Two-headed shark embryos discovered

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A pair of two headed shark embryos have been found preserved in formalin in a private collection in Argentina nearly 80 years after they were first caught.

The sharks are thought to be male Tope shark Galeorhinus galeus embryos dating from 1934 when they were removed from a pregnant female caught in the Mar del Plata coastal waters, Argentine Sea.

The sharks measured 162 and 174mm long and were fused at the level of the the fifth gill opening. Each head had a pair of nostrils, a pair of eyes, a mouth, four pairs of gills and five pairs of gill openings. There were also duplicated dorsal fins but only a single pair of both pectoral and pelvic fins.  

This is the first recorded case of dicephaly in this species and a fairly rare find for sharks with only a handful of cases ever having been reported.

Galeorhinus galeus is a medium-sized shark that occurs in coastal and shelf temperate waters in the north-east and south-east Pacific Ocean, north-east and south Atlantic Ocean, Mediterranean Sea, southern Australia and New Zealand.

The causes of the embryonic abnormality are unknown but previous suggestions for elasmobranchs have included parasitic infection, arthritis, injury, tumours, bad nutrition or a congenital abnormality as well as pollution and unfavourable environmental conditions during embryonic development.