Survival rates and the occurrence of larval malformations, including Siamese twins, following fertilization of post-ovulatory aged oocytes in ide Leuciscus idus.

Embryonic development of lower vertebrates can be influenced by many factors, especially when outside of the mother's body and under the influence of variable abiotic and biotic factors. In this study, the effects of fertilization of post-ovulatory aged (overmature) oocytes of ide Leuciscus idus on the risk of developing anomalies in ide larvae were assessed under controlled conditions. There was a negative effect of delayed fertilization of post-ovulatory aged (overmature) oocytes in the ovary on the quality of offspring, which caused reduced survival 3 days post-fertilization and at the hatching stage (68.3 vs. 81.2% and 47.2 vs. 79.7% for the overmature and mature groups, respectively). It also increased the occurrence of numerous developmental defects (19.3 vs. 0.8% for the overmature and mature groups, respectively), including the development of Siamese twins. This is the first reported case of the occurrence of Siamese twins in ide. The ide Siamese twins look like hen eggs with 2 heads and are different from 'typical' Siamese twins in fish.