Ocean acidification may ground swimming skates | Science News for Students

Ocean acidification may ground swimming skates

In warmer and more acidic seas, little skates develop stronger jaws but weaker fins
Feb 28, 2019 — 6:35 am EST
an underwater photo of a little skate

This is a little skate, a fish closely related to sharks and rays. As the oceans get more acidic, little skates may become better walkers.

Page Valentine/USGS; adapted by L. Steenblik Hwang

The more carbon dioxide people pump into Earth’s atmosphere, warming the planet, the more carbon dioxide and heat slip into the oceans. Warmer and more acidic water is thinning the shells of oysters. But they aren’t the only ocean dwellers facing unwanted home renovations. Anything with a shell is at risk. So, too, may be certain fish. A new study finds that as seawater becomes more acidic, the skeleton of a fish called the little skate stiffens in some places — and gets bendier in others. That could leave this fish better built for walking than swimming.

Burning fossil fuels, such as oil and coal, releases carbon dioxide, or CO2. In the air, this and other greenhouse gases act like a sweater, trapping heat and raising near-surface temperatures around the globe.

The oceans also absorb CO2. And there it combines with a molecule known as calcium carbonate. This produces a new molecule, bicarbonate. The reaction leaves calcium and hydrogen atoms stranded. Those free hydrogen atoms act as an acid to lower the ocean’s pH.

This ocean acidification strongly affects organisms that make shells from calcium carbonate, such as oysters and lobsters. Fish have a built-in system for dealing with changes in ocean pH. Vertebrates — animals with backbones — tend to keep their blood’s pH levels under strict control, says Sean Bignami. He’s a marine biologist at Concordia University in Irvine, Calif. These animals, he notes, “can’t survive with a lot of variation [in blood pH].” To prevent their blood pH from changing, fish have to be pH micromanagers.

If a fish’s blood gets too acidic, it extracts a molecule, calcium phosphate, from its food. (This is a calcium atom attached to a phosphate group: a phosphorus atom and four oxygen atoms.) The phosphate is alkaline. And this tends to neutralize the pH of the fish’s blood. Fish are so good at this that they can adjust to acidic waters in only a few hours.

Skating by?

a CT image of a little skate showing the cartilage in its skeleton
This is a CT image of a little skate. The close-up images on the right show the cartilage in its skeleton.
V. Di Santo/Proceedings of the Royal Society B 2019

Valentina Di Santo is a physiologist — someone who studies how living organisms function. She works at Harvard University in Cambridge, Mass. Many physiologists had assumed that fish could easily deal with warmer, more acidic waters. But she was not sure. So she launched studies using little skates.

Closely related to rays and sharks, skates are shaped like round, flat discs with pointy tails. (“They are so sweet!” Di Santo says — “very social. And they get used to you and like to get tickled.”)

Little skates grow to about 50 centimeters (20 inches). Like sharks, they don’t actually have bones. Instead, their skeletons are made of cartilage, like the strong connective tissue in your nose. This cartilage is surrounded by an outer layer of tiny, calcium-filled tiles called tesserae (TESS-er-ay).

Di Santo wanted to examine the calcium in these tesserae. She used a scanning technique called computerized tomography, or CT. Tesserae stand out nicely on the CT scans. This allowed De Santo to see how they changed over time.

Little skates lay flat, rectangular eggs in a leathery covering, sometimes called a “mermaid’s purse.” Di Santo gathered 40 eggs and exposed some to normal seawater. Others got warmer and more acidic ocean water — what scientists predict the seas may be like by 2100 if people keep pumping CO2 into the air. The whole time, Di Santo scanned them regularly with a CT scanner. Their leathery coverings allowed her to keep an eye on the skates’ skeletons as they developed.

Skinny skates, higher stakes

Skates from eggs that grew and hatched in warmer, more acidic waters became wider and flatter adults than did hatchlings raised in normal water. The jaws of skates raised in the more acidified water became tougher, with more hardened tesserae. Their wings, though, were more flexible. Di Santo published her findings January 9 in Proceedings of the Royal Society B.

a photo of the egg case of a little skate against a white background
This is the egg case of a little skate, sometimes called a “mermaid’s purse.”
Invertzoo/Wikimedia Commons (CC BY-SA 3.0)

A flatter skate is not a better skate. “They’re skinny,” and that’s bad, Di Santo notes. What’s more, she adds, “They need to eat more at high temperatures.” And extra snacking comes with a potential cost. “Eating more means exploring the environment more often and being more exposed to [predators].”

