Spitting archerfish shoot at prey above and beneath the water

By Andy Coghlan

Fire at will

University of Bayreuth

Spit and it’s a kill. Archerfish are famous for shooting mouthfuls of water at insects to dislodge them from vegetation above the water.

New experiments show that they also use the jets to hunt underwater – disturbing sediment where prey is lurking and snapping up the spoils.

“Our study adds support to the view that archerfish use their jets as tools,” says study leader Stefan Schuster of the University of Bayreuth in Germany. “They’re not simple all-or-nothing events, but the jets can be adapted to serve both in aerial and underwater hunting.”

Schuster’s team decided to investigate after observing the behaviour of wild fish on arrival in the lab. “We usually get wild-caught fish from Thailand, and when they first arrive in the lab we often see them ‘shooting’ at things on the ground of their tanks, such as leaves or small fragments of wood,” he says.

The team presented five fish in a lab with prey hidden in bowls of various types of sediment, and then filmed the behaviour. They found that the fish use the same mouth manoeuvres underwater as they do at the surface to produce jets that can dislodge insects as far as 2 metres away.

But their jets don’t travel as far underwater. They’re only used at close quarters – between 6 and 40 millimetres from the target – to blast sediment into a cloud that exposes potential prey.

To their surprise, the researchers found that the archerfish were able to alter the length and type of water blast to suit the type of sediment. Their shots were shortest if the sediment was coarse-grained and increased in length as the sand became finer.

“The big question is: how did they know beforehand which type of silt was which, and so how long they should blast it for?” asks Schuster. The answer might be that they are adept underwater shooters in the wild, too.

Which came first – aerial or underwater shooting – also remains to be established.

“Perhaps some tendency to produce underwater jets might have been there first, because this is widespread among fish,” says Schuster. Triggerfish use jets to turn round sea urchins to get access to their soft parts, for example, and lionfish use jets to orient small prey fish for easier swallowing.

“Many other fish and invertebrates forage by disturbing the ground, and this is probably the ancestral condition,” says Alex Kacelnik of the University of Oxford. “Archerfish probably thus started with this ordinary skill then transitioned to targets probably at, or narrowly above, the surface and this created new selective pressures to focus and aim water jets at ever higher targets.”

“It’s a lovely example of the incremental and interactive process of evolution of complex traits through natural selection,” he says.

Schuster says the two techniques might have evolved in parallel, with the fish building on and adapting their skills according to their habitat.

“Using the same manoeuvre in both contexts might seem silly at first, because jets in water and air face very different constraints,” says Schuster. “But it has a big advantage – that improvements in one context can be adapted to the other.”

Next, the team plans to find out if the fish use their water jets for other purposes, such as defence or to manoeuvre and trap smaller fish prey. They also plan to investigate whether other species adapt their jets for alternative uses.

Journal reference: Journal of Experimental Biology, DOI: 10.1242/jeb.146936