Anti-predator mechanisms in evolutionarily predator-naive vs. adapted fish larvae

It is a fundamental feature of evolution that natural selection acts on individuals to adapt to predation pressure via the development of anti-predator mechanisms. As such mechanisms are costly in terms of energy and time, species living in habitats where predators are rare or absent were expected to show reduced predator responses. Such a reduction was expected for larvae of the native whitefish (Coregonus wartniann) in Upper Lake Constance, as these pass their initial stage in a historically predator-free pelagic habitat and only start to encounter predators in a later ontogenetic stage during which they shift to littoral habitat. However, a recent invader of the pelagic waters of the lake, the three-spined stickle-back (Gasterosteus aculeatus), is implicated in a severe decline in whitefish recruitment through larval predation. The predator-naivety effect and its consequences on predation were experimentally tested by comparing predator defenses of different age classes of whitefish with those of roach (Rutilus rutilus) and perch (Perca fluviatilis), two species whose larvae grow up in a predator-holding habitat. As predator, the three-spined stickleback (Gasterosteus aculeatus) was selected. Sticklebacks actively hunted all prey species but predation was high on whitefish only. The addition of zooplankton (Daphnia) as alternative prey for sticklebacks in the experimental setup did not reduce predation on the whitefish larvae, indicating a clear preference for whitefish in this invasive predator. The escape responses of roach and perch were found to be clearly more complex and diverse to those of the predator-naive whitefish larvae. Particularly striking was that whitefish often did not show any apparent behavioral response to the approaching predator. Only large whitefish larvae (Table 1: length = 40.8 +/- 0.6 mm) appeared capable of escaping predation, and this was correlated with a change in complexity of predator escape responses. Thus, differences in predation pressure for the different larvae species were highly related to the observed interspecific differences in predator defenses, and with the ontogenetic intraspecific change in whitefish from poor and inefficient predator escape responses to complex and more efficient predator escape responses. The evolutionary consequences of stickleback as an invasive predator on whitefish larvae behavior are discussed.