Today, scientists have described more than 540 unique species of sharks; of different shapes and sizes, with varying skin colours and patterns, contrasting diets and behaviours, living in all kinds of habitats, in every single ocean, all around the world. As they are all separate "species", by definition, this means that individuals from different species are not able to breed together, but is this always the case? In fact, there are certain places where some sharks are able to intermingle and have offspring. But how does this happen? And why? And what does it mean for the future of these species?
Plenty of Fish in the Sea
When scientists talk about different "species" of sharks, we are discussing more than just different skin colours or varying shapes to the fins. In fact what make any species a unique species is the fact that they are so fundamentally different from each other that they are not able to interbreed (Morgan et al, 2012; Pazmiño et al 2019).
This separation can happen for many different reasons. The cause may be physiological (with the anatomy of the contrasting species meaning they are not able to mate) or it may be genetic (if they have different numbers of chromosomes then the DNA from the two parents will not be able to align properly during fertilisation), or behavioural (with each species not finding the other to be attractive because they do not perform important mating rituals that a member of their own species would) or it could also simply be geographical (with two species living in isolated habitats, so they never meet) (Morgan et al, 2012; Pazmiño et al 2019).
Same Same, but Different
In some cases, certain closely-related are animals are able to breed, despite technically being different species. This is known as "hybridisation" and the offspring are known as "hybrids". However, hybridisation very commonly produces offspring that are sterile. For instance, in ligers, which are a mix of lion and tiger parents, or mules that are donkey-horse mixes (Morgan et al, 2012; Barker et al, 2019; Pazmiño et al 2019).
Hybridisation happens in areas where two very closely related and/or very similar species coexist. It is especially common when one of the parents species is especially rare, so individuals do not often encounter their own kind and they settle for a mate from a different species (Pazmiño et al 2019).
BLACKTIP SHARK FOOTAGE
Mixing It Up
The phenomenon of hybridisation is very little-studied in sharks and rays, but recently scientists have learned that in rare cases certain species are not only able to reproduce with an individual of another species, but sometimes they are actually able to produce viable offspring that can go on to breed themselves (Morgan et al, 2012; Barker et al, 2019; Pazmiño et al 2019).
Scientists have discovered viable hybrid sharks swimming around off the coast of Australia. Interestingly though, these hybrids are not simply a 50/50 mixture of their blacktip shark (Carcharhinus limbatus) and Australian blacktip (C. tilstoni) ancestors. Instead, the hybrids have then gone on to successsfully breed with either of their founding species (a process known as "backcrossing"). So this means that some hybrids are half and half, but others might be only 25/75 or even less. This population is refered to as the "C. limbatus–C. tilstoni species complex" because these individuals are technically neither, but also both of the parent species (Morgan et al, 2012).
Similarly, scientists have found hybridisation between dusky sharks (C. obscurus) and Galapagos sharks (C. galapagensis) occurs at several different sites around the world where their ranges overlap. Again, thanks to backcrossing, the hybrid offspring have a range of proportions of genetic material from their forbearers, with some being more dusky, some more Galapagos, and others closer to a 50/50 split between the two. As these diverse hybrids have continued to backcross with each of the parent species, we now see a whole spectrum of genetic mixtures (Pazmiño et al 2019).
The Best of Both?
For some species, hybridisation could be the next evolutionary stepping stone that will allow them to adapt to a changing world. By breeding with a completely different species, the offspring will have more genetic variation and so may be better adapted, or more flexible, to be able to cope in changing habitats. They may even be able to exploit a completely different ecological niche to their parents. That could be very important in a world that is being so drastically altered by climate change (Morgan et al, 2012; Pazmiño et al 2019).
However, in some cases, the mixing could actually lead to one of the parent species dying out. For example, scalloped hammerheads (Sphyrna lewini) and Carolina hammerheads (S. gilberti) are hybridising in the western Atlantic. The scalloped hammerheads are highly migratory, where Carolina hammerheads are "endemic" to a limited region and cannot be found anywhere else in the world, so they are especially sensitive to any kind of habitat degradation. Scientists are concerned that repeated backcrossing and introgression into their population could eventually wipe out all the genetically pure Carolina species (Barker et al, 2019).
As shark populations are in decline around the world, it seems likely that hybridisation between rare species could become more common in the future. The silver lining in that is that eventually we could see the process give rise to a whole new species (Morgan et al, 2012; Barker et al, 2019; Pazmiño et al 2019).
References
Barker AM, Adams, DH, Driggers III WB, Frazier BS & Portnoy DS (2019). Hybridization between sympatric hammerhead sharks in the western North Atlantic Ocean. Biology letters, 15:4, 20190004.
Morgan JA, Harry AV, Welch DJ, Street R, White J, Geraghty PT, Macbeth WG, Tobin A, Simpfendorfer CA & Ovenden, J. R. (2012). 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, 13, 455-463. Access online.
Pazmiño DA, van Herderden L, Simpfendorfer CA, Junge C, Donnellan SC, Hoyos-Padilla E. M, Duffy CAJ, Huveneers C, Gillanders BM, Butcher PA & Maes GE (2019). Introgressive hybridisation between two widespread sharks in the east Pacific region. Molecular Phylogenetics and Evolution, 136. Access online.
By Sophie A Maycock for SharkSpeak
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