Abstract
Population density can profoundly influence fitness-related traits and population dynamics, and density dependence plays a key role in many prominent ecological and evolutionary hypotheses. Here, we evaluated how individual-level changes in population density affect growth rate and embryo production early in reproductive maturity in two different asexual lineages of Potamopyrgus antipodarum, a New Zealand freshwater snail that is an important model system for ecotoxicology and the evolution of sexual reproduction as well as a potentially destructive worldwide invader. We showed that population density had a major influence on individual growth rate and early-maturity embryo production, effects that were often apparent even when comparing treatments that differed in population density by only one individual. While individual growth rate generally decreased as population density increased, we detected a hump-shaped relationship between embryo production and density, with females from intermediate-density treatments producing the most embryos and females from low- and high-density treatments producing the fewest embryos. The two lineages responded similarly to the treatments, indicating that these effects of population density might apply more broadly across P. antipodarum. These results indicate that there are profound and complex relationships between population density, growth rate, and early-maturity embryo production in at least two lineages of this important model system, with potential implications for the study of invasive populations, research on the maintenance of sex, and approaches used in ecotoxicology.
Highlights
Potamopyrgus antipodarum, a freshwater snail native to New Zealand, has achieved prominence as the focus of a large body of research aimed at understanding the maintenance of sexual reproduction (e.g., [1,2,3]), as a destructive invasive species [4], and as an emerging ecotoxicology model system (e.g., [5,6,7])
We found that specific growth rate of juvenile P. antipodarum was markedly decreased at higher population densities and that embryo production by adult female P. antipodarum was lowest in the relatively high and relatively low population density treatments and highest at intermediate population densities
There was no difference between the two P. antipodarum lineages in how SGR and embryo production responded to the different population density treatments
Summary
Potamopyrgus antipodarum, a freshwater snail native to New Zealand, has achieved prominence as the focus of a large body of research aimed at understanding the maintenance of sexual reproduction (e.g., [1,2,3]), as a destructive invasive species [4], and as an emerging ecotoxicology model system (e.g., [5,6,7]) These studies typically use traits like reproductive output and individual growth rate to, for example, estimate the relative fitness of sexual vs asexual individuals (e.g., [8]), measure responses to environmental pollutants (e.g., [9]), and evaluate sensitivity of invasive populations to nutrient limitation (e.g., [10]). Because most studies of the relationships between fitness-related traits and environmental variables in P. antipodarum do not include individuals from multiple genetically distinct asexual lineages (e.g., [10,11,12,13]), the extent to which the conclusions of these studies apply across this notably diverse species [20], [21] remains unclear
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