Ocean News

Sleeping beauties?

By Lauren De Vos, 14th December 2023

The sight of grey reef sharks resting in Seychelles changes what we know about how they breathe

‘We were diving on a deepish reef and came to a little overhang where I spotted some tail fins poking out.’


Dr James Lea is describing an extraordinary observation on a dive around D’Arros Island in Seychelles that, at first glance, seemed all too ordinary. But it was, as he puts it, ‘a pure chance encounter that blew our minds’. While an island in Seychelles may well be the kind of place where it’s easy to imagine rest and relaxation, the image of sharks at rest – indeed, sharks staying still at all – is contrary to much of what we picture for most of these stereotypically swift predators. ‘I initially assumed that they were nurse sharks, because that’s what we usually see hiding under rocks, but as I got closer it was clear that these were actually the tails of grey reef sharks!’

James is the CEO of the Save Our Seas Foundation (SOSF) and one of three authors of a landmark report that blows what we know about grey reef sharks – how they breathe and how they move – out of the water. ‘We all got very excited, and here we are now,’ he says of the paper ‘Just keep swimming? Observations of resting behaviour in grey reef sharks Carcharhinus amblyrhynchos (Bleeker, 1856)’, which was published this month in Journal of Fish Biology.

The grey reef shark is an Endangered reef dweller that looks like the quintessential perpetually-in-motion sleek shark. With its classic requiem shark shape, this species from the Carcharinid family is a familiar sight at coral reef dive sites in the Pacific and Indian oceans. ‘On routine survey dives around D’Arros we found grey reef sharks resting under coral reef ledges. This is significant because this is not something we believed they could do,’ explains Dr Robert Bullock, the director of research at the Save Our Seas Foundation D’Arros Research Centre (SOSF-DRC).

Photo © Christopher Leon

The science of how sharks sleep and breathe is linked, and while all sharks breathe using gills, there are two ways that they move oxygen-rich sea water over those gills. Some sharks, called obligate ram ventilators, ‘ram’ the sea water over their gills and need to keep moving to do so. But other species, called buccal pumpers, actively pump sea water over their gills while stationary. Science today tells us that sharks can be still – and there is some suggestion that they sleep too. But there is no solid evidence of sleep behaviour in ram ventilators. Scientists have hypothesised that they don’t sleep at all, or they sleep using half their brain (like sperm whales and bottlenose dolphins) or they sleep facing into ocean currents.

‘Grey reef sharks have been generally considered the stereotypical ram-ventilating shark, unable to rest, so to find them resting really turns the fundamentals of our understanding of these sharks on its head,’ says Robert. ‘The finding also has knock-on impacts for the assumptions we make when doing science with these sharks.’ What the researchers have found updates what we know about how grey reef sharks breathe. It was assumed that, based on their morphology (their body form and structure), grey reef sharks – and most Carcharinidae sharks – must keep swimming to push oxygen over their gills. But this latest observation suggests that they can somehow ventilate their gills for at least 40 minutes and that they are capable of periods of rest. These two changes in our understanding will affect how we study their metabolism, energy consumption and behaviour in the future. And in the scant science on shark sleep, the fact that these resting sharks did not respond to the presence of the divers adds to the hypothesis that to accept that sleep occurs in sharks, our definitions should perhaps include factors like increased arousal thresholds as an indicator of sleep behaviour.

‘I love things that challenge our current thinking, and I’ve always thought of grey reef sharks as a clear example of a shark species that needs to swim to breathe. Clearly not from this discovery!’ James is enthusiastic about these observations, not least because, as he puts it, ‘Sharks and sleep are two are my favourite things! And there’s always room for more of both.’ He is excited about where this finding might lead. ‘It raises all kinds of other questions. How are they coping? For how long do they sleep? How often? We have so much to learn still, and to me that’s really exciting.’

For Robert, the observations were confirmation once again of the value of the SOSF-DRC and its remarkable location. ‘D’Arros is an incredible place for observing nature, and having the facilities and ongoing research here certainly promotes these kinds of discoveries,’ he says. ‘I call it accidental science. Some of the most interesting findings in science have come from accidental discovery – the “poster boy” being penicillin!

‘It’s cool because this is a well-studied species and we are working hard to uncover more intricate things about its biology, physiology, behaviour and ecology – and yet an observation like this can come along to remind us just how much we still don’t know.’ It’s an exciting discovery, but is it really an important one? Robert believes it is. ‘Conservation is founded on a thorough scientific understanding, so interesting discoveries such as this always have the potential to inform and interact with conservation in the future. In this instance in particular, this update to our understanding of how grey reef sharks breathe could help us to better interpret this species’ physiological responses to environmental change.’

James agrees and expands on the idea. ‘For the conservation of sharks, it’s really key to apply our understanding of how they use their environment, and also how this may change in response to shifts in environmental conditions. How important is this rest, or possible sleep, for the sharks? And what’s the impact on them if they can’t get that rest if conditions change – if oxygen levels, for example, rise or fall – due to climate change?’

Craig Foster, founder of the SeaChange Project and one of the paper’s authors, adds another philosophical perspective on what shouldn’t be interpreted as purely a ‘cool shark science discovery’ (although that it certainly is!). For Craig, it’s clear that the better we understand the natural world, the better we know ourselves. ‘Our relationship with the natural world defines us as humans,’ he says. ‘To know our wild kin is to know our true nature, to know ourselves and to know our original mother, nature.’ And knowing ourselves relative to our wild kin, and how we are woven into this planet’s tapestry of life, is both vital and healing for us. ‘Nature is the mirror of our wild psyche,’ he reflects. ‘Without her we dwindle, we become a shadow of our ancestral form.’

Photo © Christopher Leon

On a planet beset with conservation challenges, in the midst of catastrophic biodiversity loss and in the face of our rapidly changing climate, it’s this intimacy with other wild beings that Craig feels is fundamental to conservation success; without kinship, we have no real hope of meaningful action. ‘Knowing how our shark kin sleep is to be closer to their fascinating world and to wake up from our own slumber and realise we cannot live without these magnificent marine beings.’

The observations also serve as a humbling reminder of what we have left to learn about our blue home planet. Somewhere in the brash arrogance of our space conquests and the heedless speed at which we hurtle towards an AI future, we risk losing sight of where our greatest treasures and insights lie. NASA’s BioSCape project is an example of how our eyes being constantly fixed on distant horizons has blurred what’s right in front of us. Given all the resources that NASA has trained on exploring the stars and distant planetary systems, its most recent endeavours reflect the realisation that our richest reserves of diversity and interest still lie right here on earth. NASA has turned its satellites back onto our biospheres. With that in mind, Craig’s philosophy of finding what’s relatable between us and our wildest counterparts might help us not only to see that we’re tied to a common future, but to understand that our common ancestries and histories are too strong to be strained by talk of interstellar escapes.

‘We know so little about most animals in the sea, it’s not surprising to me that we are finding out these things,’ says Craig. ‘And there is something very special about “tiptoeing” around underwater at a depth of 25 metres, looking into the open eyes of sleeping sharks and moving carefully so as not to wake the peaceful beauties.’

The authors all agree that this discovery says a lot about our own relationship with the natural world. ‘I hope that these findings serve as a reminder of how much we still do not know and how exciting that is. Science is about being wrong quite a lot. And that’s OK,’ says Robert. And if science is the lens, then storytelling is the mode of communication that will translate findings like this into empathy – and action. ‘I hope,’ concludes Craig, ‘that publishing stories and scientific information about these beautiful sharks makes people care about them and that we can collectively establish reservoirs of hope where these animals can breed and thrive.’