Biological characteristics of three Brachyuran crab species in the Lagoon of Elbibane, South-Eastern Tunisia

This study is a first contribution to the knowledge of the biology of three crab species, Carcinus aestuarii, Eriphia verrucosa and Pachygrapsus marmoratus, occurring in the intertidal zone of Elbibane lagoon, south-eastern Tunisia. A total of 1264 individuals from the three brachyuran species were collected monthly from April 2010 until March 2011 and their reproduction and growth parameters were determined. The seasonal distribution of carapace width showed for the three species that males have always a greater size than females. The length-weight relationship showed in the three species a negative allometry in males, and in non-ovigerous females. The von Bertalanffy growth model (VBGF) parameters for C. aestuarii females and males were: CL_∞= 49.35 mm, K = 0.87 year^-1 and CL_∞ = 55.70 mm, K = 0.75 year^-1, respectively, whereas infernales and males of P. marmoratus, the parameters were CL_∞ = 42.95 mm, K= 0.56 year^-1 and CL_∞ = 44.08 mm, K = 0.52 respectively. In E. verrucosa, it was possible to estimate only the parameters in females (CL_∞ = 73.50 mm, K = 0.66 year^-1). In all, the VBGF values indicate that the crabs are fast-growing species, with a fishable lifespan of 3-4 years identified by Bhattacharya's method. Moreover, the sex-ratio of the three decapods was found to be male-biased along most of the year; the abundance of females appeared to increase in reproduction periods. In addition, the fecundity of the three species was found to be correlated with body size and weight.