Phenotypically plastic responses to predation threat in the mangrove rivulus fish (Kryptolebias marmoratus): behavior and morphology

Early life environments have important effects on phenotype development, but it can be difficult to disentangle the relative influences of genotype and environment on phenotypic variation within and among populations. Mangrove rivulus fish (Kryptolebias marmoratus) reproduce by self-fertilization and can generate isogenic lineages, which provides opportunities to resolve how the environment shapes the phenotype independent of genetic variation. Rivulus' ecology is not well understood, but mangrove water snakes (Nerodia clarkii compressicauda) are thought to be a major predator. To test developmental responses to predator-related cues, four rivulus lineages (two that naturally co-exist with snakes; two that do not) were exposed to one of three treatments for 30 days post-hatching: cues from snakes that were fasted, fed rivulus, or fed heterospecifics. One week after exposure, fear and boldness responses were quantified. Individuals were photographed at 2 and 6 months of age for body size, growth, and body shape analysis. Animals that have historically encountered snakes were more risk averse and had wider heads than animals that historically have not encountered snakes. Rivulus exposed to cues from snakes fed conspecifics or heterospecifics grew faster than those exposed to fasted snake cues. Body shape was more streamlined in animals exposed to cues from snakes fed conspecifics, which may facilitate increased jumping performance as a way to escape aquatic predators. Our results suggest that rivulus exhibit phenotypic plasticity in response to cues associated with predator threat and that historical effects from selection or other evolutionary processes also are important determinants of behavioral and morphological variation.