How to find out if the biggest fish in the world is waiting for children? It turns out not because of her bulging belly.
Scientists believed that the enlarged area on the lower part of female whale sharks was a sign of pregnancy. But a technique used for the first time on free-swimming animals, showed only skin and muscles. Instead, these humps may be a secondary sex characteristic of mature females, like breasts in humans, researchers report in study of endangered species from March 23.
The ultrasound is part of a suite of new techniques, including underwater “jet packs” and blood tests, that scientists hope can unlock the creature’s reproductive secrets.
The International Union for Conservation of Nature classifies whale sharks ( Rhincodon typus) as endangered. According to estimates, there are only 100,000 to 238,000 individuals left in the world, which is more than 50 percent less in the last 75 years.
In part because whale sharks are relatively rare, their reproductive biology remains largely a mystery. Almost everything biologists think they know is based on the study of a single pregnant female caught by a commercial fishing vessel in 1995.
“Protecting organisms without knowing their biology is like trying to catch a fly with your eyes closed,” says Rui Matsumoto, a fisheries biologist at the Okinawa Churashima Foundation in Japan. The organization studies subtropical animals and plants to maintain or enhance natural resources in national parks.
To learn more about these gentle giants, Matsumoto and shark biologist Kiyomi Murakumo of Japan’s Okinawa Churaumi Aquarium had to figure out how to keep up with them. Like comic book superheroes, the biologists used underwater jetpacks — propellers attached to a scuba tank — to swim alongside the fish, which average 12 meters in length and move at about five kilometers per hour.
The researchers then had to maneuver a 17-kilogram briefcase with a waterproof ultrasound wand on the underside of 22 females swimming off the Galapagos Islands and use syringes to draw blood from their fins. Prior to this study, the ultrasonic wand had never been used outside of an aquarium for free-swimming wildlife.
Conducting these two tests on whale sharks is particularly challenging, says study co-author Simon Pearce, a whale shark ecologist at Marine Megafauna Foundation , a nonprofit organization that uses research for marine conservation. Fish have “the thickest skin of all animals – up to 30 centimeters thick.”
Another problem is seawater itself, which can contaminate blood samples. The researchers developed a two-syringe system where the first syringe creates a vacuum and allows the second syringe to draw only blood.
Back in the lab, blood plasma from six females showed hormone levels similar to those obtained from an immature captive female in an aquarium, indicating that these wild females were not mature enough to reproduce.
The ultrasound image showed eggs in two of the 22 female sharks, meaning the females were mature enough to breed but were not pregnant. Biologists were unable to find a pregnant whale shark.
The pioneering of these non-invasive techniques on whale sharks has opened the door to the possible study of other endangered marine animals. Waterproof pole-mounted ultrasonic wands, Pearce says, are now being used for tiger sharks in areas where the predators are drawn in with bait.
Rachel Graham agrees that the development of these underwater sampling techniques is an “impressive feat”. But a marine conservation scientist and founder MarAlliance a non-profit marine wildlife conservation organization, doubts that most wild, free-roaming marine animals, including faster-swimming sharks or marine mammals, would survive such an ordeal.
“What makes whale sharks quite unique … is that they sometimes move relatively slowly, have the ability to stay still and tolerate the presence of other animals – like us – in the vicinity,” says Graham, who has studied shark species around the world and was not involved in new research.
Combined with satellite tracking, the new techniques could eventually show us where whale sharks give birth, Pierce says. Little is known about whale shark pups, including whether they are born in shallow or deep water, and whether pups are born singly or mothers gather to give birth together. “Assuming that they do have some kind of breeding area or pelagic nursery that we can identify … then that obviously helps sustain the population for quite a long time.”
