Barriers in a sea of sharks and rays: The role of genetic connectivity in the ocean

When we think of elasmobranchs (sharks, rays and skates), most people picture large predators like great white sharks or the peaceful plankton-feeders like oceanic manta rays that roam vast and interconnected oceans. But elasmobranchs come in various body sizes and shapes and live in diverse habitats, from shallow tropical reefs to polar waters and the deep sea. 

Spotted eagle ray © Harriet Gardiner

As humans we often perceive the ocean as one giant body of water, where marine species can move wherever they want to. But in fact, that is not true for most animals living in the oceans. The ocean is a patchwork of diverse aquatic environments subdivided by landmasses, changes in ocean depth, temperature and salinity. Different marine species are adapted to patches of specific environments. For example, leopard seals are adapted to and can only survive in the cold waters of the Antarctic, and clownfish can only survive in the tropical waters of the Indo-Pacific region. Patches of similar in environments are often separated by so-called ‘marine barriers’. In the same way that a population of Galapagos land iguanas on Baltra island can’t breed with land iguanas on Santiago island because there is a barrier of water between them – transitions between aquatic environments pose barriers to the movements of marine animals and the exchange of genetic information between populations of the same species that live similar environments at a different location. 

Max Hirschfeld is a tropical ecologist and science partner on Galapagos Conservation Trust’s (GCT) Endangered Sharks of Galapagos Programme. He is looking into the barriers of genetic connectivity in sharks and rays. This blog explains a little more about his work and the importance of his findings for sharks and rays found in the Galapagos Marine Reserve.   

A scientific poster by Max summarising this newly published research

What is genetic connectivity? 

Genetic connectivity is created when individual animals from one population move to another to breed. In this way they carry their genes, in the form of DNA, across the landscape (or seascape) and contribute them to the population where they breed. This so called “gene flow” is vital to maintain the genetic diversity of natural populations. Populations with a higher genetic diversity are more likely to cope with threats, such as climate change, overfishing and pollution. Marine barriers affect the movement, breeding, and gene flow among populations of the same species. On the one hand, these barriers occur naturally, generate diversity among populations, and are a driving force behind the evolution of species. However, small populations that are isolated through barriers, typical for species that live in oceanic islands, may have lower genetic diversity and can’t be replenished from other populations, making them more vulnerable to man-made threats, such as overfishing.

Golden rays - © Sophie Stanek
Golden rays – © Sophie Stanek

What are marine barriers? 

A new study by Max and his colleagues looked at over 173 papers on the genetic structure of 70 species of sharks and 32 batoids (rays and skates). They found over 45 unique marine barriers. Some of these barriers include deep ocean trenches, drastic changes in temperature and salinity, ocean currents and even large river deltas. These barriers can limit genetic connectivity in sharks and rays at large, to surprisingly small distances. Some large oceanic species, like the basking shark, can maintain global connectivity. But smaller species that live close to the seafloor, like the Pacific angel shark, can’t move across deeper ocean to breed with other animals within a 100 km distance. The impact of barriers on connectivity also depends on the ecology of individual species. The study found that ecological factors like the habitat a species lives in, how deep it can dive, and its body size, are good indicators for its capacity to move across potential barriers. 

Why is this study important? 

Studying the connectivity of elasmobranch populations and marine species, in general, is central to fisheries and conservation management because it provides an estimate of long-term resilience to, and capacity to recover from, exploitation and climate change. These issues affect many of the sharks of Galapagos, such as the Endangered whale shark and Critically Endangered scalloped hammerhead shark, which may be caught by international fishing fleets when they travel beyond the boundaries of the Galapagos Marine Reserve . This study provides a guide for future research that will help to provide sound scientific evidence to protect Galapagos’ shark and ray species and support GCT’s work on the Galapagos-Cocos Swimway.  

Read the full paper in Global Ecology and Biography.  doi.org/10.1111/geb.13379

Find out more 

Learn more about GCT’s work to protect the Endangered Sharks of Galapagos. Adopt a scalloped hammerhead shark, or adopt Marti the hammerhead shark, the lead character in our very first children’s storybook.