Etude des paramètres microscopiques de la biologie de reproduction chez squille Squilla Mantis des eaux tunisiennes

Abstract

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.