Scientists discover hybrid sharks

One of the sharks found in the wild that has both Australian blacktip and common blacktip shark DNA. (Photo: Pascal Geraghty/New South Wales Department of Primary Industries)

A team of marine biologists from the Queensland Department of Primary Industries and Fisheries announced the discovery of shark hybrids in Australian waters. The hybrids are the offspring of two related but genetically distinct species.

The discovery, published last month in the journal Conservation Genetics, is the only known instance of shark interbreeding and has implications for shark species across the globe.

During a survey of Australian marine life, lead author Jess Morgan and his colleagues found 57 sharks that physically resembled one species, but had genetic markers consistent with another species. Further genetic analysis revealed that the animals were hybrids of Australian blacktip sharks and common blacktip sharks.

Some of the sharks were first generation hybrids, meaning one parent was a common blacktip and the other was an Australian blacktip. Others were backcrossed, the result of a hybrid mating with either a common blacktip or Australian blacktip. The scientists could not determine just how many generations of hybrids were out there, but it was obvious that their numbers were healthy.

The Australian blacktip is slightly smaller than the common blacktip and can only survive in warmer tropical waters. Its hybrid offspring, however, were found in cooler seas, 1,200 miles closer to the Antarctic.

Unlike many hybrid species, the shark hybrids are viable and capable of reproduction. Animals of different species usually produce infertile hybrids, especially if they do not share the same number of chromosomes. Horses and donkeys, for instance, can mate and produce mules. But because their parent species do not have the same number of chromosomes, mules are generally infertile. The two blacktip shark species are very closely related (they are termed sister species) and this is likely why their hybridization is successful.

Many of the news articles about this discovery are emphasizing the claim, never made in the original scientific article, that the sharks are hybridizing to protect themselves from climate change. An AFP article picked up by other media sources goes so far as to say "the Australian blacktip could be adapting to ensure its survival as sea temperatures change because of global warming." This contention is not supported by the data.

Although climate change is a serious threat to many species, its role in this story is not clear. The wording of the AFP article seems to imply that the Australian blacktip is purposefully hybridizing to prepare for future warmer waters, a very unlikely prospect. Furthermore, the hybrids are able to survive in more temperate waters than the Australian blacktips. It is difficult to see how this is then an adaptation to the increasing ocean temperatures caused by global warming.

Global warming, however, is behind the formation of hybrid offspring in other animals. In the past two decades, numerous Arctic mammal hybrids have emerged where the polar ice cap has melted. These include grizzly-polar bear hybrids, beluga whale and narwhal hybrids, and various seal mixtures. The rapid disappearance of the icy barrier that has kept species isolated for thousands of years is resulting in animals meeting and mating. For rare species, this could be disastrous. For example, red wolves in the southern U.S were lost through hybridization. In the mid-twentieth century, the dwindling red wolf population began breeding with coyotes. It resulted in the loss of nearly all genetically pure red wolf populations.

The discovery of hybrid sharks is scientifically important, even if it's not due to global warming. These are the first shark hybrids to be identified by scientists, but the extent of interbreeding between shark species is not at all known. Further research into the mating habits of sharks could reveal more about how new species form.

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