When Dutch fisherman pulled in their net off the coast of the Netherlands last month to find a dead porpoise among their haul, they quickly threw the illegal bycatch back into the water. What they didn't realize was that they were throwing away an extremely rare scientific find. Luckily, they snapped a few photos first, allowing researchers to document the first-ever case of harbour porpoise conjoined twins.

Harbour porpoises are actually the most abundant cetaceans in the northwest European continental shelf, but only one case of twins has ever been recorded, and not a single case of conjoined twins. While the specimen itself has been lost to science, luckily the photos were passed along to Erwin Kompanje, an honorary curator-taxidermist at the Natural History Museum Rotterdam.

In running a ruler over the images, Kompanje and his fellow researchers were able to deduce a few important facts about the conjoined porpoises. He could tell they were babies because the dorsal fin was not yet erected and lacking firmness, and that they were probably male and most likely born alive but didn't live for long.

While conjoined twins occur more often in humans and domestic and laboratory animals, they are very rare among wild mammals, according to the researchers. Published cases predominantly consist of fetuses and embryos taken from dead pregnant females. Even rarer again are conjoined twins in whales and dolphins, with only nine published cases that the researchers are aware of.

Even though the scientists can blame the angler's adherence to fishing laws for the lack of a specimen to study, they can be thankful the fishermen had a camera on-hand to capture the moment. Kompanje along with other researchers from the Royal Netherlands Institute of Sea Research have published a paper describing the discovery, which can be accessed online here.

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