Stratégie De Reproduction Et De Croissance De Deux Espèces Sympatriques Du Genre Porcellionides (Isopoda, Oniscidea) De Tunisie

Abstract

The biology and population dynamics of two sympatric species of Oniscidea, Porcellionides pruinosus and P. sexfasciatus from the population of Garat Nâam (Kasserine, Tunisia) have been studied based on monthly sampling carried out from July 1996 to June 1998. P. pruinosus was more abundant on the sampling locality. Both species show a seasonal reproduction period. The population of P. pruinosus showed two distinct breeding peaks, while P. sexfasciatus exhibited only one spring breeding, from March to May. In P. pruinosus, larger ovigerous females were present during the first reproduction period, while gravidity in smaller females was restricted to the second breeding wave. Juveniles derived from different breeding waves showed remarkable differences in size. Also, a striking variability in the size of juveniles of the same age was observed between these two species. During the study period, analysis of size frequency distributions allowed the identification of 14 cohorts in each species. Three of these cohorts were followed in P. sexfasciatus, and four in P. pruinosus.The growth rate varied with developmental stage and with season. In winter, low temperature reduces the growth rate by decreasing moulting frequency, while in spring the increase in temperature induces growth. Sexual maturation of females is reached at approximately 2.5 months and at an average length of 4.23 ± 0.21 mm in P. pruinosus, and 5.5 months or 6.83 ± 0.23 mm in P. sexfasciatus.The total number of hatched eggs or embryos is positively correlated with body length of ovigerous females. In P. pruinosus, ovigerous females show distinct strategies of reproduction: the first one in spring with large females and high fecundity and fertility; the second in autumn characterized by small females and low fecundity and fertility. Reproduction strategies, K for P. sexfasciatus and r for P. pruinosus, the reproduction potential, and effort could explain the abundance of P. pruinosus at the station of Garat Nâam. Also, this difference in abundance was due to the growth difference between the two species, the presence of two breeding peaks, and a 3 ) e-mail: mohamed_achouri@voila.fr © Koninklijke Brill NV, Leiden, 2006 Crustaceana 79 (7): 843-864 Also available online: www.brill.nl 844 MOHAMED S. ACHOURI & FAOUZIA CHARFI-CHEIKHROUHA female-biased sex ratio, which was often in equilibrium in P. sexfasciatus, but which was always skewed in favour of females in P. pruinosus. RÉSUMÉLa biologie et la dynamique des populations de deux espèces sympatriques d'Oniscidea, Porcellionides pruinosus et P. sexfasciatus, ont été réalisées à partir d'un échantillonnage mensuel, effectué pendant deux années consécutives, de juillet 1996 à juin 1998, à Garat Nâam (Kasserine, Tunisie).L'analyse du cycle biologique de ces deux espèces a montré que P. pruinosus est plus abondant dans la station de récolte et que les deux espèces sont caractérisées par une reproduction saisonnière. Au cours de cette période, deux poussées reproductives distinctes ont été notées chez P. pruin...