Last week, a team of 10 Australian scientists announced that they had found the world’s first “shark hybrids”, offspring of individuals from two different shark species which had interbred. During a routine survey of Australian marine life, 57 sharks were found that physically resembled one species of shark, but had genetic markers inconsistent with that species. Subsequent genetic investigation revealed that these 57 animals were hybrids between common blacktip sharks (Carcharhinus limbatus) and Australian blacktip sharks (C. tilstoni).
Some of these hybrids were “F1″, meaning that one parents was a common blacktip and one was an Australian blacktip. Others were “B+”(backcrossed), which means that one parent was a common blacktip/Australian blacktip hybrid, and the other was a “purebreed” of one of those two species. According to the study’s lead author, Dr. Jess Morgan of the University of Queensland, ”our genetic marker tells us that these hybrids are ‘at least’ F1, and that these animals are reproductively viable and can produce an F2…the hybrids may be generations past F2 but the existing genetic markers can’t distinguish how many generations past the second cross have occurred.”
Genetic evidence and morphological characters have long supported the hypothesis that Australian and common blacktips are closely related, but distinct, species. They have detectable differences in their mitochondrial DNA, length at birth, length at reproductive maturity, and number of vertebrae. According Dr. Morgan, “The two blacktip species are very closely related (termed sister species) and this is probably why their hybridization has been successful. Over time the genes of species diverge away from each other due to random mutations.”
Common blacktips have a much wider distribution and are found worldwide, including throughout the more restricted range of the Australian blacktip. The area where the ranges of two species capable of interbreeding overlap is called a “hybrid zone”. Scientists expect to see more hybrid zones as climate change alters the ranges of numerous species.
Unlike most other species of fish, which reproduce by releasing eggs and sperm into the water column, sharks reproduce via copulation and internal fertilization. This means for each F1 hybrid, a male of one species physically mated with a female of the other. Different shark species often have varied mating behaviors, which was thought to make such hybridization extremely unlikely. Though hybrids have long been known in many other groups of organisms, this is the first time that hybrid sharks have ever been detected.