The world’s largest living fish – the whale shark – is a mysterious creature. Despite their size and popularity with ecotourism, little is known about the ecology and behavior of this majestic marine species. Now a new observation develops our understanding of the diet and feeding behavior of these impressive animals.
According to the WWF, whale sharks typically feed on shrimp, fish, and plankton, and are the also world's largest omnivores. During feeding aggregations, two behaviors are known to occur. The first is that the whale sharks will feed in a passive manner, opening their huge mouths to filter feed. The second is a more active feeding method where individuals swim level with the surface of the water, and repeatedly open and close their mouths to actively suck prey inside and over their gill rakers, internal mouthparts that help with feeding on small ocean life.
Until now no evidence of whale sharks feeding on the bottom of the ocean has ever been scientifically recorded. At an aggregation site in Baja California Sur, Mexico, researchers received footage and observed an individual whale shark eating the sediment on the sea floor. They suggest that the whale shark could have been looking to consume smaller sea creatures living within the sediment itself.
Recorded on a GoPro by a swimmer during a whale shark viewing tour on December 11, 2022, the video shows the whale shark feeding on the sea bed at approximately 6 meters (19 feet) deep. The shark turns almost 90 degrees (90 seconds into the video) and exhibits a sucking action similar to a surface feeding event before removing excess sand from its mouth and repeating the foraging behavior.
The team suggest that the shark is purposely targeting the benthic creatures under the sediment, and while no record of whale sharks exhibiting this behavior before now exists, some previous research on their diet has given rise to speculation about whether whale sharks do feed on species that live on or in the seabed.
The team go on to suggest that the shark could be actively looking to feed on amphipods, tiny crustaceans that are commonly found in the same coastal waters as the whale sharks, and are known to live around the area where the novel feeding behavior was observed. It is not know whether these amphipods live in this exact area where the feeding was recorded, and the team suggest this as a next step for future study.
The researchers also stress the importance of safe and respectful ecotourism practices and highlight the role of citizen scientists in observing new behaviors. In 2022, megamouth sharks were spotted from a fishing boat, leading to the first observation of what is thought to be courtship behavior.
The study is published in the Journal of Fish Biology.