Morphological criteria of egg quality in marine fishes: Activation and cleavage of eggs of Zebrasoma scopas (Acanthuridae)

Abstract

Morphological changes of the eggs of Zebrasoma scopas obtained just after ovulation and after the storage of ovulated oocytes in the ovarian cavity of the female (in vivo) or in the external medium (in vitro) are described. The dynamics of cortical reaction is followed in noninseminated oocytes, as well as in inseminated oocytes of various quality. The average proportions of eggs exhibiting normal cleavage in the control series (comprised the eggs inseminated just after ovulation) and in the series stored in vivo for 2 and 4 h at 27°C are 83, 30, and 35%, respectively. The average proportions of eggs with normal cleavage in the control series and in the series stored in vitro for 2 and 4 h at 25°C are 88, 79, and 34%, respectively. The storage of oocytes in vitro at 7°C for 2 h leads to the abrupt decrease of their quality with the proportion of normally cleaved eggs reaching only 15%. The types of abnormal cleavage leading to subsequent degradation of cells of the blastodisc and total mortality of eggs are as follows: (1) mitotic cleavage of nuclei accompanied by incomplete cytotomy, (2) mitotic cleavage of nuclei without cytotomy, (3) a weak adhesion between cells, and (4) desynchronization of cell divisions. The methods for the assessment of egg quality just after ovulation and at initial developmental stages are suggested for marine teleost fishes.