Reproductive cycle and size at first sexual maturity of Hexaplex trunculus (Gastropoda: Muricidae) in the Gulf of Gabès (southern Tunisia)

Abstract

The reproductive cycle and size at first sexual maturity of the banded murex (Hexaplex trunculus) from the tidal areas of the Gulf of Gabès (southern Tunisia) were investigated from June 2007 to May 2008 through macroscopic analysis of the gonads and monthly variations in the general condition (K) and gonadosomatic indices (GSI). This muricid gastropod is gonochoristic (separate sexes throughout life) and lacks external sexual dimorphism. The overall sex-ratio of the population was significantly unbalanced, with a predominance of females (M:F = 1:1.6). The monthly variation in the gonad maturation stages, K and GSI revealed that H. trunculus has a yearly reproductive cycle, with both sexes showing a long period of gonadal activity and slightly asynchronous gametogenic cycle. Gametogenesis occurred during most of the year (11 out of 12 months examined) and almost every gametogenic stage was observed throughout the study period. Ripe males were found mainly from November to January, preceding the main period of gametic emission (between January and May). Ripe females occurred mainly from February to April, preceding the main spawning season (between April and May), apparently triggered by the increasing daylength and seawater temperature during this period (from 13.2 to 14.2 h and from 19.3 to 23.4°C, respectively). Both sexes displayed a relatively short resting phase (males: June—July; females: June—September), coincident with the highest daylength and seawater temperature during summer in the Gulf of Gabès. The first sexual maturity was reached at a smaller size in males (SL50 = 36.9 mm SL) than in females (SL50 = 50 50 40.3 mm SL). The information gathered in this study allowed preliminary management measures for the fishery to be proposed, including a closed season during the main spawning season (April—May) and the establishment of a minimum landing size (of at least 40 mm SL) for H. trunculus from the Gulf of Gabès.