After analyzing samples of their blood and tissue from their fins, they discover that they eat more plants and algae than previously thought and can spend long periods without eating
With its 12 meters in length and its 21 tons of weight (equivalent to an urban bus or three African elephants), the whale shark ( Rhincodon typus ) is considered the largest fish in the world.
This species was discovered 183 years ago and, although it is quite widespread on the planet – it lives in warm and tropical seas – many features of its biology are still unknown to scientists. For example, young individuals are rarely sighted and it has never been possible to record this species by mating or reproducing.
“Whale sharks are globally threatened but they are a little known species, and in spite of their large size, we know relatively little about their diet, reproduction and migrations, ” comments to THE WORLD Alex Wyatt, a scientist at the Institute of Ocean Research and Atmosphere of the University of Tokyo and main author of a study that this week reveals more details about the way of life and diet of this enigmatic shark.
Sharks in captivity and wild
Unlike other observational studies, his research has been based on obtaining samples of blood and tissue from their fins taken from both captive specimens and sharks that live freely in Japanese waters. Their findings are published this week in the journal Ecological Monographs and, according to Wyatt, they have been a surprise. Their analysis revealed that they eat many more plants than previously thought and that they can spend large periods of time without eating .
“My research has focused on trying to understand the relationship between the changing conditions of the ocean and threatened marine species, using mainly the chemical footprints of the diet that these animals follow, these chemical markers, for example, the isotopes present in the tissues , offer an ideal method to clarify the mystery about what whale sharks eat and where they feed, “explains Wyatt through an email.
It is a migratory species that travels long distances in the ocean, which makes its monitoring difficult. Traditionally, in the study of these animals, scientists have taken samples of different tissues from their bodies to analyze their isotopes. But to correctly interpret the levels of these isotopes, they need to know about the history of that shark and its diet over time, something that is not viable in the long term with wild sharks. Therefore, in this study they complemented tissue analysis with blood samples, which were analyzed immediately on the ship. “By combining blood tests with tissue isotopes, we can do a complete health check of the shark, ” says Wyatt.
Thus, in this research they compared the analyzes of five sharks living in the Okinawa Churaumi aquarium with those of eight individuals that live in the wild and that were accidentally captured and later returned to the sea. “We collected tissue from their fins and blood samples from most of these individuals although, as some of the samples were taken before the start of the investigation, it was not possible to obtain tissue from the fins of two of the sharks,” he says.
Blood tests also revealed that some of the wild sharks had probably not eaten food for weeks or months . According to Wyatt, this could be due to the fact that they did not find food or that they do not eat while traveling long distances.
“Finding evidence that they are herbivores from the amino acid isotopes in wild sharks was a surprise, as it was assumed that whale sharks are consumers of zooplankton, ie, animal plankton, and small fish.” On the eight whale sharks wild were obtained between 2.2 and 2.6 (on a scale in which 2 is a pure herbivore and 3, a carnivore), “explains the scientist.
According to details, the diet of the sharks that live in the aquarium is composed mainly of two species of krill (a small crustacean), to which are added small amounts of other animals such as prawns, squid, small fish and vitamins. ” In the aquarium they are not fed with plants , which was reflected in our results, because the amino acid level of the sharks that lived in captivity was higher than 3″.
“Although we expected that wild whale sharks would have levels of 3 or more, stranded sharks that had ingested algae have been found.” Another recent study showed that the shovel-head shark (Sphyrna tiburo ), which was thought to be strictly carnivorous, may Ingest plants when you live in captivity, so omnivorous sharks may be more common than we had thought so far . “
YOU CAN COME TO LIVE 100 YEARS
Whale sharks are fish, not mammals, they filter their food through their large gills and they are easily identifiable by the white spots and streaks on their skin, with a unique pattern that distinguishes each individual. Their life expectancy is difficult to estimate but scientists believe that they can live between 70 and 100 years , reaching their sexual maturity in their thirties, according to Wyatt.
However, today it is not very likely that a shark will become so long-lived. And, like so many other shark species, the whale is also highly coveted by fishermen because of its fins , with which the famous shark soup is prepared, considered a delight in China. The usual procedure, called finning or finning and banned in the EU, consists of capturing the animal, cutting its fins and throwing the rest of the body into the sea, where it bled to death. In this way, fishermen can store more fins on the boat, which is the most profitable part.
In addition, as noted by the conservation organization WWF , are also very appreciated in the international market their guts, their meat and oil , for which they pay large amounts of money.
As Wyatt recalls, this species has recently gone from being considered vulnerable to threatened in the Red List of the International Union for the Conservation of Nature (IUCN). “Although the whale shark is very protected in some countries, because it is a migratory species that crosses waters of different nations, it is still fished in some places voluntarily or accidentally”.
Source: El Mundo