How do gills work? Tully, aged 7
Great question, Tully! Animals on land breathe air, which is made up of different gasses. Oxygen is one of these gases, and is made by plants (hug a plant today and say thanks). All animals need to breathe in oxygen to survive.
When the air goes into our lungs, oxygen goes into our blood and is delivered all around the body. Air is light, so it’s easy to move around. This makes it pretty easy to breathe air back and forth — a bit like blowing up balloons and letting them deflate.
Things are different for fish. Fishes also need oxygen, but rather than getting it from air, they have to get it from water.
But there is less oxygen available in water than air. And to make matters worse for the poor fishes, water is thicker than air, so it takes much more work to move it around. This makes the problem of getting that oxygen in the fishes’ body even harder.
Rather than breathing in and out through the mouth, fish use a one-way system, passing water in one direction over their gills.
Water goes in the mouth, across the gills and out through the opercula (the bony covering protecting their gills).
But gills and lungs are more similar than you might think. Both have really big surface areas which increases the amount of water or air that touches the gill or lung tissue, and so increases the amount of oxygen available.
What’s more, the walls of the lungs and gills are very thin and loaded with tiny tubes that transport blood (called “capillaries”).
This means the capillaries come into close contact with the air or water outside, letting oxygen pass across the thin walls and into the blood. At the same time, carbon dioxide, which is a waste product from our bodies, passes out.
Gills are also important for controlling how much salt is in the body, but let’s leave that story for another day.
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Picture the scene. Swimming off Scotland’s west coast during a summer holiday you notice a large dark shark nearly 10 metres long headed towards you. A prominent triangular dorsal fin cuts the surface, the powerful rhythmically beating tail driving it silently through the cloudy green depths. You’re transfixed by a cavernous mouth large enough to swallow a seal.
Musing this may be your last swim, it might be surprising to learn this leviathan of the deep is a harmless yet endangered gentle giant. It has little interest in humans, focusing on some unseen bounty of the warmer summer waters: zooplankton, the tiny creatures found near the surface of the ocean.
This is the basking shark (Cetorhinus maximus), once common off western Europe, feeding on the annual plankton bonanza of the European shelf.
Our recent study suggests holidaymakers and basking sharks have much in common. They make temporary forays into these higher latitudes, travelling familiar routes with extended family, feeding on local fare at well-known places visited on previous trips.
Areas supporting high densities of zooplankton are like tourist traps, drawing basking sharks from across the Atlantic in late spring and summer. Hundreds converge in inshore surface waters on the Scottish west coast, Ireland and Isle of Man.
Once hunted for its oily liver across all oceans, basking sharks in the the north-east Atlantic were primarily targeted, with more than 80,000 slaughtered in the second half of the 20th century. This earned the world’s second biggest fish (after the whale shark) a place on the International Union for Conservation of Nature´s Red List. A critical indicator of biodiversity, this catalogue of species under threat of global extinction makes depressing reading.
Conservation management of the basking shark demands knowledge of its ecology and movement patterns. These slow-swimming coastal predators easily traverse the equator and ocean basins, moving from one legislative domain to another. Identifying important feeding sites and routes popular for annual migrations can therefore help countries enact effective protection.
Difficult to track and observe, satellite tagging has revealed shark movements, showing use of the ocean throughout a year. One study suggests that basking sharks have an attachment to particular areas, returning annually to feeding sites, a behaviour known as seasonal site fidelity.
Such localities are candidates for protection, designated Marine Protected Areas (MPAs), and ensure sharks remain undisturbed during sensitive and important life stages. But tagging informs us mainly about individual movements, leaving crucial conservation questions unanswered.
Our study focused on developing genetic markers to identify individuals and establish their migration routes, population connectivity and size. We also wanted to explore basking sharks’ genetic diversity – an indicator of a species’ ability to future proof against environmental change, and kinship of feeding clusters.
But developing tools removed only one obstacle. Another was lack of routine DNA sampling of basking shark groups. A breakthrough came when, in desperation, we discovered skin mucus from a tail swipe against a boat was a DNA source. Routine swabbing of basking shark groups – quickly and with minimal disturbance – provided genetic profiles of more than 400 individuals and a snapshot of those travelling together.
