A rare case of cetacean conjoined twins Henk Tanis

Fishers off the coast of the Netherlands got quite a shock when they caught what has now been confirmed as the first case of conjoined twin harbour porpoises (Phocoena phocoena).

With a single body and two fully grown heads, this is a case of partial twinning, or parapagus dicephalus. The sighting is extremely rare: these male porpoises are only the 10th known case of conjoined twins in cetaceans, a group of animals that also includes whales and dolphins.

“The anatomy of cetaceans is strikingly different from terrestrial mammals with adaptations for living in the sea as a mammal. Much is unknown,” says Erwin Kompanje at the Erasmus MC University Medical Center in Rotterdam, and one of the authors of the paper describing the find. “Adding any extra case to the known nine specimens brings more knowledge on this aspect.”


The fishers who made the discovery returned the twins – which were probably already dead when caught – to the ocean. They believed it would be illegal to keep such a specimen, but were able to produce a series of photographs useful for research.

Kompanje says although it is a shame that researchers will not be able to directly examine the twins, we can still learn from the photographs provided.

Newborns

We know that the twins died shortly after birth, because their tail had not stiffened – which is necessary for newborn dolphins to be able to swim, says Kompanje.

Other signs of their age were a flat dorsal fin that should have become vertical soon after entering the ocean water, and hairs on the upper lip, which should fall out shortly after birth.

Partial twinning can happen in one of two ways: two initially separate embryonic discs can fuse together or the zygote can only partially split during the early development process.

“Normal twins are extremely rare in cetaceans,” adds Kompanje. “There is simply not enough room in the body of the female to give room to more than one fetus.”

It is likely that most conjoined twins will go unnoticed by science because of the vast size of our oceans.

“Conjoined twins will be more common than the 10 cases we know at this moment, but we are unaware of them because they are born at sea and are never found,” says Kompanje.

Journal reference: Deinsea, 17: 1-5 (2017)