杏吧原创

A whale shark has been observed feeding at the seabed for the first time, rather than filter-feeding on plankton at the sea surface.

The unusual behaviour was spotted during a whale shark tour in Baja California Sur, Mexico, by an ecotourism guide who sent the footage (above) to researchers for analysis.

A 5-metre-long juvenile was observed sucking at the sand at around 6 metres deep, appearing to gulp down material from the substrate.

鈥淣o one鈥檚 ever seen this behaviour before,鈥� says Darren Whitehead at , who studied the footage.

Whale sharks (Rhincodon typus) are the biggest fish in the sea, but many aspects of their lives are mysterious.

Previous toxicology work has suggested bottom feeding could form a significant part of a whale shark鈥檚 diet, but this is the first time it has been directly observed.

The researchers don鈥檛 yet know whether the substrate provides nutrients the sharks cannot get from surface feeding, or if the shark was unable to get enough energy from plankton alone and went into survival mode, says at Imperial College London, who also worked on the study. 鈥淓xactly what they鈥檙e attempting to feed on is a mystery for now,鈥� he says.

Understanding why whale sharks do this is vital for conservation, the researchers say. If bottom feeding makes up a major part of the sharks鈥� diet, it further highlights the importance of protecting the deep-sea ecosystems threatened by trawling and mining. If they are responding to a decrease in plankton availability, there could also be catastrophic consequences for other species that can鈥檛 switch between food sources.

Although this hasn鈥檛 been seen before, 鈥渢here鈥檚 no reason to suggest it鈥檚 a rare behaviour鈥�, says Gayford. These huge fish migrate thousands of kilometres and can dive to nearly 2000 metres, so human observations are limited to an extremely small part of their range, he says.

While ecotourism is a hotly debated topic, there are huge benefits to having citizen scientists capturing data and sharing it with academics, says Whitehead.

, director of the conservation foundation , who wasn鈥檛 involved in the study, says this highlights the value of citizen science. 鈥淎s researchers in the field have limited time, members of the public can help answer pending questions about elusive species,鈥� he says.