Etude des parametres microscopiques de la biologie de reproduction chez la 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 oocytes were performed using an image analysis system (Optimas 6.5 software). In this study, identification of representative histological development stages is based on oocytes, nucleus and cytoplasm sizes, cytoplasm homogeneity, inclusions type and quantity and cumulative oocytes size frequency distributions. Indeed, post-ovulatory follicles and atretic oocytes stages are used as indicator of previous spawning activity. The ovarian cycle was determined histologically based on the relative frequency of the different oocyte developmental stages and post-ovulatory follicles. Seven growth substages of oocytes were distinguished: Early primary oocytes, St; Late primary oocytes, St2; Primary vitellogenic oocytes, St3; Early secondary vitellogenic oocytes, St4; Late secondary vitellogenic oocytes, St5; Hyaline oocytes, St6; and Atretic oocytes, Atr. 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 oogenesis 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 oocytes were distinguished; a fairly synchronous population of large vitellogenic oocytes, and a heterogeneous population of smaller oocytes from which the next clutch will be recruited. The presence of only one clutch of yolked oocytes 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 and it was significantly different between the three gulfs of Tunisia. The average of the relative fecundity is about 1618 plus or minus 632 eggs / g of mantis shrimp flesh.