Phenotypically plastic response of an invasive species to temperature but not a predator

Background/Question/Methods

Many cladoceran zooplankton exhibit plastic responses to changes in their biotic and abiotic environment, particularly through morphological and life history modification to predators and water temperature. Previous field studies show that the morphology and life history of an invasive cladoceran in the Great Lakes, Bythotrephes longimanus (the spiny water flea), strongly vary, but the cause of this variation is not known. In particular, Bythotrephes possess a conspicuous tail spine, which can reach up to 80% of total body length, that varies in absolute and relative (to body size) length across systems and within growing seasons. This morphological variation correlates with and is theorized to be a defense against gape-limited fish predation. Morphological variation also correlates with variation in water temperature. We tested the effect of fish (Perca flavescens) cues (presence / absence) and temperature (9C and 15C) on the morphology and life history of Bythotrephes raised in clonal lines to the F2 generation. We predicted that Bythotrephes raised in fish cues would adopt a strategy concordant with out-growing gape-limited predators and that Bythotrephes raised in the higher temperature would develop faster and reach a larger overall size.

Results/Conclusions

We found no significant effect of fish cues on Bythotrephes morphology (absolute spine and body length; relative spine-to-body length) or life history (days to first reproduction; number and size of offspring). Higher temperature affected life history (fewer days to reproduction) and led to longer absolute spine and body lengths, decreased clutch size, and increased size of progeny. There was no effect of temperature on morphometry (i.e., spine-to-body length). The fish cue results may be surprising given previous research documenting morphological and life history plasticity of cladoceran zooplankton in response to fish predators. Although it is difficult to interpret non-significant results, we had sufficient statistical power and confirmed the biological activity of our fish cues using a Daphnia bioassay. Further, experimental variation in morphology and life history fell within natural field variation, suggesting that Bythotrephes were not constrained by an experimental condition (e.g., poor nutrition). Our findings thus suggest that Bythotrephes temporal and spatial phenotypic variation may be due to temperature variation, or alternatively, non-plastic mechanisms such as local genetic adaptation to fish predation.