This register identified individuals arriving at summer feeding sites, revealing that sharks were re-sighted within seasons and again in later years, sometimes around the same date at sampling locations only kilometres apart. This supports findings of basking sharks repeatedly visiting feeding sites in the recently designated Sea of the Hebrides MPA. Ominously, our study also indicates the Irish Sea is an important migration route – an area of increasing human activity.
We expected the roaming and mixing of cosmopolitan, filter-feeders that live long lives to erode genetic differences between populations. But regular sampling of feeding groups revealed basking sharks off the coast of Ireland in spring (perhaps having wintered near the US) were genetically distinct from north-east Atlantic populations. This differentiation was explained when genetic snapshots made up family albums.
We found that basking shark groups consist of related individuals, indicating a tendency to travel prescribed seasonal migration routes as extended family parties. It would seem the family that feeds together, stays together.
Cetaceans often travel as kin groups, perhaps facilitating learning of migration routes and encouraging cooperative behaviours. This could mean that basking shark groups also exhibit complex behaviours. Certainly, they don’t fit the lone shark stereotype.
Until our study, the perception was that they moved into warmer waters from widespread locations, sniffing out a plankton meal, collecting as groups of unrelated individuals – like gourmands headed into the city, chancing on finding a good restaurant by smell.
Now it looks like basking sharks carry “road maps” of gourmet venues, taking the family along. Perhaps travelling together allows young kin to learn accurate navigation, and maybe many noses are better at sniffing out a meal of densely packed zooplankton.
Conservation biologists fret about genetic variation of threatened species. Large marine creatures have low rates of reproduction and consist of small populations. This means they accumulate genetic variation more slowly than the tiny, populous, rapidly reproducing plankton they eat. That lack of evolutionary currency slows responses to environmental change. In an important conservation milestone, our genetic estimates suggest a north-east Atlantic basking shark population not exceeding 10,000 individuals.
Worse still, most variation is distributed amongst families, so loss of kin groups erodes genetic variation rapidly – as when basking sharks were hunted, and as occurs now during accidental bycatch, when fishing vessels trap unwanted marine creatures in their nets.
Such population size and structure, coupled with tendencies to frequent inshore feeding areas earmarked for development of marine renewables such as windfarms, may not produce a happy outcome without intelligent management of such environments.
When it comes to basking shark conservation we have to remember that in a rapidly changing world, family matters.
Catherine S Jones, Senior Lecturer, Biological Sciences, University of Aberdeen; Leslie Noble, Professor of Aquatic Biosciences, Nord University, and Lilian Lieber, Research Fellow in the Bryden Centre, School of Chemistry and Chemical Engineering, Queen’s University Belfast
They may have been around for hundreds of millions of years — long before trees — but today sharks and rays are are among the most threatened animals in the world, largely because of overfishing and habitat loss.
Climate change adds another overarching stressor to the mix. So how will sharks cope as the ocean heats up?
Our new research looked at Port Jackson sharks to find out. We found individual sharks adapt in different ways, depending where they came from.
Port Jackson sharks from cooler waters in the Great Australian Bight found it harder to cope with rising temperatures than those living in the warmer water from Jervis Bay in New South Wales.
This is important because it goes against the general assumption that species in warmer, tropical waters are at the greatest risk of climate change. It also illustrates that we shouldn’t assume all populations in one species respond to climate change in the same way, as it can lead to over- or underestimating their sensitivity.
But before we explore this further, let’s look at what exactly sharks will be exposed to in the coming years.
In Australia, the grim reality of climate change is already upon us: we’re seeing intense marine heat waves and coral bleaching events, the disappearance of entire kelp forests, mangrove forest dieback and the continent-wide shifting of marine life.
The southeast of Australia is a global change hotspot, with water temperatures rising at three to four times the global average. In addition to rising water temperatures, oceans are becoming more acidic and the amount of oxygen is declining.
Any one of these factors is cause for concern, but all three may also be acting together.
One may argue sharks have been around for millions of years and survived multiple climate catastrophes, including several global mass extinctions events.
To that, we say life in the anthropocene is characterised by changes in temperature and levels of carbon dioxide on a scale not seen for more than three million years.
Rapid climate change represents an existential threat to all life on Earth and sharks can’t evolve fast enough to keep up because they tend to be long-lived with low reproductive output (they don’t have many pups). The time between generations is just too long to respond via natural selection.
