An encounter by divers with grey reef sharks in Seychelles might have just transformed the science of sharks as we know it.
There is a pervasive idea among ocean lovers that sharks can never stop swimming, as they must continue to keep water passing across their gills in order to extract from it the life-giving supply of oxygen.
There is some partial truth to this. Most species of shark are ‘ram ventilators’; they ‘breathe’ by continuously swimming to force oxygen-rich water across their gills. Anybody who has dived with nurse sharks, wobbegong and whitetip reef sharks, however – all famous for lounging about not doing very much – knows that not all sharks need to keep moving.
It is also worth pointing out that sharks are negatively buoyant, so should they stop swimming while traversing the ocean depths, they would sink and suffocate in the low-oxygen content realm of the deep ocean.
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Once they’ve found a nice spot to rest, however, most ram-ventilating species of shark can continue to breathe by switching to a process known as buccal pumping – rhythmically opening and closing their jaws to force water through their gills.
Some large pelagic species of shark, such as the whale shark and great white, have lost this ability. They are – together with a handful of other species – known as ‘obligate ram-ventilators’, and they must, indeed, keep swimming to keep breathing; capable of resting only when their snouts face into the current.
Until very recently, grey reef sharks (Carcharhinus amblyrhynchos) were thought to be members of the obligate ram-ventilating club. That is until a group of research divers from the Save Our Seas Foundation spotted them resting, motionless, and apparently breathing by buccal pumping – under ledges in the coral reefs of Seychelles. The sighting has completely rewritten the scientific understanding of obligate ram-ventilators, and poses new questions about how sharks ‘sleep’.
There is very little research into sharks’ sleep behaviour – it is, after all, an incredibly difficult topic to study. There are some theories that sharks might sleep using just half their brains, a phenomenon observed in sperm whales and bottlenose dolphins, but no hard evidence as to how obligate ram-ventilators take their rest.
A paper published in light of the new observations has shed new light on the subject, which – as the authors write – has ‘implications for [many] studies that rely on the assumption that Carcharhinid sharks never stop swimming.’
‘On routine survey dives around D’Arros we found grey reef sharks resting under coral reef ledges,’ said Dr Robert Bullock, director of research at the Save Our Seas D’Arros Research Centre (SOSF-DRC) in Seychelles and lead author of the paper. ‘This is not something we believed they could do. The grey reef shark has been considered a ram-ventilating species, unable to rest, so to find these ones resting turns our fundamental understanding of them on its head.’
The researchers encountered grey reef sharks resting both alone and in groups at different sites around Seychelles. The sharks were opening and closing their jaws, but otherwise remained still and appeared to be unaware of their observers, an especially important observation as it implies the sharks – which would normally take flight when approached by divers – were exhibiting a pattern of sleep, rather than just rest.
Equally important was the observation that the sharks were facing in different directions, which puts to rest the idea that they might have been maintaining their ram-ventilation breathing by facing into an oncoming current.
And through it all, the sharks seemed blissfully unaware of their observers. It’s a key observation: scientists think that increases in arousal thresholds count as the hallmark of sleep rather than just rest. The sharks remained still, except for lower jaw movements that suggest these ram-ventilating sharks can switch to buccal pumping behaviour. With little to no currents at the sites and sharks at rest facing in all directions, it seems the idea that the only way they’d rest is facing into currents holds little water for grey reef sharks. Craig Foster, founder of the SeaChange Project, was one of the divers and authors of the paper. ‘
There is something very special,’ said Craig Foster, founder of the Sea Change Project and one of the divers and authors of the paper, ‘about “tiptoeing” around underwater at a depth of 25 metres and looking into the open eyes of sleeping sharks, moving carefully so as not to wake the peaceful beauties. ‘I love things that challenge our current thinking, and I’ve always thought of the grey reef shark as a clear example of a species that needs to swim to breathe. Clearly not from this discovery!’
The implication from this discovery is that other species of obligate ram-ventilating sharks may also be capable of switching to buccal pumping while resting and/or sleeping, which poses many further questions. It might be plausible to imagine a relatively small shark such scalloped hammerhead – of a similar size to grey reef sharks – taking a break under a ledge, but what about it’s larger cousin, the great hammerhead? Or great whites? Where would a fully-grown, 13m-long whale shark take a nap?
‘I hope that these findings serve as a reminder of how much we still do not know and how exciting that is,’ said Dr Bullock. ‘Science is about being wrong quite a lot. And that’s OK.’
The complete paper ‘Just keep swimming? Observations of resting behavior in gray reef sharks Carcharhinus amblyrhynchos‘ by Robert W Bullock, Craig Foster, and James S E Lea is published in the Journal of Fish Biology. For more from the work of the Save Our Seas Foundation, visit www.saveourseas.com
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