Morphological characteristics of the micropyle and micropylar cone in red spotted grouper (Epinephelus akaara) and sevenband grouper (Epinephelus septemfasciatus) during fertilization process.

In order to understand the phenomenon of cross-species hybridization between grouper species, we investigated the morphological characteristics of the micropyle and micropylar cone during fertilization process of Epinephelus akaara and E. septemfasciatus using scanning electron microscopy (SEM). This species has one micropyle, which is found at the animal pole of the eggs. Various size pores (0.11-2.17 µm in diameter) were observed in a concentric circle only around the micropyle. The outer aperture of the micropyle and various size pores were in an open state before fertilization. The micropylar diameter of E. akaara and E. septemfasciatus was 5.91 ± 0.57 µm and 6.22 ± 0.69 µm respectively. The micropyle was a circular flat crateriform and type I, and consisted of a vestibule and canal. The ridge of the micropylar vestibule was reinforced in a clockwise eight to nine spiral arrangement. After fertilization, the vestibular ridge initially swelled by activation of the ridge. The micropylar cone with a pillar or dome shape was projected from the micropyle and completely plugged the micropylar aperture. The pores were closed and nodules were not observed on the chorionic surface. These results suggest that the micropylar size and the structure of the grouper species are similar but induced the prevention of polyspermy by micropylar cone. [ABSTRACT FROM AUTHOR]