A dead two-headed dolphin that washed ashore in Turkey earlier this week has made waves across the Internet.

Is a two-headed dolphin really being examined in Turkey? http://t.co/kSWNQrorTF pic.twitter.com/mctAHzLIt7 — The Independent (@Independent) August 13, 2014

The unusual marine mammal, discovered by a tourist in the city of Izmir (map), is only the fifth known case of conjoined twins in the dolphin family—and only the third that is known to have lived beyond the fetal stage, according to a 2008 study in the Journal of Wildlife Diseases. (Also see “Three-Eyed Crab and More Freaks of Nature.”)

Because conjoined twins usually die while in the womb, how often the condition occurs in dolphins and whales is unknown, explained Kelli Danill, a research biologist with the marine-mammal and turtle division of the National Oceanic and Atmospheric Administration.

Though conjoined twins are common in humans, wild birds, reptiles, amphibians, fish, and domestic and laboratory animals, the fetal abnormality is extremely rare in wild mammals: Between 1671 and 2006 there were only 19 recorded cases. Danill estimates the rate of conjoined twins in small marine mammals is less than one percent.

Most of the time, people find conjoined marine mammals after dissecting a pregnant female, according to the journal study. (See “Two-Headed Shark Found by Fishermen.”)

Conjoined Twins Explained

Scientists also don’t know the reasons for these mutations, which can take several forms, noted Shawn Johnson, director of veterinary science at the Marine Mammal Center, a nonprofit based in Sausalito, California.

Twins’ bodies can fuse at eight different points. The Turkish dolphin pair is classified as Parapagus dicephalus, the most common manifestation of conjoined twins, in which the animals share one body but have two heads. (Also see “Deformed Dolphin Accepted Into New Family.”)

“These types of twins would not be able to survive long in the wild due to the difficulties they would have in swimming and surfacing to breathe,” Johnson said.

Added Sarah van Schagen, spokesperson for the MMC: “The condition of the carcass and location where it washed up makes it difficult to determine what species it is—which then makes any guesses about age tricky as well.”

Other marine mammals, like minke and sei whales, are often conjoined at the chest, sharing two heads and two tails. This past January, conjoined gray whales, the first ever to be seen, were found floating in the shallow waters of Laguna Ojo de Liebre in Baja California, Mexico. This type of fusion often means the whales’ blowholes point to the left and right, submerged underwater, and prevent any chance at survival.

Fetus Within a Fetus

And then there’s “fetus in fetu,” said the Marine Mammal Center’s van Schagen. A congenital abnormality, fetus in fetu causes one conjoined fetal twin to be completely enclosed within the body of the other.

In 1999 the Marine Mammal Center rescued a harbor seal on Monterey County’s Pebble Beach with a large cranial mass. Upon further examination, researchers discovered the mass was encompassing the seal’s fetal twin.

“Our veterinary experts performed surgery to remove the twin, but the host harbor seal, Honeydew, died a few hours after the surgery,” said van Schagen. (Also see “Pictures: Rare ‘Cyclops’ Shark Found.”)

High levels of PCBs and DDTs, types of industrial chemicals, were later found in the blubber of the host pup; however, the exact cause of the anomaly was never determined.

In the curious case of Turkey’s “Siamese fins,” the dolphins’ bodies are currently missing, according to the AP—but if they’re found, a local university plans to put the oddity on display.

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