Research Article: Profound Effects of Population Density on Fitness-Related Traits in an Invasive Freshwater Snail

Date Published: November 21, 2013

Publisher: Public Library of Science

Author(s): Nicholas Zachar, Maurine Neiman, Lilach Hadany.

http://doi.org/10.1371/journal.pone.0080067

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.

Partial Text

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]–[3]), as a destructive invasive species [4], and as an emerging ecotoxicology model system (e.g., [5]–[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]).

We established our experiment with 207 arbitrarily selected juvenile P. antipodarum (<2 mm in shell length (e.g., [17]) from each of two different asexual triploid lineages. One lineage was descended from a single asexual female sampled in January 2009 from Lake Waikaremoana (North Island, New Zealand, “Waikaremoana”), and the other lineage was descended from a single asexual female sampled in 2008 from a stream entering Lake Ontario, New York (“Ontario”). Potamopyrgus antipodarum is not an endangered or protected species, and necessary permits were granted by the New Zealand Department of Conservation, the New York State Department of Environmental Conservation, and the Iowa Department of Natural Resources. Population density significantly affected the SGR of juvenile P. antipodarum (Fig. 1, Table 1). Spearman's correlation analyses evaluating the association between population density and SGR indicated that this effect was driven by decreased SGR in higher-density treatments relative to lower-density treatments (Fig. 1; both lineages, Spearman's ρ = −0.466, p<0.0001; Ontario: Spearman's ρ = −0.529, p = 0.003; Waikaremoana, Spearman's ρ = −0.593, p = 0.001). There was also a significant main effect of lineage on SGR, demonstrating the existence of genetic variation for growth rate (also see [16]). There was not a significant density by lineage interaction, indicating that growth rate in the Waikaremoana and Ontario lineages responded similarly to the population density treatments (Fig. 1, Table 1). 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. While direct inferences to P. antipodarum as a whole and to natural populations are impossible given that the study included only two laboratory-raised lineages and took place in a laboratory setting, the similarity in response of growth rate and especially embryo output of these different two lineages suggests that our results might be informative with respect to other P. antipodarum lineages and/or to natural P. antipodarum populations (also see [11]).   Source: http://doi.org/10.1371/journal.pone.0080067