Unlike the helmet of Perseus, thehelmet of the water flea (Daphniaspp.) does not make it invisible, butit does confer some protectionagainst predators by making the fleasharder to catch. Water fleas do notnecessarily begin life with a helmetbut can form one as a morphologicaldefence in response to chemicalsreleased by their predators. Theimages above show three pairs ofwater fleas from different species;the non-helmeted morph is on theleft in each pair of fleas. Clockwisefrom the top left these are: Daphniacucullata; the Australian speciesDaphnia longicephala, whose hugecrests keep their predator, thebackswimmer, from getting a firmgrasp on them; and the Asian/African

species Daphnialumholtzi, whosedistinctive helmetsprotect them againsteven fish.

But being able todevelop a helmetis no good to thewater flea if it is attacked before ithas done so. In Daphnia, females whohave grown helmets can transmit thisbenefit to their offspring, therebyimproving their reproductive success.Their offspring not only start life witha form that provides better protectionbut, in the face of predatorial threat,can go on to develop larger helmetsthan the offspring of nonhelmetedfemales in the same environment.This bequeathing of a helmet is an

example of amaternallyinduced(transgenerational)adaptation; genesactivated in the motherare expressed as aphenotype in their offspring.For further details, seeAgrawal A.A., Laforsch C. andTollrian R., Nature 1999, 440011:60-63. Images provided by RalphTollrian and Christian Laforsch,Ludwig–Maximilian University,Zoological Institute, Karlstrasse 25,80333 München, Germany.

Magazine R3

Biology in pictures

Safety helmets

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