First Conjoined Twin Harbor Porpoises Found In The North Sea

This is only the 10th example of conjoined twins ever described in cetaceans. Henk Tanis/ Kompanje et al. 2017 

Scientists have recorded the first ever case of conjoined harbor porpoises. Discovered in the North Sea off the coast of the Netherlands, the specimen was caught by fishermen by accident, who took photos of the incredibly rare occurrence, before throwing it back into the sea.

This specimen is important not only as it is the first conjoined harbor porpoise ever discovered, but it represents only the 10th conjoined cetacean ever found globally. While it is likely that more cases do occur in the wild, it is highly improbable that they are ever found due simply to the vastness of the ocean in which they are swimming. Unfortunately for science, after taking the images the fisherman threw the twins back overboard, as they thought it would be illegal for them to land the catch.

But researchers are grateful for the photos, as they can still give some insight into the rare event. While the twins were clearly fully developed, being neonates rather than fetuses, the fact that the tail fluke and dorsal fin were floppy suggests that they died soon after birth, as it is during this period that they stiffen. The presence of whiskers on the snout of the carcass is also indicative that it was a neonate, as these usually fall out quite soon after birth, as was the open umbilicus.

The specimen shows what is known as parapagus dicephalus. Henk Tanis/ Kompanje et al. 2017

“The anatomy of cetaceans is strikingly different from terrestrial mammals with adaptations for living in the sea as a mammal. Much is unknown,” explains Erwin Kompanje, one of the co-authors of the study published in the Online Journal of the Natural History Museum Rotterdam describing the twins, to New Scientist. “Adding any extra case to the known nine specimens brings more knowledge on this aspect.”

While conjoined twins are clearly incredibly uncommon, even normal twins in cetaceans are rare. Even though there are thought to be 700,000 harbor porpoises, there is only one other account of this species having fully developed twins. It is thought that across all whales and dolphins generally, only around 0.5 percent of births produce twins. This is thought to be due to the fact that the uterus is simply not large enough to accommodate them, and if they are born, the mothers will struggle to produce enough milk to support two infants.

This marks only the tenth example of conjoined cetaceans globally, and only the fourth known case of parapagus dicephalus in this group. This refers to the twinning occurring in which they are fused side by side sharing a single pelvis and parts of the thorax, but two distinct developed heads.

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