Featured interviews, news coverage, and public engagement in marine science and conservation.
From 2040 onwards the average year for marine ecosystems is likely to be more extreme than the worst years experienced up until 2015, researchers say
Rachel Moore Imagine watching your favourite nature documentary. The predator lunges rapidly from its hiding place, jaws wide open, and the prey … suddenly goes limp. It looks dead. For some animals, this freeze response – called “tonic immobility” – can be a lifesaver. Possums famously “play dead” to avoid predators. So do rabbits, lizards, snakes, and even some insects. But what happens when a shark does it? In our recent study, we explored this strange behaviour in sharks, rays and their relatives. In this group, tonic immobility is triggered when the animal is turned upside down – it stops moving, its muscles relax, and it enters a trance-like state. Some scientists even use tonic immobility as a technique to safely handle certain shark species. But why does it happen? And does it actually help these marine predators survive? The mystery of the ‘frozen shark’ Despite being well documented across the animal kingdom, the reasons behind tonic immobility remain murky – especially in the ocean. It is generally thought of as an anti-predator defence. But there is no evidence to support this idea in sharks, and alternative hypotheses exist. We tested 13 species of sharks, rays, and a chimaera — a shark relative commonly referred to as a ghost shark — to see whether they entered tonic immobility when gently turned upside down underwater. Seven species did, but six did not. We then analysed these findings using evolutionary tools to map the behaviour across hundreds of million years of shark family history. So, why do some sharks freeze? Tonic immobility is triggered in sharks when they are turned upside down. Rachel Moore Three main hypotheses There are three main hypotheses to explain tonic immobility in sharks: Anti-predator strategy – “playing dead” to avoid being eaten Reproductive role – some male sharks invert females during mating, so perhaps tonic immobility helps reduce struggle Sensory overload response – a kind of shutdown during extreme stimulation. Our results don’t support any of these explanations. There’s no strong evidence sharks benefit from freezing when attacked. In fact, modern predators such as orcas can use this response against sharks by flipping them over to immobilise them and then remove their nutrient-rich livers – a deadly exploit. The reproductive hypothesis also falls short. Tonic immobility doesn’t differ between sexes, and remaining immobile could make females vulnerable to harmful or forced mating events. And the sensory overload idea? Untested and unverified. So, we offer a simpler explanation. Tonic immobility in sharks is likely an evolutionary relic. A case of evolutionary baggage Our evolutionary analysis suggests tonic immobility is “plesiomorphic” – an ancestral trait that was likely present in ancient sharks, rays and chimaeras. But as species evolved, many lost the behaviour. In fact, we found that tonic immobility was lost independently at least five times across different groups. Which raises the question: why? In some environments, freezing might actually be a bad idea. Small reef sharks and bottom-dwelling rays often squeeze through tight crevices in complex coral habitats when feeding or resting. Going limp in such settings could get them stuck – or worse. That means losing this behaviour might have actually been advantageous in these lineages. So, what does this all mean? Rather than a clever survival tactic, tonic immobility might just be “evolutionary baggage” – a behaviour that once served a purpose, but now persists in some species simply because it doesn’t do enough harm to be selected against. It’s a good reminder that not every trait in nature is adaptive. Some are just historical quirks. Our work helps challenge long-held assumptions about shark behaviour, and sheds light on the hidden evolutionary stories still unfolding in the ocean’s depths. Next time you hear about a shark “playing dead”, remember – it might just be muscle memory from a very, very long time ago. Jodie L. Rummer receives funding from the Australian Research Council. She is affiliated with the Australian Coral Reef Society, as President. Joel Gayford receives funding from the Northcote Trust.
