Plastic response to a proxy cue of predation risk when direct cues are unreliable

Abstract

Responses to proximate cues that directly affect fitness or cues directly released by selective agents are well-documented forms of phenotypic plasticity. For example, to reduce predation risk, prey change phenotype in response to light level (e.g., moon phase) when light affects predation risk from visual predators, and to chemical cues (kairomones) released by predators. Less well understood is the potential for organisms to perceive predation risk through "proxy cues": proximate cues that correlate with, but do not directly affect predation risk. Previous field studies indicate that body and spine length of an invasive cladoceran in Lake Michigan, Bythotrephes longimanus (the spiny water flea), increase during the growing season, coincident with a decrease in clutch size. Although the cause of seasonal trait changes is not known, changes are associated with warmer water temperature and increased predation risk from gape-limited fish (i.e., fish whose ability to consume Bythotrephes is limited by mouth size). Using a laboratory experiment, we found no effect of fish (Perca flavescens) kairomones on Bythotrephes morphology or life history. In contrast, higher water temperature led to longer absolute spine and body length, increased investment in morphological defense of offspring (measured as the ratio of spine-to-body length), and decreased clutch size and age at reproduction. These plastic responses are unlikely to be adaptive to temperature per se, but rather our findings indicate that temperature serves as a proxy cue of fish predation risk. Temperature correlates with risk of gape-limited fish predation due to growth of fish from larval stages incapable of consuming Bythotrephes early in the season, to larger sizes by midseason increasingly capable of consuming Bythotrephes, but limited by gape size to consuming smaller individuals. We argue that for Bythotrephes, temperature is a more reliable cue of predation risk than fish kairomones, because fish kairomones are present throughout the season due to continual presence of non-gape-limited adult fish, to which plastic response would have little effect. Organisms may, therefore, not only respond to changes in an environmental factor because the factor directly affects risk, but also when the environmental factor serves as a proxy signaling change in predation risk.

Mother and offspring fitness in an insect with maternal care: phenotypic trade-offs between egg number, egg mass and egg care

“…Second, it may reflect an adaptive strategy of the females which favors investment into either egg number or pre-hatching care [16]. For instance, low risks of egg predation and thus low benefits of egg guarding could favor females allocating more resources into egg number than egg care [32]. A third hypothesis is that the level of maternal expenditure on pre-hatching care might not be directly affected by egg production, but by the impact of egg production on egg quality.…”

Gape-Limited Predators as Agents of Selection on the Defensive Morphology of an Invasive Invertebrate

“…B), providing a control for potentially confounding effects of phenotypic plasticity (Pigliucci ). Previous work (Miehls ; Miehls et al ) indicated that Bythotrephes mothers at higher temperature produce offspring with longer distal spines. If first and second instar Bythotrephes collected on the same sampling trip experienced different water temperatures during development, measurements of selection could be confounded by temperature‐induced plasticity in distal spine length.…”

Evolutionary stasis despite selection on a heritable trait in an invasive zooplankton

“…Miehls et al . () found that Bythotrephes mothers plastically induced longer distal spines in offspring in response to warmer water temperature. It is therefore possible that a genetic response to selection for longer distal spines could have been countered at the phenotypic level by a general cooling trend in water temperatures, but no such trend in water temperatures in Lake Michigan has been documented.…”

“…This measurement of selection within a cohort is important, as it provides a control for the potentially confounding effects of phenotypic plasticity. Previous work (Miehls et al ., ) indicated that Bythotrephes mothers at higher temperature produce offspring with longer distal spines. Individuals of the same cohort should have experienced the same temperature environment during development and thus are free from possible temperature‐induced differences in distal spine length, which a comparison of individuals between cohorts (i.e.…”