The effects of predation risk on phenotypic and hatching time variation in fathead minnow (pimephales promelas) embryos
Kusch, Robin Christine
Predation has long been recognized as a strong selective force influencing the behaviour, morphology and life history traits of prey species. Some prey species have the ability to alter their phenotypes in response to predation threat, to decrease their chances of being detected, caught and/or consumed by predators. It has also been demonstrated that some species have control over the timing of transition between one specific life stage and the next. This ability gives the prey control over how much time it spends in a risky stage. A particularly vulnerable stage for many prey fishes is that of the embryo, as they are a major prey item for many invertebrate and vertebrate predators. As such, strong selection pressure should exist for the development of anti-predator defenses specific for this time period. In a series of four experiments I assessed the ability of fathead minnow embryos to alter their hatching time and/or phenotype in response to various predation threats. In the first three experiments injured embryo cue was used to simulate a predation threat, as it has been shown to represent a general predation risk for many aquatic animals. In the fourth experiment predator odour was used in conjunction with injured embryo cues. Results of a power analysis conducted on the first three experiments determined with 95% confidence that fathead minnow embryos do not alter their hatch time in response to injured embryo cues. However, the embryos in the predation treatment did hatch with an altered phenotype; fry were significantly smaller (total body length) when exposed to predation cues. In the fourth experiment the embryos hatched with the same altered morphology in response to injured embryo cues combined with predator cues. Moreover, in this experiment the embryos hatched faster in the predation treatment than the control treatment. This is the first empirical evidence that fish can alter their hatching time in response to predation.