Rachel Moore From hand-sized lantern sharks that glow in the deep sea to bus-sized whale sharks gliding through tropical waters, sharks come in all shapes and sizes. Despite these differences, they all face the same fundamental challenge: how to get oxygen, heat and nutrients to every part of their bodies efficiently. Our new study, published today in Royal Society Open Science, shows that sharks follow a centuries-old mathematical rule – the two-thirds scaling law – that predicts how body shape changes with size. This tells us something profound about how evolution works – and why size really does matter. What is the two-thirds scaling law? The basic idea is mathematical: surface area increases with the square of body length, while volume increases with the cube. That means surface area increases more slowly than volume, and the ratio between the two – crucial for many biological functions – decreases with size. This matters because many essential life processes happen at the surface: gas exchange in the lungs or gills, such as to take in oxygen or release carbon dioxide, but also heat loss through skin and nutrient uptake in the gut. These processes depend on surface area, while the demands they must meet – such as the crucial task of keeping the body supplied with oxygen – depend on volume. So, the surface area-to-volume ratio shapes how animals function. Whale sharks are as big as buses, while dwarf lanternsharks (pictured here) are as small as a human hand. Chip Clark/Smithsonian Institution Despite its central role in biology, this rule has only ever been rigorously tested in cells, tissues and small organisms such as insects. Until now. Why sharks? Sharks might seem like an unlikely group for testing an old mathematical theory, but they’re actually ideal. For starters, they span a huge range of sizes, from the tiny dwarf lantern shark (about 20 centimetres long) to the whale shark (which can exceed 20 metres). They also have diverse shapes and lifestyles – hammerheads, reef-dwellers, deep-sea hunters – each posing different challenges for physiology and movement. Plus, sharks are charismatic, ecologically important and increasingly under threat. Understanding their biology is both scientifically valuable and important for conservation. Sharks are ecologically important but are increasingly under threat. Rachel Moore How did we test the rule? We used high-resolution 3D models to digitally measure surface area and volume in 54 species of sharks. These models were created using open-source CT scans and photogrammetry, which involves using photographs to approximate a 3D structure. Until recently, these techniques were the domain of video game designers and special effects artists, not biologists. We refined the models in Blender, a powerful 3D software tool, and extracted surface and volume data for each species. Then we applied phylogenetic regression – a statistical method that accounts for shared evolutionary history – to see how closely shark shapes follow the predictions of the two-thirds rule. Sharks follow the two-thirds scaling rule almost perfectly, as seen in this 3D representation. Joel Gayford et al What did we find? The results were striking: sharks follow the two-thirds scaling rule almost perfectly, with surface area scaling to body volume raised to the power of 0.64 – just a 3% difference from the theoretical 0.67. This suggests something deeper is going on. Despite their wide range of forms and habitats, sharks seem to converge on the same basic body plan when it comes to surface area and volume. Why? One explanation is that what are known as “developmental constraints” – limits imposed by how animals grow and form in early life – make it difficult, or too costly, for sharks to deviate from this fundamental pattern. Changing surface area-to-volume ratios might require rewiring how tissues are allocated during embryonic development, something that evolution appears to avoid unless absolutely necessary. But why does it matter? This isn’t just academic. Many equations in biology, physiology and climate science rely on assumptions about surface area-to-volume ratios. These equations are used to model how animals regulate temperature, use oxygen, and respond to environmental stress. Until now, we haven’t had accurate data from large animals to test those assumptions. Our findings give researchers more confidence in using these models – not just for sharks, but potentially for other groups too. As we face accelerating climate change and biodiversity loss, understanding how animals of all sizes interact with their environments has never been more urgent. This study, powered by modern imaging tech and some old-school curiosity, brings us one step closer to that goal. Jodie L. Rummer receives funding from the Australian Research Council. She is affiliated with the Australian Coral Reef Society, as President. Joel Gayford receives funding from the Northcote Trust.
Jodie Rummer: For the love of sharks - Oceanographic Oceanographic Magazine
These parasitic sharks glow in the dark and are known to have a go at anything they come across, despite their small size
Experts fear for health of corals and other marine life as about 1m sq km of ocean experience prolonged elevated temperatures