When it comes to dealing with rising water temperature, sharks have two options: they can change their physiology to adapt, or move towards the poles to cooler waters.
Moving to cooler waters is one of the more obvious responses to climate change, while subtle impacts on physiology, as we studied, have largely been ignored to date. However, they can have big impacts on individual, and ultimately species, distributions and survival.
We collected Port Jackson sharks from cold water around Adelaide and warm water in Jervis Bay. After increasing temperatures by 3℃, we studied their thermal limits (how much heat the sharks could take before losing equilibrium), swimming activity and their resting metabolic rate.
While all populations could adjust their thermal limits, their metabolic rate and swimming activity depended on where the sharks were originally collected from.
With a rise in water temperature of just 3℃, the energy required to survive is more than twice that of current day temperatures for the Port Jackson sharks in Adelaide.
The massive shift in energy demand we observed in the Adelaide sharks means they have to prioritise survival (coping mechanisms) over other processes, such as growth and reproduction. This is consistent with several other shark species that have slower growth when exposed to warmer waters, including epaulette sharks and bonnethead sharks.
On the other hand, a 3℃ temperature rise hardly affected the energy demands of the Port Jackson sharks from Jervis Bay at all.
Discovering what drives responses to heat is important for identifying broader patterns. For example, the decreased sensitivity of the Jervis Bay sharks likely reflects the thermal history of the region.
Australia’s southeastern coastline is warmed by the East Australian Current, which varies in strength both throughout the year and from year to year. With each generation exposed to these naturally variable conditions, populations along this coastline have likely become more tolerant to heat.
Populations in the Great Australian Bight, in contrast, don’t experience such variability, which may make them more susceptible to climate change.
So why is this important? When sharks change their behaviour it affects the whole ecosystem.
The implications range from shifts in fish stocks to conservation management, such as where marine reserves are assigned.
Sharks and rays generally rank at the top or in the middle of the food chain, and
have critical ecosystem functions.
Port Jackson sharks, for example, are predators of urchins, and urchins feed on kelp forests — a rich habitat for hundreds of marine species. If the number of sharks decline in a region and the number of urchins increase, then it could lead to the loss of kelp forests.
There’s little research dedicated to understanding how individuals from different populations within species respond to climate change.
We need more of this kind of research, because it can help identify hidden resilience within species, and also highlight populations at greatest risk. We have seen this in action in coral bleaching events in different parts of Australia, for example.
We also need a better handle on how a wide range of species will respond to a changing climate. This will help us understand how communities and ecosystems might fragment, as each ecosystem component responds to warming in different ways and at different speeds.
Steps need to be taken to address these holes in our knowledge base if we’re to prepare for what follows.
A recent global assessment of shark populations at 371 coral reefs in 58 countries found no sharks at almost 20% of reefs and alarmingly low numbers at many others.
The study, which involved over 100 scientists under the Global FinPrint project, gave New Zealand a good score card. But because it focused on coral reefs, it included only one region — Rangitāhua (Kermadec Islands), a pristine subtropical archipelago surrounded by New Zealand’s largest marine reserve.
It is a different story around the main islands of New Zealand. Many coastal shark species may be in decline, and less than half a percent of territorial waters is protected by marine reserves.
In New Zealand, there are more than a hundred species of sharks, rays and chimaeras. They belong to a group of fishes called chondrichthyans, which have skeletons of cartilage instead of bone.
Some 55% of New Zealand’s chondrichthyan species are listed as “not threatened” by the International Union for Conservation of Nature (IUCN). Not so encouraging is the 32% of species listed as “data deficient”, meaning we don’t know the status of their populations. Most species (77%) live in waters deeper than 200 metres.
Seven species are fully protected under the Wildlife Act 1953. They are mostly large, migratory species such as the giant manta ray. Some are threatened with extinction according to the IUCN, including great white sharks, basking sharks, whale sharks and oceanic white tip sharks.
Historically, basking sharks were caught as bycatch in New Zealand fisheries, and seen in their hundreds in some inshore areas. Sightings of these giant plankton-feeders suddenly dried up over a decade ago. We don’t know why.
Eleven chondrichthyan species are fished commercially in New Zealand under the quota management system. Commercial fisheries for school shark, rig and elephant fish took off from the 1970s and now catch around 8,000 tonnes per year in total.
