Two-headed porpoise caught in North Sea

Kompaje observed in his study that scientists lack a complete understanding of what causes conjoined twins.

Ian Marsh | Jun 15, 2017

The first recorded example of a two-headed porpoise was accidentally caught in May by Dutch fishermen in the North Sea.

Although the fishermen, who worried it might be illegal to keep the dead porpoise, threw it back into the sea, they first took photos and then contacted researchers.

From the photos, scientists know that the porpoise was a newborn male and died soon after birth, a report by National Geographic said. Its dorsal fins were not erect, its tale had not stiffened, and an umbilical opening could be seen.

The porpoise was a symmetrically conjoined twin, a condition thought to occur when a zygote only partly splits or two separate embryos fuse together.

"Descriptions of conjoined twin in whales and dolphins are extremely rare," wrote Erwin Kompanje of the National History Museum Rotterdam, in a study recently published online by the Journal of the National History Museum Rotterdam. "We were aware of only nine [other] published cases."

Kompaje observed in his study that scientists lack a complete understanding of what causes conjoined twins.

Porpoises generally give birth to a single offspring every year or two, according to National Geographic. Some 700,000 of the marine mammals are found around the globe, with about 345,000 making their home in the North Sea.

Accidental catch in trawling nets poses the greatest threat to porpoises, says the International Union for the Conservation of Nature.

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