Featured Image Credit: Henk Tanis
By Alice Morris
Dutch fishermen witnessed the catch of a lifetime when they hauled aboard an extremely rare two-headed porpoise in late May.
It was the first confirmed case of conjoined twins in harbor porpoises and only the tenth documented case of conjoined cetacean twins.
The twins each had a fully developed head, but they shared a body, an anomaly called partial twinning, or parapagus dicephalus.
Erwin Kompanje of the Erasmus MC University Medical Center in Rotterdam has been studying cetaceans for twenty years and has never come across anything quite like this.
“Normal twins are extremely rare in cetaceans. There is simply not enough room in the body of the female to give room to more than one fetus,” said Kompanje.
“The anatomy of cetaceans is strikingly different from terrestrial mammals with adaptations for living in the sea as a mammal. Much is unknown. Adding any extra case to the known nine specimens brings more knowledge on this aspect.”
Kompanje co-authored a paper with several other marine researchers that describes the recent discovery.
The paper was published earlier this month in the Journal of the Natural History Museum of Rotterdam.
Unfortunately for the researchers, they never got to see the conjoined twins in person.
The fishermen, believing it to be illegal to keep the porpoises, returned them to the ocean, but not before taking several photographs that provided scientists with useful information about the specimens.
Researchers believe the porpoises, which measured about two feet long, likely died soon after birth.
The animals’ tail hadn’t stiffened and their dorsal fin was flat, which would make it impossible for the animals to swim. There were also hairs present on the porpoises’ lips that would have normally fallen off shortly after birth.
Kompanje said the porpoises’ anatomical deformities doomed them to drown from the beginning.
“As cetaceans are mammals anatomically adapted to complete life in the sea, we are curious how the anatomy in conjoined twins is, especially the development of the spinal musculatures,” said Kompanje. “Cetaceans have highly developed spinal musculature as they use their spinal column for movement.”
Partial twinning can happen in one of two ways. Either two separate embryonic discs can fuse together or a zygote may split partially early on in development.
The rarity of conjoined twins is unknown because the vast majority will never be found out at sea.
Kompanje says he doesn’t expect to see another case like this in his lifetime.
“For a cetologist,” he said, “this is a real horror.”
The International Union for the Conservation of Nature, an international conservation organization, says that by-catch is one of the biggest threats to porpoises and they have taken action to monitor the effects fishing has on porpoise populations.