Behind the Science: Elasmobranch Movement and Climate change
Author Natasha Hynes
Climate change has many effects on the planet, from extreme temperatures to rising sea levels to loss of biodiversity. Two effects that have a major impact on the ocean environment are rising temperatures and ocean acidification. An increase in carbon dioxide and other greenhouse gasses has led to higher temperatures around the world, including in the ocean. The ocean also acts as a “carbon sink”, absorbing extra carbon dioxide from the atmosphere. Unfortunately, this uptake of carbon dioxide results in a product called carbonic acid. Increased carbonic acid in the ocean has caused a phenomenon called ocean acidification. Researchers recently reviewed the ways increased temperature and ocean acidification can affect elasmobranchs.
Figure from Vilmar and Di Santo 2022. Climate stressors and their possible effects on elasmobranchs.
Ocean warming can affect different elasmobranch species in various ways. A study on little skates (Leucoraja erinacea) showed that warmer ocean temperatures can cause a decrease in the mineralization of their pectoral fins, which makes them less stiff. With more flexible pectoral fins, skates need to spend more energy to achieve the same speed.
Warmer ocean temperatures may also lead to smaller body sizes in juvenile epaulette sharks (Hemiscyllium ocellatum)! While it’s thought that smaller elasmobranch species could be more tolerant to increased heat than larger species, it can still cost them more energy to move around, like in the example of the little skate. For larger species that may be fast swimmers, heterothermic (warm-blooded), or ram-ventilators, warmer temperatures could cause an evenmore pronounced effect. With all those extra metabolic demands, it’s thought that they might not have enough energy to deal with the stress that comes with increased ocean temperatures.
Elasmobranchs are millions of years old. They evolved during a time when there was more carbon dioxide in the atmosphere than there is today. Because of this, some people think that elasmobranchs may be able to handle ocean acidification better than we think. Could this be true?
Coming back to our little skate: under higher atmospheric carbon dioxide, mineralization of the jaws and crura increased. The crura are modified appendages that help skates “walk” across the seafloor. Higher mineralization of the crura might help them “walk” better. But there’s a catch - higher mineralization increases body weight, which will cost the skate more energy to swim. Another effect of ocean acidification is the erosion of dermal denticles. Dermal denticles are the teeth-like scales on shark skin. The erosion of these scales can cause the shark to become less hydrodynamic and lose swimming efficiency.
So what’s a shark to do? Ocean warming and acidification is happening at such a fast pace. It’s unlikely that elasmobranch species will be able to evolve to adapt to these changes. Therefore, they will have to deal with these stressors through changes in behaviour and distribution. You may have heard that certain species will move towards the poles in response to warmer ocean waters, but researchers note that some species may actually move to deeper waters. These location shifts can result in changes in food web dynamics and restricted home ranges. A change in location also assumes that elasmobranch species are able to make these migrations and that there is a more suitable habitat out there. While large pelagic sharks can “easily” cross vast distances, smaller, benthic species can’t move as far. These smaller species, however, might be more tolerant to changes in temperature and pH due to their lower metabolic rates.
Figure from Vilmar and Di Santo 2022. Climate stressors and their possible effects on elasmobranchs swimming.
As the climate continues to change, it will be necessary to keep studying how it can affect our oceans. In particular, there needs to be more research on individual effects of climate change stressors (i.e. ocean warming, acidification, etc.) as well as the combined effects of these stressors on different elasmobranch species. A better understanding of our changing world gives us access to the most cutting-edge information that can be used to aid in the conservation of vulnerable species, like many elasmobranch species.
Vilmar M and Di Santo V. 2022. Swimming performance of sharks and rays under climate
Change. Rev Fish Biol Fisheries. https://doi.org/10.1007/s11160-022-09706-x.