The whale shark (Rhincodon typus) is the largest fish known. In fact, it is not a cetacean, but a real shark, even if, like whales, it feeds primarily on plankton. For various reasons, the entire species is at risk of extinction, which is why scientists have been studying their eating habits for some time with the aim of developing suitable conservation strategies. With a study published in the scientific journal Fishesa group of researchers from the University of Western Australia and the Australian Institute of Marine Sciences has discovered that these habits can be investigated by taking and analyzing certain parasites of the marine giant. And the new method appears to be not only less invasive towards sharks than taking skin samples, but also capable of providing more accurate indications. Curiously, whale sharks seem to appreciate the procedure of parasite sampling by researchers, with whom they "collaborate" by slowing down or stopping to facilitate their work.
A polka dot shark
The whale shark can measure up to 12-20 meters in length, and, even if less than 10% survive to adulthood, some specimens can even reach 150 years of age. They mainly feed on plankton, shrimps and small fishes, which they obtain by filtering thousands of liters of seawater a day through their gills. They are easily recognizable not only for their size but also for the white dots that cover their back and which, just like our fingerprints, form unique patterns that allow, in principle, to recognize and catalog each single individual.
Like other shark species, whale sharks are also at risk of extinction for several reasons: one is certainly climate change, which causes the ocean warming depleting them of the species they feed on. The other reason is the deliberate capture of these fish to obtain their fins, considered as a real delicacy in some Asian countries, where they are used to cook typical soups. Finally, also theplastic pollution constitutes a risk factor for the survival of this species, as for that of many other fish and seabirds.
The good news, we said, is that we may have learned a new method for studying their eating habits, which will perhaps allow us to develop new conservation strategies. "Direct observation of whale sharks - explains Brendon Osorio, first author of the study - can reveal only a part of their feeding activity at a given moment, while the biochemical analysis of the specific parasites of the host allows to obtain more complete information on the 'whale shark feeding on weekly and monthly time scales'.
In fact, these animals tend to cover long distances to find the food necessary for their survival, a fact that has contributed to making the study of their eating habits difficult, at least until now. In addition, it is indeed a rare species and often feeds at night or in very deep waters.
"A better understanding of the life history and ecology of copepods (the parasites analyzed in this study, ed) - explains Mark Meekanthe study's last author and senior researcher at the University of Western Australia - is necessary to fully understand the relationship between the whale shark and its parasite, but the recent study has shown that copepods provide an accurate and less invasive method for the analysis of the diet, compared to biopsies". And, as Meekan himself recounts in an article in the magazine ABC newswhale sharks do not mind these samplings at all: some of them, especially those that have already been approached in the past by researchers, seem to want to collaborate, slowing down their movement in the water and allowing scientists to reach the parts of the body from which they are made the withdrawals, especially the sides of the mouth and the fins.