, Universidad National Experimental Francisco de Miranda, Plaza La Antillana, La Vela de Coro, Falcdn, Venezuela Received 9 May 1988 and in revisedform 23 December 1988 Keywords: mangrove crab; population dynamics; coastal lagoon; Venezuela 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 ,OO. Ovigerous females were continuously present during the study, fluctuating from 23,2?b to 82.70i0 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 it 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. Introduction Aratus pisonii (H. Milne Edwards) (Brachyura:Grapsidae) is commonly found in the borders of mangrove swamp forests, from the supralittoral zone of roots to the canopy. This species is widely distributed in the western Atlantic, from eastern Florida to northern Brazil and throughout most of the Caribbean Islands and also in the eastern Pacific, from Mexico to Peru (Rathbun, 1918; Chace & Hobbs, 1969). Many of the life history features of A. pisonii are documented through the work of Hartnoll (1965), Warner (1967, 1968, 1977), von Hagen (1977), Diaz and Bevilacqua (1986, 1987), and Diaz and Conde (1989). The feeding habits and trophic role of Aratus have been studied by Beever et aE. (1979), de Lacerda (1981), Wilson (1981), Mattson 0272-7714/89/060639+ 12 $03.00/O @ 1989 Academic Press Limited