Adaptive armour: water-fleas get geared up

Water fleas get geared up in their ongoing fight for survival, growing a range of impressive anti-predator weapons, from spines to helmets. And not only that, they can tailor their defensive weaponry to the particular type of predator present.

The growth of these defensive structures in water fleas (Daphnia) has been well documented. But until now, researchers have only been able to make educated guesses on how exactly the growth of different armour types is triggered.

Chemical cues are key, allowing the fleas to detect what threats are nearby. Be it fish, predatory larvae, or voracious water boatmen (Notonecta glauca), many things are happy to take the fleas as prey. If they can get through the fleas’ predator-specific protection, of course.

Researchers have established that specific neurotransmitters allow the flea to quickly translate cues from the environmental into a different hormonal response, which in turn drives spectacular changes to body shape. If the flea detects water boatmen lurking nearby, for example, it develops an enlarged, bulbous head – part crest, part helmet – and elongated tail spines. In contrast, if chemical cues mean a stickleback is in the vicinity, the fleas get spiky, with magnificent long spines forming at each end of their body.

Spines and spikes, adaptive armour that may be enough to put off a predator, make the Daphnia more than a mouthful, or at least allow the innovative flea to make a getaway.


Originally published in Biosphere Magazine Issue 18.

Image: Undefended Daphnia lumholtzi (left) is compared with the defended phenotype (right). The defended phenotype has remarkably elongated head and tail-spines in response to chemical cues from the three-spined stickleback Gasterosteus aculeatus. Credit: Dr Linda Weiss



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