Two-headed sharks are being raised and studied by scientists.

Two-headed sharks sound like something out of a B-list horror film, yet scientists are discovering more of them worldwide.

Some have speculated that the increase in mutants is the result of genetic .def.ects caused by overfishing.

The perplexing pattern began in 2008, when Christian Johnson, an Australian fisherman, captured a two-headed blue shark embryo off the coast of Australia. A crew of Floridian fishermen struggled to catch a massive Bull shark in 2013, but when gutting it, they discovered that its u.te.rus contained a two-headed baby.

Because they carry big litters of up to 50 kids at a time in the womb, blue sharks have generated the most two-headed offspring thus far.

More recently, while breeding hundreds of sharks for human-health studies, Spanish researchers discovered a two-headed Atlantic saw-tail catshark embryo.
It is the first instance of a two-headed shark being produced by an oviparous shark species (a shark that lays eggs). The researchers gently opened the egg to examine the unusual embryo.

Professor Valentn Sans-Coma, the study’s leader, is doubtful if the embryo would have survived if it had been let to hatch spontaneously.

These em.bryos are unlikely to survive long after hatching, which might explain why two-headed egg-laying sharks have never been discovered. Scientists still still know what sparked this recent surge of two-headed shark findings.

While their numbers are increasing, sightings are few and far between, making it impossible for experts to determine what causes the legendary mutation.

And a 2011 study described conjoined twins discovered in blue sharks caught in the Gulf of California and northwestern Mexico. Blue sharks have produced the most recorded two-headed embryos because they carry so many babies—up to 50 at at time, says study leader Felipe Galván-Magaña, of the National Polytechnic Institute in Mexico.

Now, Spanish researchers have identified an embryo of an Atlantic saw tail catshark with two heads, according to a new study in the Journal of Fish Biology. While raising sharks for human-health research in the laboratory, a team noticed an unusual embryo in a see-through shark egg.

Nicolas Ehemann, a marine biologist, has discovered the Caribbean Sea’s first two-headed shark.

According to Ehemann, the significant occurrence of two-headed sharks in nature suggests that overfishing is the most likely cause. Ehemann, a master’s student at Mexico’s National Polytechnic Institute, agrees that the smaller shark gene pool caused by fishing would almost certainly result in an increase in birth abnormalities.

This week, researchers in Malaga discovered a two-headed Atlantic saw tail catshark, the first of its type.
The catshark embryo was not your average two-headed beast—it’s the first such specimen known from an oviparous shark species or a shark that lays eggs.
Galván-Magaña, who authored the 2011 study, doesn’t think two-headed sharks are more common—but rather that there are more scientific journals around to publish accounts.

Galván-Magaña has seen other bizarre sharks, too, including a “cyclops” shark, caught off Mexico in 2011, with a single, functioning eye at the front of its head. The dusky shark fetus’s single eye is the hallmark of a congenital condition called cyclopia, which occurs in several animal species, including people.

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