The great hammerhead shark

Sharks are an ancient and extremely diverse lineage of animals comprising more than 500 species. One of the most novel body designs in this fascinating group is that of the hammerheads, which have curiously expanded and flattened heads and eyes that, cartoon-like, are spaced far apart. Indeed, the hammerhead’s shape conjures up a caricature of a creature from outer space! But evolution works in unpredictable ways, and this peculiar head shape offers a clear benefit by improving the shark’s field of vision. It is also thought to enhance its sense of smell and its electroreceptivity. Based on these features, one would think the hammerheads were well equipped to thrive, but they also have an Achilles heel – and one that is not driven by evolutionary selection. Unfortunately, their survival is threatened by the fact that their large fins are highly valued in the world’s shark-fin markets.

Among the many shark populations, and sometimes even entire species, that have been fished unsustainably, the large-bodied hammerhead species are prime examples. To improve the management of shark fisheries, we need two crucial pieces of information: which species are involved in trade, and the geographic origin of shark products. But getting this information has been difficult. In the first place, shark products are typically traded as detached fins, or as carcasses without fins, heads or tails. Secondly, the fact that our knowledge about their global genetic structure, genetic connectivity and population dynamics is limited hampers our ability to understand local and international patterns of exploitation and to identify regional hotspots of overfishing for individual species and their populations.

The great hammerhead shark Sphyrna mokarran is the largest of the nine hammerhead species and is found around the world in tropical coastal seas. Although it is not typically targeted by commercial fisheries, there is cause for significant management and conservation concern. Great hammerheads are often caught incidentally as by-catch and, as they are extraordinarily sensitive to the trauma of being captured, the mortality rate is high. Additionally, the high market value of their large fins provides incentive for opportunistic fishing. In fact, our previous research on the shark-fin trade has shown that fins from more than two million hammerhead sharks, mainly the great, scalloped and smooth hammerheads, have been traded in the global fin market each year. And of these three species, the great hammerhead’s fins appear to be preferred, fetching the highest prices.

Concern about the future of the large-bodied hammerheads is clearly warranted. Studies indicate that all hammerheads have declined by 89% in the north-western and west–central Atlantic and by 99% in the Mediterranean. The abundance of the great hammerhead specifically has declined by about 80% off the coast of South Africa and in the eastern Atlantic. The species is listed as Endangered on the International Union for Conservation of Nature (IUCN) Red List of Threatened SpeciesTM and in March 2013 it was listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), with a deadline for implementing this listing by September 2014. Given its dire status worldwide and the limited scientific information that is available to guide its conservation and management, there is a pressing need to gather more information about the biology of the great hammerhead, as well as about its genetic structure and diversity, migration patterns, mating systems and breeding and pupping grounds.

At the SOSSRC, research on the genetics of the great hammerhead is a priority. We start with conservation-related questions that can be uniquely addressed by the in-depth analysis of DNA taken from individuals across the species’ global distribution. Our results are revealing the genetic structure of great hammerhead populations and to what extent regional populations are connected. They have also enabled us to estimate current genetic diversity, which is being used to get an idea of trends in the genetic health of the population. In addition, we are gaining insight into the migratory behaviour of male and female great hammerheads, particularly whether females return repeatedly to specific places to breed or give birth, a behaviour known as philopatry.

With this basic information in hand, we plan to move into the realm of wildlife forensics by designing rapid molecular tests to identify the geographic origin of great hammerhead products in the marketplace. We will be able to use these forensic tools to evaluate fishing patterns, which may indicate disproportionate harvest levels in certain regions and tell us whether great hammerheads are being fished illegally in protected areas or are being transported globally in contravention of international laws or agreements. This information will indicate where additional national and international protection needs to be put in place.

Another area being actively investigated at the SOSSRC is the mating system of great hammerheads. Understanding the sex lives of endangered animals is important for effectively managing their populations. For example, a species’ genetic diversity – and ultimately its ability to adapt to changing environmental conditions – can be directly influenced by whether females mate with multiple males (polyandry) or whether more than one male fathers the offspring in a single litter (multiple paternity). Although shark mating is rarely observed in the wild, DNA taken from mothers and their offspring can provide insight into the sharks’ sex life. Earlier research conducted in our lab has shown that female bonnetheads (a small hammerhead species) have an unexpectedly low incidence of multiple paternity in their litters. Even more surprisingly, females of this species can reproduce by virgin birth, also known as parthenogenesis! One wonders what other surprises lie in wait as we continue to examine the mating behaviour of great hammerheads.