Study system
In this study, we focus on one of the most iconic predator-induced defences in water fleas (Daphnia species), the ‘neckteeth’ ofD. pulex (Fig. 1). The neckteeth defence comprises a swollen area on the back of the head (neck-pedestal) and spikey bits which grow on top. The defence grows in response to predator cues (kairomones) released by Chaoborus midge larvae . The onset of kairomone sensitivity begins in the late embryonic stage, when the neck-pedestal begins to grow, and the spikey bits develop later in the early juvenile stages . The maintenance of the defence requires consistent exposure to predator cues and usually lasts until the third instar, after which theDaphnia are large enough to escape size-selective predation . Also, it is well known that the induction of the defence increases with the level of predation risk . Although the exact mechanism is unclear, it has been shown that the neckteeth defence increases prey survival by up to 50% .