The dense aggregations of scalloped hammerheads (Sphryna lewini) that form off remote islands in the Eastern Tropical Pacific (ETP) are one of nature’s greatest spectacles. Thousands of individuals, mostly females, can be seen at any one time. When seen from beneath it gives the appearance of a seemingly endless canopy of sharks, like clouds drifting across the sky. The species itself is iconic; a large, active, predatory shark, with an extremely distinctive t-shaped head (known as a ‘cephalofoil’) which, as the name suggests, has a scalloped edge.
Unfortunately, there are concerns that one day these aggregations might become features of the past. Scalloped hammerheads are a Critically Endangered species, fished extensively for their meat and fins across the region. Several measures are currently in place to protect them. On an international scale, they are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), which means that global trade in this species is regulated. They are also on Appendix II of the Convention on Migratory Species (CMS), an international treaty between governments to coordinate conservation efforts of species that cross jurisdictional borders. Regionally, the sites within the ETP where scalloped hammerheads commonly aggregate – Darwin and Wolf islands in the Galápagos, Malpelo island in Colombia, and Coco Island in Costa Rica – are all UNESCO World Heritage Sites.
However, scalloped hammerheads are a highly migratory species, travelling over 1000km between habitats. This includes nursery grounds. In the ETP, females give birth in coastal waters, where the young will remain for the first years of their lives. This propensity to move regularly and over long distances often takes them outside Marine Protected Area boundaries, where they become vulnerable to high levels of Illegal, Unreported and Unregulated (IUU) fishing. It is therefore vital that conservation management strategies for the species look at not only aggregations in one specific place, but how scalloped hammerheads move between areas within their entire range and how their population is structured over time – which, in a very basic sense, is known as population dynamics. If we understand this, we can make better inferences into the genetic health of a population (which will be explained later on) and make informed decisions about how to protect them across their entire distribution.
A team of scientists led by Sydney Harned, graduate student in the SOSF Shark Research Centre and Guy Harvey Research Institute at Nova Southeastern University, Florida, USA, set out to use population genetics to fill in these knowledge gaps relating to scalloped hammerheads in the ETP. The dataset was collected in collaboration with local researchers in Ecuador, Colombia, Costa Rica and Panama, a regional effort coordinated by scientists at the Charles Darwin Foundation in the Galapagos Islands. This is the largest sample size-based investigation of population structure, distribution and genetic diversity of this species in the region to date. For the study, which has just been published in the journal Ecology and Evolution, scientists gathered genetic data from tissue samples of scalloped hammerheads from 14 different locations across the globe, 12 of which were within the ETP. Analysis revealed some core findings that hold important implications for the conservation and management of the species.
One key finding was that adult scalloped hammerheads from the ETP, including those aggregating in the Galápagos Marine Reserve, had a higher genetic diversity than those from other regions. Why is this important? Every single living thing has DNA, which contains the instructions that dictate how organisms are built. Differences in the DNA result in different species, but smaller differences can also produce variations within species. It’s why we have different coloured eyes, for example. The more differences between individuals in a species, or within a population of that species, the higher we say the genetic diversity is. This is important because it means that there are many different characteristics within a species, which gives a greater chance that some individuals in the population will be able to adapt if the environment changes. For example, if a species has a high genetic diversity and the temperature rises, there is a higher chance at least some individuals will have the genetic makeup that allows them to cope with warmer conditions. This means that the species as a whole has a greater potential to survive. With a low genetic diversity, most individuals will have very similar traits and so they are less able to adapt to change, meaning the species is at greater risk of dying out.
The fact that the scalloped hammerheads in the ETP have comparatively higher genetic diversity, despite being a Critically Endangered species, is an optimistic sign. It means they are more able to adapt and potentially be more resilient in the face of IUU fishing and other threats, like climate change.
Another question the scientists set out to answer in the paper was whether female scalloped hammerheads were exhibiting regional philopatry. Regional philopatry is a behaviour where animals habitually return to the region of their birth, to breed as an adult. The analysis showed a high genetic connectivity, meaning that females were actually moving around coastal nursery sites rather than returning to the same area every time to give birth. To understand the benefits of this, we need to go back to the DNA. We’ve already established genetic diversity is a good thing, and gene flow – the movement of genetic material between populations – contributes to it. Imagine mixing food colouring. If you keep adding drops of blue dye into a bowl filled with blue water, you’ll end up with gradually darkening shades of blue. But, drop your blue dye into a bowl filled with red water, and you might end up with purple. If we translate this to the scalloped hammerheads, by travelling among sites they are widening their gene pool, bringing their own genes to a population that might not have been exposed to them and creating the possibility for new genetic combinations (increasing genetic diversity).
But this also carries important considerations from a practical perspective. If scalloped hammerheads are utilising areas across the whole ETP – which stretches from southern Mexico to northern Peru – then they are crossing jurisdictional borders. This means that all countries within the ETP need to coordinate conservation and management actions in order for protective measures to be effective.
The study, although primarily focussed on scalloped hammerheads within the ETP, also compared the genetic data from this population with that of others from additional parts of the species’ global range. The results demonstrate that scalloped hammerheads globally can be divided into three distinct evolutionary lineages based on their geography: Atlantic, Western Indian and Eastern Indian-Pacific. These lineages correspond to three major ocean basins where scalloped hammerheads are found. This means that, although they are all the same species and share the same common ancestor, the populations in each ocean basin have their own unique evolutionary history and diversity. A bit like your family tree – you and your cousin may share some of the same ancestral relatives, but in between them and you are a bunch of different descendants.
The new research also highlights that not only are the populations different in their evolutionary history, but they are also different in levels of genetic diversity. This means that management strategies will have to reflect these differences, and be tailored to the individual lineage, rather than the species as a whole.
In summary, this new research carries a message of hope for the legendary scalloped hammerheads of the ETP. The comparatively high genetic diversity and connectivity of these hammerheads suggests that they may be better equipped – genetically speaking – to face threats to their survival. But they cannot survive based on genetics alone. The findings also have clear implications for conservation strategies, in that they must reflect the widespread habitat use of such a mobile species, and aim to combat IUU fishing practices outside of reserve boundaries. Hopefully, we might ensure that the iconic aggregations of this species in the ETP continue for many years to come.
**References: Harned, Sydney; Bernard, Andrea; Salinas de Leon, Pelayo; Mehlrose, Marissa; Suarez, Jenifer; Robles, Yolani; Bessudo, Sandra; Ladino, Felipe; López Garo, Andrés; Zanella, Ilena; Feldheim, Kevin, Mahmood S. Shivji. 2022. Genetic population dynamics of the critically endangered scalloped hammerhead shark (Sphyrna lewini) in the Eastern Tropical Pacific. Ecology and Evolution.