Finning of sharks has been illegal throughout New Zealand since 2014.
Most of New Zealand’s shark fisheries are considered sustainable. But a sustainable fishery can mean sustained at low levels, and we must tread carefully. School shark was recently added to the critically endangered list after the collapse of fisheries in Australia and elsewhere, and there’s a lot we don’t know about the New Zealand population.
We do know sharks were much more abundant in pre-European times. In Tīkapa Moana (Hauraki Gulf), sharks have since declined by an estimated 86%. An ongoing planning process provides some hope for the ecosystems of the gulf.
Not surprisingly, the global assessment found a ban on shark fishing to be the most effective intervention to protect sharks. Several countries have recently established large shark sanctuaries, sometimes covering entire exclusive economic zones.
These countries tend to have ecotourism industries that provide economic incentives for protection — live sharks can be more valuable than dead ones.
Other effective interventions are restrictions on fishing gear, such as longlines and set nets.
Waters within 12 nautical miles of the Kermadec Islands have been protected by a marine reserve since 1990. In 2015, the Kermadec Ocean Sanctuary was announced but progress has stalled. The sanctuary would extend the boundaries to the exclusive economic zone, some 200 nautical miles offshore, and increase the protected area 83-fold.
A large population of Galapagos sharks, which prefer isolated islands surrounded by deep ocean, thrive around the Kermadec Islands but are found nowhere else in New Zealand. Great white sharks also visit en route to the tropics. Many other species are found only at the Kermadecs, including three sharks and a sex-changing giant limpet as big as a saucer.
What makes the Global FinPrint project so valuable is that it uses a standard survey method, allowing data to be compared across the globe. The method uses a video camera pointed at a canister of bait. This contraption is put on the seafloor for an hour, then we watch the videos and count the sharks.
Baited cameras have been used in a few places in New Zealand but there are no systematic surveys at a national scale. We lack fundamental knowledge about the distribution and abundance of sharks in our coastal waters, and how they compare to the rest of the world.
Satellite tags are another technological boon for shark research. It is difficult to protect sharks without knowing where they go and what habitats they use. Electronic tags that transmit positional data via satellite can be attached to live sharks, revealing the details of their movements. Some have crossed oceans.
Sharks have patrolled the seas for more than 400 million years. In a few decades, demand for shark meat and fins has reduced their numbers by around 90%.
Sharks are generally more vulnerable to exploitation than other fishes. While a young bony fish can release tens of millions of eggs in a day, mature sharks lay a few eggs or give birth to a few live young. Females take many years to reach sexual maturity and, in some species, only reproduce once every two or three years.
These biological characteristics mean their populations are quick to collapse and slow to rebuild. They need careful management informed by science. It’s time New Zealand put more resources into understanding our oldest and most vulnerable fishes, and the far-flung subtropical waters in which they rule.
It might surprise you to learn that nuclear bomb tests during the Cold War are now helping conserve whale sharks, the largest living fish.
Growing up to 18 metres – longer than the average bus – whale sharks live in all tropical oceans. In Australia, they are found off tropical coasts in the north, particularly in Western Australia.
Whale sharks face a number of threats. Globally they are listed as endangered, and their numbers continue to decline.
Until recently, key information about the life history of whale sharks was missing, which prevented informed choices about how they were managed. In particular, scientists were not able to accurately assess their age and growth patterns.
Our research, published today in Frontiers in Marine Science, changes that. We examined the skeleton of whale sharks, using carbon from Cold War atomic bomb testing as a “time stamp” to reveal their true age. The findings will help protect these beautiful animals into the future.
Whale sharks are placid “filter feeders”, which basically means they eat by opening their massive mouths and straining small fish and plankton that pass through the gills.
They are covered in a pattern of stripes and spots that provide camouflage as they bask at the surface. Whale sharks’ gentle nature and striking appearance has made them a drawcard for tourists who pay to snorkel or dive with the animals.
Whale shark ecotourism is big business. At Ningaloo Reef off Western Australia, the industry is worth an estimated A$12.5 million per year.
The industry is also valuable for small island nations such as the Maldives and developing countries including the Philippines and Indonesia. It has lifted thousands of villagers from poverty and provided an impetus for governments to protect whale sharks.