The skate’s new skeletons might also change how they behave, she suspects. Little skates use parts of their jaws to “walk” along the seafloor. A stiffer jaw with more calcium could make them better hikers. And a heavier skeleton with more calcium could also keep them on the seafloor more often, she notes, because it would take more energy to lift off.

And skates raised in warm, acidic water might have to use more energy to keep their extra-bendy wings stiff as they swam. Di Santo worries that this extra effort might mean that skates “aren’t likely to fare well in future conditions.”

Past studies had looked at how ocean acidification might affect skeletal features in fish with bones, Bignami notes, and “didn’t find much.” But the CT scans Di Santo used gave her much more information than the methods used in those other studies.

The effects she saw, such as a slightly harder jaws and softer fins, are small, Bigmani admits. But “those can end up being pretty meaningful when you pile small effects on each other.” Together with fishing, habitat loss or other stressors, the ocean’s pH changes might break a skate.

Power Words

(more about Power Words)

acidic     An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.

acidification     A process that lowers the pH of a solution. When carbon dioxide dissolves in water, it triggers chemical reactions that create carbonic acid.

alkaline     An adjective that describes a chemical that produces hydroxide ions (OH-) in a solution. These solutions are also referred to as basic — as in the opposite of acidic — and have a pH above 7.

atmosphere     The envelope of gases surrounding Earth or another planet.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

biology  The study of living things. The scientists who study them are known as biologists.

calcium     A chemical element which is common in minerals of the Earth’s crust and in sea salt. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.

calcium carbonate     The main chemical compound in limestone, a rock made from the tiny shells of ancient marine organisms. Its formula is CaCO3 (meaning it contains one calcium atom, one carbon atom and three oxygen atoms). It’s also the active ingredient in some antacid medicines (ones used to neutralize stomach acids).

carbonate     A group of minerals, including those that make up limestone, which contains carbon and oxygen.

carbon dioxide (or CO2)    A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

cartilage     (adj. cartilaginous) A type of strong connective tissue often found in joints, the nose and ear. In certain primitive fishes, such as sharks and rays, cartilage provides an internal structure — or skeleton — for their bodies.

computerized tomography     (CT, for short). A special kind of X-ray scanning technology that produces cross-sectional views of the inside of a bone or body.

connective tissue     Certain groups of cells that attach to form the boundaries for — and interfaces between — many structures throughout the body.

egg     The unfertilized reproductive cell made by females.

element     A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

embryo     The early stages of a developing organism, or animal with a backbone, consisting only one or a few cells. As an adjective, the term would be embryonic — and could be used to refer to the early stages or life of a system or technology.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

habitat     The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

hydrogen     The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues. It’s made of a single proton (which serves as its nucleus) orbited by a single electron.

marine     Having to do with the ocean world or environment.

marine biologist     A scientist who studies creatures that live in ocean water, from bacteria and shellfish to kelp and whales.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

pH     A measure of a solution’s acidity or alkalinity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, called bases, have a pH higher than 7; again, the farther above 7, the stronger the base.

phosphate     A chemical containing one atom of phosphorus and four atoms of oxygen. It is a component of bones, hard white tooth enamel, and some minerals such as apatite.

phosphate group     A bound group of  five atoms — one phosphorus and four oxygens — that plays an important role in many reactions within living things.

phosphorus     A highly reactive, nonmetallic element occurring naturally in phosphates. Its scientific symbol is P. It is an important part of many chemicals and structures that are found in cells, such as membranes, and DNA.

physiologist     A scientist who studies the branch of biology that deals with how the bodies of healthy organisms function under normal circumstances.

predator     (adjective: predatory) A creature that preys on other animals for most or all of its food.

rays     (in biology) Members of the shark family, these kite-shaped fish species resemble a flattened shark with wide fins that resemble wings.

scanner     A machine that runs some sort of light (which includes anything from X-rays to infrared energy) over a person or object to get a succession of images. When a computer brings these images together, they can provide a motion picture of something or can offer a three-dimensional view through the target. Such systems are often used to see inside the human body or solid objects without breaching their surface.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

tissue     Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

vertebrate     The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes amphibians, reptiles, birds, mammals and most fish.


Journal:​ ​​V. Di Santo. Ocean acidification and warming affect skeletal mineralization in a marine fish. Proceedings of the Royal Society B. Vol. 268, January 9, 2019. doi: 10.1098/rspb.2018.2187.