Dr Lauren Peel and the team from the Save Our Seas Foundation D’Arros Research Centre (SOSF-DRC) set about testing what reef manta rays were eating. They found that 45% of the rays' diet consisted of pelagic zooplankton, tiny animals near the ocean’s surface. Another 38% of their diet was made up of emergent zooplankton that rise from the seabed at night, while the remaining 17% comprised mesopelagic plankton, found in deeper waters further offshore.
Lauren’s study showed that reef mantas were munching pelagic zooplankton in the St Joseph Channel during the day, and then socialising at a coral reef cleaning station to the north of D’Arros Island. The researchers believe that while these mantas fraternise and are being deep-cleaned, they deposit their faeces that may be packed with nutrients picked up from feeding at night, in deeper waters, and in different areas. Thus reef manta faeces could be a fertiliser of sorts, helping corals to flourish. While reef manta rays are not the only factor contributing to the health of these coral reef systems, studies like this help us tease apart a complex web of life that may be disrupted if we push its resilience too far.
But what do reef manta rays do for us? Coral reefs are incredible ecosystems worth billions to our economies, feeding people with the fish that live there and providing ecotourism income. Healthy coral reefs can protect coastlines from huge storms and they are important to many people for recreational, cultural and spiritual reasons. If reef manta rays are contributing in any way to the better functioning of these beleaguered systems, then we can be grateful for the indirect effects of these ocean gardeners.
As we continue to gather incredible stories of how sharks, rays and their kin function in the oceans, there are other non-ecological reasons why sharks are important players in our marine ecosystems.
The long period of time that sharks have ruled the oceans has given them an evolutionary advantage over the relative ‘blink of an eye’ that human-beings have dominated the earth. What has enabled these animals to persist so successfully and for so long? We still have much to learn from them. Biomimicry experts (those innovators who look to nature to find sustainable solutions to modern-day problems) have used shark skin as inspiration for streamlining in aeronautical engineering and Olympic swimwear. Prof. Mahmood Shivji, the director of the Save Our Seas Foundation Shark Research Center in Florida, has shown that understanding the genetics of sharks might advance our own medical research and help us gain new insights into human disease, wound healing and how our own genetics function. We stand to lose a lot more than we know when we lose shark species from our oceans: we lose not only their current function, but their potential too.
Many philosophers and environmentalists would argue that we need sharks in our oceans simply because they have what is called an intrinsic value. They argue that nature has a value, irrespective of whether it benefits human-beings or not. The persistence of sharks in our oceans should have nothing to do with whether we need them or not: they have their own, intrinsic right to exist.
However, we can also show that sharks have a numeric value. We rely on sharks for income as fishers or tour operators, for recreation (many divers love to seek out the diversity of sharks while snorkelling or on scuba-diving) and for cultural or spiritual ties. Researcher Dr Ruth Leeney has detailed the deep ties that many communities still have with sharks and rays, celebrating their existence in art and stories and revering them in their belief systems. These are links to sharks and rays that are sometimes more difficult to make sense of than hard numbers and empirical evidence, but these animals' ability to help us make sense of our place in the natural world and keep us connected to it may be equally important.
Perhaps there’s something to be said for those who support the intrinsic value of nature. What might our world look like without the eccentricity of a wobbegong shark sailing, tassled and camouflaged, along the seafloor? What else do we stand to lose when the last whale shark earns a dive operator a final paycheque, or a reef manta no longer brings its well-travelled fertiliser to a reef? Could our planet, the only known one in our solar system with such vibrant life and bewildering diversity, simply just feel poorer for losing one more of the 500 species of sharks that have found how to make their way in the oceans over millennia?
Would we even notice?
A lot more funding and effort are needed to support research into exactly what role sharks play in the myriad of habitats and depths of the ocean. The trouble is, sound science and conservation action don’t always act on the same timescale. At times, the speed at which we’re damaging ecosystems outstrips our ability to investigate thoroughly how they function.
No one wants to jump to conclusions and most good scientists hold it is a matter of pride to conduct sound scientific studies. In law and philosophy, we are familiar with the idea of the precautionary principle. The idea is to pause, reflect and accumulate more information before making decisions that may have consequences that we cannot have foreseen.
Sharks matter. We might not yet know exactly how they all matter, or what role they play in every one of the diverse ocean ecosystems on earth, but the question to ask yourself after reading these snippets is whether you now too have an intuitive sense that we need sharks in our seas? The reasons may be as varied as the sharks themselves, and they might feel as personal as your own connection to the natural world. While our understanding of sharks grows and evolves, it appears that we’re going to need them if we’re to make sense of our changing world, and our place in it.
Lauren R. Peel, et al., 2019, Stable isotope analyses reveal unique trophic role of reef manta rays (Mobula alfredi) at a remote coral reef, Royal Publishing Society.
Michael R. Heithaus, et al., 2014, Seagrasses in the age of sea turtle conservation and shark overfishing, Frontiers in Marine Science.
Melanie Fyda, 2019, Foraging ecology and behavior of batoids and their influence on coastal sandflats .