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What hybrid sharks mean (and don’t mean) for climate change and evolution: fact-checking the media coverage

Photo credit: study author Pascal Geraghty, New South Wales Department of Primary Industry

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.

Additionally, both species are an important part of the Australian inshore shark fishery, and the discovery that they can hybridize has implications for the management of both species. Hybrids are often less reproductively fit, and if that is the case here, it means that both species may be less able to support a fishery than previously believed. However, Dr. Morgan says, “If the hybrids turn out to be fitter than their parent species then over time the two species will merge into one species, simplifying management.”

Finally, hybridizing with the more temperature-tolerant common blacktips has allowed for a range expansion beyond the tropical waters where Australian blacktips are commonly found:

Figure 1 from Morgan et al. 2011. This shows the distribution of parental (P), hybrid (F1) and backcrossed (B1) sharks at five sample sites along the Northeast coast of Australia. Note: The contents of this graph are more complex than they appear and are explained in more detail in the paper, just notice that in southern, temperate waters, there are few purebreed Australian blackitps, but numerous purebreed common blacktips and hybrids.

This fascinating discovery has deservedly resulted in a great deal of media coverage. With many news stories involving sharks, climate change, or evolution, the mainstream media has a tendency misconstrue the science. This story involves all three of these issues, and the resulting news coverage hasn’t been entirely accurate. Presented below is a list of common inaccurate claims made by news reports about these hybrid sharks, and a summary of the relevant science.

Do other hybrid sharks exist?
The Huffington Post claims that these hybrid sharks are “unlike any other [shark] in existence”. While it’s true that none have ever been documented before, claiming that no other hybrid sharks exist is inaccurate because we simply don’t know.  Just because something was only discovered recently doesn’t mean that it only came into existence recently, and just because we’ve never seen any others doesn’t mean that none are out there. This point was even raised in the press release associated with this discovery, which noted “other closely related shark and ray species around the world may be doing the same thing.” These animals are the first hybrid sharks to be detected by scientists, but are likely not the world’s only hybrid sharks.

“We suspect that there may be other hybrid elasmobranchs out there, and recommend that studies investigating sister-species with overlapping distributions consider including a nuclear DNA marker for species identification as well as using a mitochondrial DNA barcode (mitochondial DNA is maternally inherited and will miss hybrids),” Dr. Morgan said.

Is mating with a different species better than not mating at all?
The MSNBC article claims that this hybridization may be a reaction to overfishing, which has reduced the populations of both species. Basically, they are saying if an Australian blacktip shark has fewer other Australian blacktip sharks to mate with, it may be less resistant to mating with a common blacktip. In other words, “as fisheries are depleted, hybridization is a way to keep reproducing.”

While this is an interesting hypothesis, there is (so far) absolutely no evidence supporting it. Sharks have long generation times, so in order for backcrossed individuals (essentially grandchildren) to exist, the original hybridization detected in this study has to have been going on for at least several decades. That long ago, shark populations were much healthier than they are now.

Additionally, there is no evidence that individual sharks within a species struggle to find mates, even with today’s greatly reduced populations. Many species of sharks have mating aggregations, so even if populations were reduced, individuals could find each other. It is not known if either species of blacktip engages in this behavior.

The study’s authors did not mention this hypothesis in the paper itself, the press release, or any interviews that I can find. It seems to have originated in an interview with another shark scientist unrelated to this research project, and that scientist was merely proposing a possible explanation.

Dr. Colin Simpfendorfer, an expert on Australian shark fisheries and one of the study’s authors, said, “Both species of blacktips are target species in fisheries across northern Australia (including the area where hybrids were found). However, assessments suggest that they are relatively productive species and that populations are not at dangerously low levels. Unfortunately we know little about mating systems in these species (or most other sharks for that matter), including if they have specific mating areas, and if they do where they occur.”

Are blacktips planning ahead to protect themselves from climate change?
A common claim in news articles about this discovery was that the sharks were hybridizing to protect themselves from climate change. This was expressed most clearly in an AFP article (picked up by both Yahoo News and Discovery News), which stated ” the Australian black-tip could be adapting to ensure its survival as sea temperatures change because of global warming”. This claim displays an all-too-common lack of understanding of how evolution works.

