Thursday, 5 January 2012

Evolutionary bouillabaisse: Hybrid sharks (but no laser beams)

To find a wild hybrid animal is unusual. To find clear evidence of F1 and backcrossed (B+) hybrid sharks in nature is extraordinary. To find 57 hybrids along 2,000 km of coastline is unprecedented. 
Morgan et al., 2011 

Who doesn’t love a good shark story? And the news of the first ever record of hybrid sharks ticks all the boxes. Sharks! In Australia! That place where everything is out to kill you! And they’re doing weird things! With their claspers! The media have been going for the climate change angle, because, well, everything that isn’t about life on Mars is a climate change story. But they’ve thrown in a bit of ‘OMG we’re doomed, hybrid sex-crazed super-sharks are ravaging our coastlines’ for colour*. But what’s been lost in most of the news pieces is the original reason for the research — management of shark fisheries.

The hybrids are the result of interbreeding between two closely related species of blacktip sharks (Carcharhinus). The Australian blacktip (Carcharhinus tilstoni) occurs in the tropical waters of northern Australia. The common blacktip (C. limbatus) has a much wider range in tropical and subtropical waters worldwide. Both species are important to the inshore shark fisheries in northern and eastern Australia. In order to manage these fisheries, it is imperative to understand population dynamics of the target species.

It can be difficult to distinguish between these two on morphology alone, so anatomical characters (such as size and pre-caudal vertebrae count) are combined with DNA analysis for a reliable identification. In this case, the initial analysis looked at mitochondrial DNA (mtDNA), which is maternally inherited.

During sampling from fisheries catch along the east coast, the research team found that 42 sharks returned odd results — the anatomical and DNA data contradicted each other. The sharks looked like one species, but had the mtDNA of the other. After ruling out other reasons for the mismatches, the team decided to explore the possibility that the two species were interbreeding.

They collected more samples and analysed both mtDNA (maternally inherited) and nuclear DNA (inherited from both parents). This technique allowed them to identify the species of both parents. They found 17 F1 (first cross or first filial) hybrids along the coast from Far North Queensland to northern New South Wales. In addition, they found a further 40 sharks that appeared to be the progeny of hybrids breeding together or of hybrids that had back-crossed (bred) with one of the parent species. Blacktip sharks had taken swinging to a whole new level.

This discovery throws up some juicy evolutionary questions. Are the hybrids as fit (in an evolutionary sense) as the parent species? That is, are they as successful in producing offspring? And do those offspring survive to reproduce in the same numbers as those of pure-bred sharks? If so, why isn’t everything a hybrid? What are the long-term impacts of hybridisation in warming waters? Does a hybrid have an advantage over pure-bred C. limbatus in tropical waters or pure-bred C. tilstoni in temperate waters? Did past hybridisation between other species lead to the high diversity of Carcharhinus we see today? The list goes on.

But this discovery also has implications for managing the blacktip fisheries. The research team identified several issues that need further investigation. Perhaps the most pressing of these is the possibility of inadvertent overharvesting. If hybrids are not as prolific in producing young as their parent species, then populations with a high proportion of hybrids could be at risk. They might not reproduce fast enough to survive commercial exploitation at the current rate. But the reverse is also true. If the hybrids are fitter, this could be good news for the industry. Maintaining both blacktip shark populations and shark fisheries requires a reassessment of stock structure, taking the hybrids into consideration.

The focus now is on mapping the geographical range of hybrids along the northern Australian coast; estimating the frequency of interbreeding; and investigating their fitness. And it might be necessary to include a third closely related blacktip species, the graceful shark (C. amblyrhynchoides), which was not studied in the initial work. Although it is not a major component of the shark fisheries, C. amblyrhynchoides coexists with the other two species along the northern coast. And who knows what it gets up to? It’s a shark! In Australia! It’s bound to do weird things with its claspers.


Morgan, J.A.T. et al. (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

Abstract available here.


*Might not accurately reflect the nature of most news stories.


Anonymous said...

You summarise it soooo much better than the journalists! Good job B.

Snail said...

Thanks, W. I loves me a good evo biol story, as you know, and if it involves hybrids, well, so much the better. *cough*Sharktopus*cough* But this paper was about fisheries and the implications of hybridisation on stocks. Even the darned press release missed that bit. With all the interest, I thought I'd present the findings for anyone wants to know, but doesn't have access to the paper.

I am very much looking forward to the evo stuff that'll be coming out of this discovery. I hope they won't make me wait too long!