But all is not plain sailing for these animals. In some parts of the world they are hunted for their fins, meat, oil and skin. The flesh resembles tofu when cooked, and is a popular menu item in parts of Asia, particularly China.
When shipping lanes are established near whale shark habitat, the animals are frequently struck by vessels and either die or suffer propeller injuries such as fin amputation. Their habit of basking at the surface of the ocean during the day puts whale sharks at particular risk of ship strike.
This combined with other threats – such as warming sea surface temperatures due to climate change – has created an uncertain future for these charismatic and valuable animals.
Just how vulnerable whale shark populations are to these threats is not clear. Growth rates of fish species – or how many years they take to reach a certain size – determine their resilience, and how fast populations are likely to recover if severely damaged.
But determining the age of whale sharks has, to date, been very difficult. Their vertebrae feature distinct bands, similar to the rings of a tree trunk, which increase in number as the animal grows older. But the bands could not conclusively be used to determine age because some scientists believed a ring formed every year, but others suggested one formed every six months.
To settle the debate, we turned to the radioactive legacy of the Cold War’s nuclear arms race – specifically, carbon-14.
Carbon-14 is a naturally occurring radioactive element. But in the 1950s and early 1960s, nuclear weapons tests by the US, Soviet Union, Great Britain, France and China released enormous amounts of carbon-14 into the air.
It travelled into the world’s oceans, and into every living organism on the planet – including the skeletons and shells of animals.
We analysed the vertebrae of two whale sharks collected many years ago in Taiwan and Pakistan. By counting back from the peak carbon-14 level, we concluded the rings were formed once per year. This meant that for the first time, the age and growth rate of a whale shark could accurately be determined; a 10-metre shark was 50 years old.
We know whale sharks can grow to almost twice the length of the animals we analysed, and have been estimated to live as long as 100 years. The results of our study makes that prediction now seem more likely.
Slow-growing species with long lifespans are typically very susceptible to threats such as fishing. This is because it takes many years for animals to reach reproduction age, and the rate at which individuals are replaced is very slow.
Our study explains why fisheries targeting whale sharks almost immediately collapse: the species is not built to cope with the added pressures of human harvests.
Whale sharks populations take a very long time to recover from over-harvesting. Governments and management agencies must work together to ensure this iconic animal persists in tropical oceans – for both the future of the species, and the many communities whose livelihoods depend on whale shark ecotourism.
Whale sharks swim near surface to keep warm
Finding a species that’s entirely new to science is always exciting, and so we were delighted to be a part of the discovery of two new sixgill sawsharks (called Pliotrema kajae and Pliotrema annae) off the coast of East Africa.
We know very little about sawsharks. Until now, only one sixgill species (Pliotrema warreni) was recognised. But we know sawsharks are carnivores, living on a diet of fish, crustaceans and squid. They use their serrated snouts to kill their prey and, with quick side-to-side slashes, break them up into bite-sized chunks.
Sawsharks look similar to sawfish (which are actually rays), but they are much smaller. Sawsharks grow to around 1.5 metres in length, compared to 7 metres for a sawfish and they also have barbels (fish “whiskers”), which sawfish lack. Sawsharks have gills on the side of their heads, whereas sawfish have them on the underside of their bodies.
Together with our colleagues, we discovered these two new sawsharks while researching small-scale fisheries that were operating off the coasts of Madagascar and Zanzibar. While the discovery of these extraordinary and interesting sharks is a wonder in itself, it also highlights how much is still unknown about biodiversity in coastal waters around the world, and how vulnerable it may be to poorly monitored and managed fisheries.
Despite what their name might suggest, small-scale fisheries employ around 95% of the world’s fishers and are an incredibly important source of food and money, particularly in tropical developing countries. These fisheries usually operate close to the coast in some of the world’s most important biodiversity hotspots, such as coral reefs, mangrove forests and seagrass beds.
For most small-scale fisheries, there is very little information available about their fishing effort – that is, how many fishers there are, and where, when and how they fish, as well as exactly what they catch. Without this, it’s very difficult for governments to develop management programmes that can ensure sustainable fishing and protect the ecosystems and livelihoods of the fishers and the communities that depend on them.
