Traces from nuclear-weapons tests offer clues to whale sharks’ ages

Researchers studying growth bands in the fish’s vertebrae find these animals can live at least a half century

Traces from nuclear-weapons tests offer clues to whale sharks’ ages

It’s been surprisingly hard to figure out how old a whale shark is. Now scientists have gleaned clues ones written in the the sharks’ spines. The clues are visible thanks to the radioactive residues. They were left by nuclear-weapons tests during the Cold War.

As they grow, the vertebrae of whale sharks (Rhincodon typus) build up alternating stripes of opaque and see-through tissue. The process is similar to how tree trunks grow rings. Until now, scientists hadn’t known how fast those whale shark vertebrae gain a new growth band. Is it each year? Every six months? Not knowing that made it hard to gauge just how fast these sharks were growing or how long they lived.

Now researchers have measured levels of carbon-14 in the vertebrae of two whale sharks. That carbon is a radioactive isotope (energy-shedding form of the element). Both whales had lived during the 20th century. Soviet and American nuclear weapons tests in the 1950s and 1960s released that carbon-14 as fallout. Over time, it built up in Earth’s atmosphere and oceans — and in the bodies of animals that lived in the oceans. The researchers compared the amount of carbon-14 in the growth bands of whale shark vertebrae with the known carbon-14 levels in surface seawater in different years. That helped the researchers estimate when each band had formed.

The results showed that bands generally grew one year apart. Steven Campana and his team shared what they learned in the April 2020 Frontiers in Marine Science.

The alternating dark and light bands in this slice of a whale shark’s vertebra are growth bands. They form as the fish ages. Here, each band is marked with a black dot. A new study finds that a new growth band forms each year. They point to a whale shark’s age. The one shown here, found in Taiwan, lived about 18 years.

Campana is a fisheries scientist at the University of Iceland in Reykjavík. His group also used the total number of growth bands in sharks’ dated vertebrae to figure out their the animals’ ages. Suggest and share, one 10-meter (33-foot) male found in Taiwan had been about 35 years old when it died. A female of about the same size, collected in Pakistan, lived to be around 50 years old.

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“It is really, really likely that there are older whale sharks out there, simply because we know there are much larger whale sharks,” says Campana. Whale sharks are known to grow up to about 18 meters (59 feet) long.

Campana and his colleagues used vertebral growth-band counts to also figure out the ages of 18 additional dead whale sharks. The animals had all been between 15 and 25 years old. They spanned between 2.7 meters (almost 9 feet) and almost 6 meters (about 20 feet) in length. Based on the lengths of sharks of different ages, the researchers estimated that young whale sharks grow an average of about 20 centimeters (8 inches) per year.

Data on the life span and growth rates of these sharks could be important for finding ways to protect this endangered species, Campana says. “Long-lived animals like whale sharks almost certainly become mature at a relatively old age,” he says. “suggest and share and their populations are relatively slow to increase their numbers.” So whale-shark populations may not bounce back from threats, such as overfishing, as

Fisheries biologist Allen Andrews has a word of caution about the new research. He works at the University of Hawaii at Manoa. There is some uncertainty in the carbon-14 dating technique that Campana’s team used, he says. That’s because radioactive carbon from nuclear tests did not spread evenly throughout the oceans. And “young [whale] sharks disappear for a big portion of their lives,” Andrews adds. If young whale sharks spend much of their time deep underwater, they may not take in the same amount of carbon-14 that is measured at the ocean’s surface.

Researchers could further investigate whale-shark growth by tagging live sharks and then recapturing them, Andrews says. That way, they could measure how much the animals grew from one time to the next.

carbon: The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules. (in climate studies) The term carbon sometimes will be used almost interchangeably with carbon dioxide to connote the potential impacts that some action, product, policy or process may have on long-term atmospheric warming.

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carbon-14 dating: A process to determine the age of material from a once-living object. It is based on comparing the relative share of any carbon-12 present to carbon-14. This ratio changes as radioactive carbon-14 decays and is not replaced. Since the decay of carbon-14 occurs at a steady rate, it provides a way to date how old an object is, but only as long as all of the carbon-14 has not decayed to carbon-12.

Cold War: A term for political and military tensions between the United States and the former Soviet Union. It began shortly after the end of World War II and lasted for decades. It is called “cold” because no major military clashes broke out. But each side engaged in lots of spying and there were the occasional international incidents that led to the threat of nuclear war.

data: Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

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.

endangered: An adjective used to describe species at risk of going extinct.

fisheries: Sites where large numbers of a fish species naturally congregate, making it a place where commercial fleets come to harvest that species. Or a term given to operations that raise young fish that will later be released into the wild.

gauge: A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.

Iceland: A largely arctic nation in the North Atlantic, sitting between Greenland and the western edge of Northern Europe.

life cycle: The succession of stages that occur as an organism grows, develops, reproduces — and then eventually ages and dies.

marine: Having to do with the ocean world or environment.

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mature: (adj.) Connoting an adult individual or full-grown and fully developed (non-juvenile) form of something. (verb) To develop toward — or into — a more complex and full-grown form of something, be it a living thing, a technology or an idea.

opaque: Unable to see through, blocking light.

population: (in biology) A group of individuals from the same species that lives in the same area.

radioactive: An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy is by a process known as radioactive decay.

residue: A remnant or material that is left behind after something has been removed.

seawater: The salty water found in oceans.

sharks: A family of primitive fishes that rely on skeletons formed of cartilage, not bone. Like skates and rays, they belong to a group known as elasmobranchs. Then tend to grow and mature slowly and have few young. Some lay eggs, others give birth to live young.

species: A group of similar organisms capable of producing offspring that can survive and reproduce.

tagging: (in biology) Attaching some rugged band or package of instruments onto an animal. Sometimes the tag is used to give each individual a unique identification number. Once attached to the leg, ear or other part of the body of a critter, it can effectively become the animal’s “name.” In some instances, a

tag can collect information from the environment around the animal as well. This helps scientists understand both the environment and the animal’s role within it.

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, suggest and share for instance, often are made from many different types of tissues.

uncertainty: (in statistics) A range of how much measurements of something will vary around an already-measured value.

vertebrae: (sing. vertebra) The bones that make up the neck, spine and tail of vertebrates. Bones in the neck are called cervical vertebrae. Bones in the tail, for animals that have them, are called caudal vertebrae.

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