Biological, Histological and Ultra-Structural Studies of Female Mullet, Mugil cephalus, Ovaries Collected from Different Habitats during Annual Reproductive Cycle
This study investigates some biological characters (oocyte diameter, fecundity, histological and ultra structural features) of female Mugil cephalus ovaries collected from three different natural habitats: marine (MW), brackish (BW) and fresh (FW) water. Monthly gonadosomatic index (GSI) values clearly showed that the time period of reproductive activity in female M. cephalus from marine and brackish water habitats was from early September to late November. No peak value of GSI in females collected from freshwater was observed throughout the year. Analysis of ovum diameter for M. cephalus in the two habitats revealed that, there are small diameter ova (less than 0.3 mm) and large ova (larger than 0.35 mm). The percentage of small ova diameter was 5±1% in marine habitat, while 27±3% for brackish water habitat. The mean oocyte diameters in fresh water fish were less than 350 μm. The oocytes did not develop enough to be differentiated into small and large diameter ova. The total number of ripe ova in marine fish varied between 0.84 ± 0.05 to 4.14±1.01 x106 for a total length ranging between 35 and 52 cm, respectively; whereas, the total number of ripe ova in brackish water fish ranged from 0.57±0.14 to 3.81±0.59 x106 for the same length groups. There was highly significant correlation (p>0.01) between the number and length of ripe ova in 37 and 50 cm length group from the two habitats. Yolky nucleus or Balbiani’s body and interstitial epithelial cells are a characteristic feature of oocytes at maturing stage, which is clearly detected in marine water fish with isolated follicular and active organelles. In brackish and fresh water fish ovary, the cytoplasm was compacted without accumulation of active organelles. Ultra structurally vacuolized oocyte wall in marine fish showed the presence of the fifth layer (cortical alveoli) while no cortical alveoli formation was observed in oocyte of brackish or fresh water females. The percentage of atretic oocytes in late vitellogenic ovary of marine water fish was about 2.5%, while in brackish water fish it was about 92±2%. In both brackish and fresh water fishes the initial stage of oocytes atresia degeneration was observed. In conclusion, the comparative study shows that ovary of marine and brackish M. cephalus morphologically overlaps from ripening to re-sorption stages. With the histological and fine structure characteristics, it was possible to understand the functional relationship between oocyte size and stage of fish maturation. This knowledge is of huge importance in establishing the reproductive status of the fish which is related to the functional expression of the folliculogenesis in female individuals.
Keywords: Ova, Mugil cephalus, Marine, Brackish and Fresh Water Fish