While climate change can indeed negatively affect marine organisms like sharks, the logic behind this particular claim is a little hard to follow. It is unlikely (at best) that Australian blacktips would know that mating with common blacktips would result in offspring that could survive in more temperate waters. Additionally, evolution is not goal oriented. Females of many species look for a variety of traits before deciding which males to mate with (strength, fighting ability, size of antlers, etc.), but to my knowledge desired traits do not include a invisible ability to survive in colder water. Also, waters are getting warmer, so it is unclear why adapting to more temperate waters would be advantageous. Climate change is not specifically mentioned in the original article or in the press release, though both reference broader “environmental change”.

“We do not believe that climate change triggered the hybridization event,” said Dr. Morgan. “We do see that hybrid Australian blacktips have extended their range south into cooler waters suggesting that they may have a wider temperature tolerance than pure Australian blacktips. This increased temperature tolerance may make the hybrids better adapted to tolerate changes in water temperature in the future.”

Reader reactions
While the media coverage of these hybrid sharks has been occasionally inaccurate, reader comments left on the bottom of news articles are absolutely terrifying. CNN actually dedicated an entire blog post to their favorite reader comments from the original news story, and comments that didn’t make this list include rants against evolution, climate change, shark conservation, and science itself. Apparently, the scientists involved in this discovery hope to use it to simultaneously promote socialism and get incredibly rich while denying the Bible.

My favorite reaction to this discovery comes from conservative provocateur James Delingpole, who wrote a mocking post for the Telegraph’s blog entitled “Can climate change create deadly mutant sharks which kill us all?” It is often difficult to tell when Delingpole, the author of books such as “365 ways to drive a liberal crazy”, is being sarcastic or serious. However, it’s easy to tell  (from this and many other posts) that he doesn’t understand science and is kind of a jerk.

Dr. Morgan has a message for these naysayers. “Please read our research article in Conservation Genetics before you attack the science. Not everything you read in the paper, see on the news or find on the internet is true,” she said. “I have enjoyed some of the rants, my current favourite is mutant sharks with lasers. ”

The bottom line
Kudos to this team of scientists for a fascinating discovery with important implications.  I look forward to seeing follow-up studies determining if hybrids have different life histories, and how these life histories impact fisheries management. I also look forward to discoveries of many more hybrid sharks, which I expect will happen soon, and studies of how the presence of hybrids impacts forensic technology like DNA barcoding. However, these sharks are probably not the only hybrids in the world, they did not intentionally hybridize for the purpose of becoming more resilient against climate change, and they are not going to evolve frickin’ laser beams on their heads.


ResearchBlogging.org

Allendorf, F., Leary, R., Spruell, P., & Wenburg, J. (2001). The problems with hybrids: setting conservation guidelines Trends in Ecology & Evolution, 16 (11), 613-622 DOI: 10.1016/S0169-5347(01)02290-X

Cheung, W., Lam, V., Sarmiento, J., Kearney, K., Watson, R., & Pauly, D. (2009). Projecting global marine biodiversity impacts under climate change scenarios Fish and Fisheries, 10 (3), 235-251 DOI: 10.1111/j.1467-2979.2008.00315.x

Lavery, S. (1992). Electrophoretic analysis of Phylogenetic relationships among Australian Carcharhinid Sharks Marine and Freshwater Research, 43 (1) DOI: 10.1071/MF9920097

Morgan, J., Harry, A., Welch, D., Street, R., White, J., Geraghty, P., Macbeth, W., Tobin, A., Simpfendorfer, C., & Ovenden, J. (2011). Detection of interspecies hybridisation in Chondrichthyes: hybrids and hybrid offspring between Australian (Carcharhinus tilstoni) and common (C. limbatus) blacktip shark found in an Australian fishery Conservation Genetics DOI: 10.1007/s10592-011-0298-6

Pratt, Jr., H., & Carrier, J. (2001). A Review of Elasmobranch Reproductive Behavior with a Case Study on the Nurse Shark, Ginglymostoma Cirratum Environmental Biology of Fishes, 60 (1/3), 157-188 DOI: 10.1023/A:1007656126281

WONG, E., SHIVJI, M., & HANNER, R. (2009). Identifying sharks with DNA barcodes: assessing the utility of a nucleotide diagnostic approach Molecular Ecology Resources, 9, 243-256 DOI: 10.1111/j.1755-0998.2009.02653.x