Cell morphology as an indicator of viability of cod eggs – results from an experimental study

Abstract

This study evaluates the use of egg malformations, at early blastula stages, as direct indicators of egg viability in cod, Gadus morhua. Eggs with visible aberrations at early stages (4–32 cells) were incubated individually, using regular eggs as controls. Irregular eggs displayed, on an average, a lower hatching rate than the controls, 35% and 80%, respectively, but with substantial variation among batches. In several examples, severe malformations at blastula stages (4–32 cells) resulted in normal hatching. There were no differences in mortality rate during the yolk-sac period between larvae hatching from irregular eggs and controls, respectively, implying similar opportunities for further survival. In addition, artificially fertilized eggs of 17 batches from 11 different females of Baltic cod were incubated after estimations of malformation rate at early blastula stages. Viable hatch varied between 14% and 97% (average 64%, SD=28) among batches and a rank correlation test revealed a significant correlation between malformation rate (average 13%, SD=17) and viable hatch. The results are consistent with those from the individual egg incubation, suggesting that malformations during early stages may affect further development. However, large variations in hatching success, between batches, both from individual egg incubation and from incubation in batches indicate that other factors may be important. This study reveals that severe aberrations occurring at early stages may recover. Thus, malformations occurring at early blastula stages should be used with prudence and not as a direct and consistent indicator of poor egg viability.