ETUDE DES PARAMETRES MICROSCOPIQUES DE LA BIOLOGIE DE REPRODUCTION CHEZ LA SQUILLE SQUILLA MANTIS DES EAUX TUNISIENNES

L’étude histologique représente la première approche pour la validation des résultats macroscopiques de la reproduction chez Squilla mantis des eaux tunisiennes. Les objectifs de cette investigation sont d’une part l'identification des différentes phases du développement ovocytaire et d’autre part de décrire la cinétique de l'ovogenèse ainsi que la stratégie de ponte de cette espèce. L’étude histologique a été réalisée sur les ovaires de 380 femelles de Squilla mantis pêchées dans les trois golfes de la Tunisie (Tunis, Hammamet et Gabès). Dans cette étude, les follicules post-ovulatoires et les ovocytes atrésie atrétiques ont été utilisés comme indicateurs d’une activité de ponte antérieure. Chaque phase histologique a été corrélée au stade macroscopique de maturité et au rapport gonado-somatique des individus étudiés et ceci afin de valider les différentes phases du cycle de reproduction chez S. mantis. L’analyse des coupes histologiques des ovaires a permis de diviser l’ovogenèse en 4 phases : pré-vitellogenèse, vitellogenèse, maturation et atrésie. Le cycle sexuel des femelles de squilles peut être composé de 6 étapes : juvéniles, repos sexuel, début de la vitellogenèse, vitellogenèse, ponte, post-ponte et récupération. La présence des follicules post ovulatoires et une faible fréquence des ovocytes à différents stades de développement permettent d’affirmer que les squilles des eaux tunisiennes appartiennent aux groupes d’espèces à ponte unique. Ce crustacé appartient au groupe des espèces dont la fécondité de la femelle croît proportionnellement moins vite que le cube de la longueur totale. Cette fécondité présente une différence significative entre les trois golfes de la Tunisie.

Microscopic parameters of the reproductive biology of the spot-tail mantis shrimp Squilla mantis caught in Tunisian waters : This histological study represents the first approach to validate the results obtained in macroscopic reproduction of Squilla mantis (L., 1758) in the Tunisian waters. The aim of this investigation was the identification of the oocyte development phases, the description the oogenesis kinetics and the spawning strategy of this species. A total of 380 female ovaries of Squilla mantis caught in the Tunisian gulfs (Tunis, Hammamet and Gabes) were exanimate. The photomicrographs of histological developmental stages and measurement of oöcytes were performed using an image analysis system (Optimas 6.5 software). Thereafter for the histological study, only a portion of the middle part of the ovary from each female was chosen to facilitate manipulations. During oögenesis, oöcytes develop continuously through different steps. Some steps can be identified as oöcyte development or maturation stages according to their microscopic features. In this study, identification of representative histological development stages is based on oöcyte, nucleus and cytoplasm sizes, cytoplasm homogeneity, inclusions type and quantity and cumulative oöcyte size frequency distributions. To determine the range of oöcytes diameters at the different developmental stages, the area equivalent diameter of at least 100 cells per stage was measured. Post-ovulatory follicles and atretic oöcytes were equally considered as ovary characteristic elements in the reproductive strategy study. Indeed, post-ovulatory follicles and atretic oöcyte stages are used as indicator of previous spawning activity. The ovarian cycle was determined histologically based on the relative frequency of the different oöcyte developmental stages and post-ovulatory follicles. Seven growth substages of oöcytes were distinguished: 1) Early primary oöcyte, St; 2) Late primary oöcyte, St2; 3) Primary vitellogenic oöcyte, St3; 4) Early secondary vitellogenic oöcyte, St4; 5) Late secondary vitellogenic oöcyte, St5; 6) Hyaline oöcyte, St6; and 7) Atretic oöcyte, Atr. The histological stages were correlated to the macroscopic maturity and to the gonad index to validate the reproductive cycle of S. mantis. The analysis of the histological sections of the female’s ovaries shows that oogenesis can be divided in 4 phases: pre-vitellogenesis, vitellogenesis, maturation and atresia. The kinetics of oögenesis of females of S. mantis is characterized by six successive ovarian development phases: Juvenile, sexual resting, beginning of vitellogenesis, vitellogenesis, spawning and post-spawning and recuperation. At the mature ovarian phases, two populations of oöcytes were distinguished; a fairly synchronous population of large vitellogenic oöcytes (St3 or St4 or St5), and a heterogeneous population of smaller oöcytes from which the next clutch will be recruited. In terms of ovarian organization, S. mantis can be considered as ‘‘group-synchronous’’. The presence of only one clutch of yolked oöcytes able to be ovulated once during the breeding period suggests that the Spot-tail mantis shrimp spawns only once during every sexual cycle, i.e., S. mantis can be classified as species with a unique spawning period. The fecundity of the female of S. mantis increases proportionally less than to the cube of the total length ant it was significantly different between the three gulfs of Tunisia. The average of the relative fecundity is about 1618 ± 632 eggs / g of mantis shrimp.