The Bureau of Meteorology this week declared a 70% chance of an El Niño developing this year. This raises concern for the health of the Great Barrier Reef, which is under continuing threat from climate change. Recent summers have shown the devastating damage heat stress can wreak on the reef. We must act urgently to protect this underwater marvel – through this likely El Niño, and beyond. We are coral reef and climate scientists, and policy experts. We’ve seen how the Great Barrier Reef is nearing its tipping point. After this point, it will become unrecognisable as a functioning ecosystem. But the scale of climate threat is beyond the tools currently used to manage the Great Barrier Reef. New measures and sustained effort are needed – at local, national, and international scales – if we’re serious about saving this natural wonder. International treasure under threat The Great Barrier Reef is internationally renowned for its biodiversity, including more than 450 species of coral, 1,600 species of fish and 6,000 species of molluscs. It is also an economic workhorse, contributing about A$6 billion to the Australian economy and providing some 64,000 full-time jobs. Many industries and coastal communities in Queensland rely on a healthy Great Barrier Reef. But Australia’s reefs are in trouble and climate change is the biggest threat – bringing heatwaves, severe cyclones and more acidic oceans. The background temperature of the Great Barrier Reef has warmed by 0.8°C since 1910. This warming can couple with ocean temperature variability, such as from El Niño and its counterpart, La Niña. But because the Great Barrier Reef is already struggling under climate change, an El Niño could mean even more pressure. What the next Australian government must do to save the Great Barrier Reef The bathtub is filling We hope this analogy helps explain the situation. Imagine a bathtub. The water inside it represents global sea-surface temperature. When the bathtub was only half-full, temporary heat variability (from El Niño) caused splashes, but they were contained in the tub. Now fast-forward to the present day. For more than a century, humans have been heating the planet by burning fossil fuels. The background temperature has risen and the bathtub is now almost filled to the brim. Add a splash of heat from El Niño and the bath spills over. These splashes bring consequences: more mass bleaching of coral and, in severe cases, widespread coral death. El Niño and La Niña have become more variable in recent decades. This has meant more frequent and stronger events – bigger splashes in the bathtub – that pose a grave threat to the Great Barrier Reef’s health and biodiversity. All the while, the bathtub keeps filling. The World Meteorological Organisation reported that the next five years will be the warmest since records began. And 2023 will almost certainly be among the ten warmest years on record. Earth’s average temperature is predicted to exceed 1.5°C of warming in at least one of the next five years. This would produce a big splash – but it doesn’t represent the bathtub level reaching the brim. Under the global climate accord known as the Paris Agreement, nations are pursuing efforts to limit the average global temperature increase to 1.5°C above pre-industrial levels. Background warming beyond 1.5°C is widely considered by climate scientists as dangerous. We’re entering an era in which hot and more frequent splashes are imminent – and the survival of coral reefs is becoming increasingly threatened. Clearly, the global warming we’re seeing now is unprecedented. We must turn off the tap. An inadequate tool kit Unless global emissions are drastically reduced, frequent severe bleaching is projected this century for all 29 World Heritage-listed coral reefs. This would cause untold ecological damage. It would also reduce the reefs’ ability to support human communities that depend on them. Coral bleaching is not the only threat to the Great Barrier Reef. Other pressing problems include poor water quality from land-based runoff, crown-of-thorns starfish and unsustainable fishing and coastal development. So how do we deal with all of this? A range of management actions exists. Banning fishing in some areas and limiting exploitation elsewhere has benefited conservation, while also enhancing fisheries. But other actions have had mixed success. And not all available tools are being applied effectively. For example, “special management areas” were intended to restrict human use of the Great Barrier Reef for conservation or management purposes. But their use has been limited. And emergency implementations of these areas, allowed under the law, have never been used. Crucially, none of the available actions were designed to respond to climate threats. The reality is, the scale of climate disturbance is beyond the available management tools. We all know the Great Barrier Reef is in danger – the UN has just confirmed it. Again Source: Australian Academy of Science. What are we waiting for? The scientific evidence is unequivocal. We must work at local, national, and international scales to help the Great Barrier Reef better cope with climate change. The likely arrival of an El Niño makes this task ever more urgent. Australian and international governments must take immediate and decisive action on emissions reduction. This includes banning new coal and gas projects and rapidly shifting to renewable energy. Communities reliant on fossil-fuel industries should be helped to transition to new livelihoods. Reef management agencies need to tackle climate threats more effectively – at a scale commensurate with the problem. This requires a new way of managing key areas. That could mean, for example, temporarily closing off parts of the Great Barrier Reef affected by coral bleaching to give them a reprieve from other stressors such as fishing and tourism. And individuals must also ensure our everyday choices – in transport, consumption and elsewhere – help tackle the climate threat. It’s time for us all to double-down and ensure the survival of the Great Barrier Reef, and the planet. There is no room for complacency. So what are we waiting for? Adapt, move, or die: repeated coral bleaching leaves wildlife on the Great Barrier Reef with few options Scott F. Heron is receiving and has received funding from Australian Research Council, as well as from international government sources. Together with Jon Day, Scott developed the Climate Vulnerability Index (CVI) for World Heritage that has also been applied to assess climate impacts upon other areas of significance. Jodie L. Rummer has received funding from the Australian Research Council. She is the current Vice President of the Australian Coral Reef Society. Jon Day previously worked for the Great Barrier Reef Marine Park Authority between 1986 and 2014, and was one of the Directors at GBRMPA between 1998 and 2014. He represented Australia as one of the formal delegates to the World Heritage Committee between 2007-2011.
