Elephantnose fish can recognise objects in the dark

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Elephantnose fish can detect the shape, size, volume and material of underwater objects in complete darkness, according to the results of a new study.

Elephantnose fish, Gnathonemus petersii, live in the muddy swamps of Africa and have poor eyesight but an acute sense of spatial awareness thanks to electrolocation.

Now a new study has revealed that the mormyrid fish are able to use their electrosensory system to tell the difference between the size, shape and volume of objects near them in the water.

Gerhard von der Emde and Steffen Fetz of the Institute of Zoology at the University of Bonn, set up an experiment to test which features of objects Elephant noses were able to discriminate between, in return for a food reward.

Their findings, which have just been published in the latest volume of the Journal of Experimental Biology, explain that all eight specimens in the study learned to discriminate between two objects of different shapes and volumes.

The authors wrote: "When new object combinations were offered in non-rewarded test trials, fish preferred those objects that resembled the one they had been trained to (S+) and avoided objects resembling the one that had not been rewarded (S").

"For a decision, fish paid attention to the relative differences between the two objects they had to discriminate. For discrimination, fish used several object features, the most important ones being volume, material and shape.

"The importance of shape was demonstrated by reducing the objects to their 3-dimensional contours, which sufficed for the fish to distinguish differently shaped objects."

Fetz and von der Emde believe that the fish tended to favour objects with a larger volume, and preferred plastic objects to metal ones, irrespective of training.

The fish were also able to detect whether an object had corners or rounded edges.

"When confronted with two unknown objects, fish weighed up the positive and negative properties of these novel objects and based their decision on the outcome of this comparison.

"Our results suggest that fish are able to link and assemble local features of an electrolocation pattern to construct a representation of an object, suggesting that some form of a feature extraction mechanism enables them to solve a complex object recognition task."

For more information see the paper: Fetz S and G von der Emde (2007) - Distance, shape and more: recognition of object features during active electrolocation in a weakly electric fish. Journal of Experimental Biology 210, 3082-3095 (2007).