Radioactive
residue from Cold War nuclear tests has given scientists a cipher to decode the
ages of whale sharks, written on the animals’ vertebrae.

Whale
sharks (Rhincodon typus) accumulate alternating stripes of opaque and
translucent tissue on their vertebrae as they age, similar to the way tree
trunks grow rings. But until now, scientists haven’t known whether whale shark
vertebrae gain a new growth band each year or every six months — making it
difficult to gauge just how fast these sharks grow or how long they live.

New
measurements of carbon-14 in the vertebrae of two whale sharks that lived
during the 20th century suggest that growth bands form annually, researchers report in the April 2020
Frontiers in Marine Science. Soviet and American nuclear weapons tests
in the 1950s and 1960s produced that carbon-14, which built up in Earth’s
atmosphere and oceans. By matching the amount of carbon-14 in different
vertebral growth bands with the known carbon-14 levels in surface seawater in
different years, the researchers estimated when each band formed — and found
that subsequent bands generally grew a year apart.

The total
number of growth bands in each dated vertebra indicated that one whale shark, a
10-meter male found in Taiwan, was about 35 years old when it died. The other
shark, a female of about the same size collected in Pakistan, was around 50
years old.

“It is
really, really likely that there are older whale sharks out there, simply
because we know there are much larger whale sharks,” says Steven Campana, a
fisheries scientist at the University of Iceland in Reykjavík. Whale sharks are
known to grow up to about 18 meters long.

Campana
and colleagues used vertebral growth-band counts to figure out the ages of 18 additional
dead whale sharks, which ranged from 15 to 25 years old. Those sharks measured
2.7 to almost 6 meters in length. Based on the lengths of sharks of different
ages, the researchers estimated that young whale sharks grow about 20
centimeters per year, on average.

Whale shark vertebrae
Whale shark vertebrae rack up growth bands as the shark ages (the alternating opaque and translucent stripes seen in this slice of a vertebra, each marked with a black dot). These growth bands, now estimated to form annually, can be used to decipher a whale shark’s age. This whale shark, found in Taiwan, lived about 18 years.J.J.L. Ong et al/Frontiers in Marine Science 2020

Information
on the life spans and growth rates of whale sharks could inform conservation
strategies to protect this endangered species, Campana says. “Long-lived animals
like whale sharks almost certainly become mature at a relatively old age, and
their populations are relatively slow to increase their numbers.” So whale
shark populations may not bounce back from threats like overfishing as quickly as species with shorter
life cycles.

Fisheries
biologist Allen Andrews cautions that there is some uncertainty in the
carbon-14 dating technique that Campana’s team used, because radioactive carbon
from nuclear tests did not spread evenly throughout Earth’s oceans. What’s
more, “young [whale] sharks disappear for a big portion of their lives,” says
Andrews, of the University of Hawaii at Manoa. If young whale sharks spend much
of their time deep underwater, they may not receive the same dose of carbon-14
as measured at the ocean’s surface.

Researchers
could further investigate whale shark growth by tagging live sharks and then recapturing
them, measuring how much the animals grew in the interim, Andrews says.