Kim Briers, Shutterstock If you’ve seen the hit animated film Finding Nemo, you might recall the character Dory singing the catchy tune “Just Keep Swimming” to help her clownfish friend Marlin make the long journey from the Great Barrier Reef to Sydney. In this case, art imitates life. Marathon swimming performances are a vital part of early life for the vast majority of coral reef fish. Baby (larval) reef fish – smaller than the size of your thumbnail – hatch from eggs laid on the reef and spend a few weeks in the open ocean before swimming back to the reef. But how does such a small creature make this impressive journey? Our research published today set out to answer this question. We found larval clownfish dramatically alter their physiology to complete their journey from the ocean back to the reef. In particular, they take in more oxygen per breath and at a faster rate than any other fish species studied to date. Essentially, this makes baby clownfish some of the smallest athletes on the planet. Just Keep Swimming from Finding Nemo. Read more: Dazzling or deceptive? The markings of coral reef fish Mini athletes swimming 10-50 body lengths per second Reef fish are vital to coral reef ecosystems. They play important roles in the food web, help keep the reef clean and recycle nutrients. Plus, their vibrant colours attract millions of tourists annually. Adult reef fish keep to a small patch. Their eggs are carried off by wave action into the open ocean, where they hatch and develop. Within a few weeks the tiny fish larvae must return to the reef. It’s a long, arduous journey that can last weeks to months. Depending on the species, they cover distances as far as 64 kilometres. So how do they do it? Until the 1990s, scientists believed the development of larval reef fish was like that of other fish such as herring, cod and flatfish. These species “go with the flow”, passively riding ocean currents until they become large and developed enough to actively swim on their own, against the currents. However, landmark studies from the early 1990s documented the impressive swimming capabilities of baby reef fish. It turns out reef fish are not passive particles after all. Previous research has provided overwhelming evidence coral reef fishes are capable of amazing swimming performance as babies. Some of these tiny athletes are capable of swimming 10-50 body lengths per second as a larva. For comparison, Olympic multi-gold medallist Michael Phelps races at just under two body lengths per second. When paired with well-developed sensory systems such as vision and the sense of smell, such impressive athletic performance enables these babies to “just keep swimming” with or against ocean currents until they find an optimal reef on which to settle. But 30 years after the discovery, we were still wondering how they manage it. Now we know. Clownfish eggs begin as tiny orange spots, but they soon start to lengthen and acquire visible eyes. Joe Belanger, Shutterstock Measuring the traits of an athlete My colleagues and I measured physiological traits required to be an athletic swimmer across the entire larval phase of a clownfish. These traits included swimming speed, oxygen uptake rates, gene expression patterns, and tolerance to low oxygen (hypoxia). Why hypoxia? At night, when it’s no longer possible to use sunshine and carbon dioxide to make energy by photosynthesis, corals and plants breathe in oxygen to make energy. This lowers oxygen levels on reefs. Larval reef fish returning home from the open ocean must prepare for such conditions. We found larval clownfish have the highest oxygen uptake rates of any fish to date. This supports elite swimming, growth and development. As they develop and swim faster, thousands of genes change. Genes that code for proteins that transport and store oxygen, such as haemoglobin and myoglobin (also found in our bodies), are especially important. They enable oxygen to be transported and stored during intense exercise and help retain oxygen in tissues when the fish experience hypoxia in their reef habitats. The changes in haemoglobin and myoglobin genes also correspond to when these baby fish start to increase their hypoxia tolerance. We’ve seen this before, in reverse. Salmon are one of the most studied fish of all time and, as adults, they’re pretty amazing athletes as well. However, baby salmon endure low oxygen conditions in the first few weeks of life, right after hatching, while they are hiding in the gravel of the freshwater riverbeds. And, sure enough, back in the 1980s, research showed salmon switch their haemoglobin too – right when the baby salmon have to transition from being hypoxia tolerant, to training to become elite swimmers. Why our research matters The changes in physiological machinery that we uncovered are key to survival for clownfish. It’s likely other coral reef fish follow similar developmental pathways. Reef fish – of all shapes, sizes, and colours – are integral for maintaining coral reef health and persistence of future coral reefs. This is crucial as climate change threatens these beautiful, delicate ecosystems. I studied what happens to reef fish after coral bleaching. What I saw still makes me nauseous Adam Downie receives funding from the University of Queensland, and the Goodman Foundation Research Grant Scheme through the Morton Bay Research station. He is a member of the Australian Society for Fish Biology. His past affiliations include the University of New Brunswick (BSc student) and James Cook University (PhD student). Jodie L. Rummer receives funding from the Australian Research Council and is the Vice President of the Australian Coral Reef Society.
Almost two-thirds of sharks and rays living on world’s coral reefs at risk, with 14 of 134 species reviewed critically endangered