Crustaceans have a reputation for vicious weapons and impenetrable defences. However, Linda Weiss from the University of Birmingham, UK, explains that one diminutive crustacean – the water flea (Daphnia) – fine-tunes its defences depending on which predator is in the vicinity. ‘D. pulex is able to distinguish the phantom midge larvae from fish’, she says, explaining that the crustaceans remain small and inconspicuous when threatened by fish, but develop neck teeth when menaced by the voracious larvae. As water fleas select their defence strategy in response to the warning odours (kairomones) exuded by specific predators, Weiss and her colleague Ralph Tollrian from the Ruhr-University Bochum, Germany, decided to focus on the responses of developing Daphnia longicephala to their archenemy, the back swimmer Notonecta glauca. The youngsters develop a helmet-like defensive crest when threatened by the back swimmers, so the duo wondered at which stage of development the minute water fleas produce the defensive structure and how they sniff the odour that triggers the lifestyle change.
However, before Weiss and undergraduate student Julian Leimann could begin testing the developing water flea's reactions to the predator, they had to produce kairomone-scented water. ‘The chemical structure of kairomones remains to be determined’, says Weiss, so she isolated a few water fleas with predatory back swimmers and collected the kairomone-laced water 24 h later. Then she began testing the effects of the spiced water on the Daphnia.
Explaining that the crustaceans shed their shell every 24 h as they move on to the next stage of development, the duo selected youngsters at each developmental stage – from the first instar to the fifth – bathed the water fleas in kairomone water and monitored their development. Scrutinising the youngster's appearance, the duo could see that the youngest life stage (first instar) was unaffected by the warning odour. Instead, the youngsters began responding to the back swimmer's kairomones during the second instar, although they only produced the distinctive protective crest during the fourth instar. And when Weiss and Leimann tested the effects of long-term exposure to the kairomone-scented water, the youngsters did not develop larger crests.
Having confirmed that the youngsters could only respond to the distinctive warning odour once they had reached the second life stage, the duo began testing which of the crustacean's antennae were sensitive to the scent of doom by dabbing a drop of glue onto the first antennae. Knowing that the crustaceans slough off the adhesive when they moult, Weiss realised that she could temporarily inactivate the crustaceans’ sense of smell at specific stages of development with a glue mask then wait to see whether the animals were able to develop a defensive crest after bathing in the kairomone. However, the duo made little headway – normal adhesives failed to set on the water flea's damp surface – until Leimann hit on the idea of using waterproof aquarium adhesive. ‘This was a great moment’, Weiss chuckles.
Impressively, the second instar water fleas were no longer able to produce the defensive display with the glue mask in place. However, once it became detached during the fourth instar, the water fleas responded to the warning odour again, developing defensive crests during the sixth instar 2 days later. So, the receptors for the predator's kairomones are located on the first antenna and Weiss is keen to understand how the tiny crustaceans convert the warning signal into defensive action.