First report of the occurrence and different types of conjoined twins in common whitefish Coregonus maraena larvae originating from the Baltic Sea.

This paper describes for the first time various forms of conjoined twins in the common whitefish Coregonus maraena originating from the brackish waters of the Baltic Sea. The occurrence of conjoined twins was observed during the hatching of whitefish under controlled conditions. Two types of twins were identified: symmetric (parapagus) and asymmetric (parasitic form). The percentage of conjoined twins among the whitefish did not exceed 0.01% of the hatched larvae. The forms of twins observed in this study were similar to conjoined twins occurring among humans. One main type, parapagus, was identified in the group of symmetric twins with 3 subtypes: diprosopus parapagus, dicephalic parapagus and dithoracic parapagus. In its parasitic form, the parasitic twin occurred as a 'cyclops' or just as an eye growing out of the other twin's head. We conclude that fish, as well as their fertilised eggs, can be used as a model system to observe how such twins are joined in humans because of the similarity with conjoined twins occurring in humans. Because of internal fertilisation in fish, as well as the size and transparency of egg cells in these animals, it is possible to observe the process of fertilisation, cell division and formation of an embryo and subsequently of twins, as a model for humans.