While the small-scale fisheries of East Africa and the nearby islands are not well documented, we do know that there are at least half a million small-scale fishers using upwards of 150,000 boats. That’s a lot of fishing. While each fisher and boat may not catch that many fish each day, with so many operating, it really starts to add up. Many use nets – either driftnets floating at the surface or gillnets, which are anchored close to the sea floor. Both are cheap but not very selective with what they catch. Some use longlines, which are effective at catching big fish, including sharks and rays.
In 2019, our team reported that catch records were massively underreporting the number of sharks and rays caught in East Africa and the nearby islands. With the discovery of two new species here – a global hotspot for shark and ray biodiversity – the need to properly assess the impact of small-scale fisheries on marine life is even more urgent.
How many other unidentified sharks and other species are commonly caught in these fisheries? There is a real risk of species going extinct before they’re even discovered.
Efforts to monitor and manage fisheries in this region, and globally, must be expanded to prevent biodiversity loss and to develop sustainable fisheries. There are simple methods available that can work on small boats where monitoring is currently absent, including using cameras to document what’s caught.
The discovery of two new sixgill sawsharks also demonstrates the value of scientists working with local communities. Without the participation of fishers we may never have found these animals. From simple assessments all the way through to developing methods to alter catches and manage fisheries, it’s our goal to make fisheries sustainable and preserve the long-term future of species like these sawsharks, the ecosystems they live in and the communities that rely on them for generations to come.
Shark skin might look perfectly smooth, but inspect it under a microscope and you’ll notice something strange. The entire outer surface of a shark’s body is actually covered in sharp, little scales known as denticles. More remarkable still, these denticles are incredibly similar to human teeth, as they’re also comprised of dentine and enamel-like materials.
Your dentist will no doubt have warned you that acidic drinks like fizzy cola damage your teeth. This is because acid can dissolve the calcium and phosphate in the enamel tooth covering. For the first time, scientists have discovered a similar process acting on the tooth-like scales of sharks in the ocean.
The carbon dioxide (CO₂) that humans release into the atmosphere doesn’t just heat the planet. As more of it dissolves in the ocean, it’s gradually increasing the acidity of seawater. In the past 200 years, the ocean has absorbed 525 billion tonnes of CO₂ and become 30% more acidic as a result. Now scientists worry that the lower pH is affecting one of the ocean’s top predators.
Over hundreds of millions of years, the denticles that make up shark skin have evolved to allow sharks to thrive in different environments. Different species have distinct denticle shapes and patterns that enable a range of remarkable functions. I’ve spent the last four years attempting to understand how the development of these scales is genetically controlled in shark embryos, and how their intricate details give each species an edge.
Denticles have highly specialised ridges which help reduce drag by up to 10%, allowing sharks to swim further and faster while using less energy. This works in a similar fashion to the ridges in the hulls of speed boats, which help the vessel move more efficiently through the water. In fact, these scales are so effective at reducing drag that scientists and engineers have long tried to create shark skin-inspired materials for boats and aircraft that can help them travel further on less fuel.
The patterning of denticles also works as a defensive armour, which protects sharks from their environment and from other predators. Some female sharks – such as the small-spotted catshark – have even developed a region of enlarged denticles which provide protection from a male shark’s bites during mating.
The changing chemistry of the ocean has been linked to coral bleaching, but its effect on other marine animals is less clear. To address this, researchers exposed puffadder shysharks – a species found off the coast of South Africa – to different levels of acidity in aquariums, and used a high-resolution imaging technique to examine the effect of acid exposure on their skin. After just nine weeks, they found that increased water acidity had weakened the surfaces of their denticles.
Corrosion and weakening of the denticle surface could degrade the highly specialised drag-reducing ridges, affecting the ability of these sharks to swim and hunt. Many shark species are top-level predators, so if they’re not able to hunt as effectively, this might have an unpredictable impact on the population size of their prey and other animals in the complex marine environment. Some species of shark need to swim constantly to keep oxygen-rich water flowing over their gills and to expel CO₂ – another process which might be hindered by increased drag.
Sharks belong to an ancient group of vertebrates known as the cartilaginous fishes, which split from the bony fishes – a lineage that later gave rise to humans – roughly 450 million years ago. Sharks, and other cartilaginous fish like rays, arose long before the dinosaurs, and have outlived multiple mass extinction events. But the rate of change in the marine environment over the last few centuries is an unprecedented challenge. These ancient predators may struggle to adapt to the fastest known change in ocean chemistry in the last 50 million years.