Sniffing Danger -- Why A Freshwater Shrimp Has To Keep On The Move
Hunters prefer to stalk from downwind to prevent odours alerting their prey. In the same way, fish hunt from downstream. But now researchers have found that fresh-water shrimps subvert this strategy by creating tiny currents to enable them to sense the chemical odour of brown trout waiting downstream to ambush them.
Jonas Dahl, a limnologist at Lund University in Sweden, placed a set of mesh enclosures in a stream north of Lund. Half the enclosures contained brown trout and half were empty. Dahl then counted amphipod shrimps, Gammarus pulex, that drifted downstream into the enclosures.
He found that fewer of the amphipods migrated into the enclosures containing trout than into those that were empty. This suggested that the amphipods could somehow detect the trout from upstream.
With colleagues Anders Nilsson and Lars Pettersson in the university's department of ecology, Dahl placed some shrimps in an artificial stream in their laboratory. They split the stream in two with a divider and put a cage housing a brown trout into one side and an empty cage into the other. As expected, more shrimps drifted into the empty side of the stream than into the side with the trout.
The team repeated the experiment with the trout enclosed in a large glass tube. The shrimps could now see the trout, but could not smell it. This time an equal number of shrimps drifted into each side of the divider. Finally the researchers gently rubbed the trout against the upstream side of its cage, to mark the cage with its chemical odour. The shrimps still avoided this cage, even though there were no trout in it (Proceedings of the Royal Society Series B, vol 265, p 1339).
Using droplets of milk to trace flow patterns in the artificial stream, the researchers showed that live shrimps could generate a backflow extending 7 millimetres downstream. As Gammarus usually drift downstream at night and trout rely on sight to hunt, this small backflow could allow the amphipods to escape predation.
Author: Jon Copley
New Scientist issue 1st Aug. p.6
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