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Many land animals will dip beneath the waves to catch and eat fish, but the reverse situation can sometimes occur too, as the titan triggerfish demonstrates. The fish, which can grow to a length of 75 centimetres, will sometimes beach itself to feed on crabs walking along the shoreline.

In 2018, on the Red Sea鈥檚 Mar Mar Island, Matthew Tietbohl, a coral reef ecologist at King Abdullah University of Science & Technology in Saudi Arabia, was surveying a beach for sea turtle tracks with some colleagues. The team heard loud splashing at the water鈥檚 edge.

鈥淲e turned to see this triggerfish launching itself into the shallows and stranding itself,鈥� recounts Tietbohl.

It soon became clear that the fish was attempting to feed on ghost crabs that were grazing on algae-covered rocks at the water鈥檚 edge. The triggerfish would slowly stalk the crabs from the water, turning on its side and lunging out of the shallows like a crocodile. At one point, the fish successfully gripped a crab, pulling it back into the water.

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The determined diner was about 35 centimetres long, but titan triggerfish (Balistoides viridescens) can exceed twice that length, and were already infamous among divers, sometimes rushing and biting divers with crushing teeth in a territorial defence of their nests. Titan triggerfish are clever, says Tietbohl, using a suite of foraging techniques, including flipping over spiky urchins to expose their vulnerable undersides.

鈥淪o this beaching behaviour does fit the titan triggerfish鈥檚 reputation as a fish that seeks out food in many ways,鈥� says Tietbohl.

Still, the observations were surprising, because none of the handful of fish species that strand themselves to eat land-based prey are closely related to triggerfish. In fact, only a small proportion of these are saltwater fish.

Tietbohl says the triggerfish may be avoiding competition with other predatory fish. Supplementing a diet with terrestrial prey 鈥渨ould open up a whole new food source [titan triggerfish] could exploit that other fishes just can鈥檛鈥�.

Tietbohl wants to know if this hunting style is used by other related species, or is widespread among titan triggerfish. The team did see other triggerfish 鈥減atrolling鈥� the island鈥檚 shallows.

Since water and air refract light differently, Tietbohl thinks it is intriguing that the triggerfish are able to spot and track terrestrial prey from below the water. Studying their visual systems in further detail could prove interesting, he says.