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

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.

Effects of polyploidy and reproductive mode on life history trait expression

“…2013a), temperature (Dybdahl and Kane 2005), and population density (Neiman et al. 2013a; Zachar and Neiman 2013) suggest that at least some of the genetic variation for growth rate observed in benign laboratory conditions may be suppressed or expressed differently in the more heterogeneous natural populations. Indirect support for this possibility is provided by Neiman et al.…”

“…Because a higher growth rate likely translates into fitness advantages (earlier reproduction) for female P. antipodarum (also see Tibbets et al 2010), our results raise the questions of why there exists substantial genetic variation for this trait in P. antipodarum and why sexual P. antipodarum grow and mature relatively slowly. With respect to the maintenance of genetic variation for growth rate, the well-documented dependence of growth rate in P. antipodarum on environmental variables like food quality (Neiman et al 2013c;Krist et al 2014), food quantity (Neiman et al 2013a), temperature (Dybdahl and Kane 2005), and population density (Neiman et al 2013a;Zachar and Neiman 2013) suggest that at least some of the genetic variation for growth rate observed in benign laboratory conditions may be suppressed or expressed differently in the more heterogeneous natural populations. Indirect support for this possibility is provided by Neiman et al (2013c), who found that the growth rate advantages experienced by tetraploid versus triploid P. antipodarum when fed a high-phosphorus diet (notably, a diet higher in phosphorus than the snails in the present study received) disappear under relatively low-P conditions.…”

“…Each snail was fed three times per week with dried Spirulina , a common laboratory food source for P. antipodarum ( e.g ., Neiman et al. 2013a; Zachar and Neiman 2013). All females isolated from the field were housed with males to ensure that sexual females, which are phenotypically indistinguishable from asexual females, would have the opportunity to become fertilized.…”

“…All founding females were individually housed in a oneliter cup filled with~200 mL carbon-filtered tap water and maintained in a room held at 16°C and under a 16:8 light/dark cycle. Each snail was fed three times per week with dried Spirulina, a common laboratory food source for P. antipodarum (e.g., Neiman et al 2013a;Zachar and Neiman 2013). All females isolated from the field were housed with males to ensure that sexual females, which are phenotypically indistinguishable from asexual females, would have the opportunity to become fertilized.…”

Phenotypic variation in mitochondrial function across New Zealand snail populations

“…Spirulina algae three times per week, as previously described 62 . We arbitrarily selected adult 114 snails from lake collections (each of which consisted of hundreds to thousands of individuals) 115 and isolated each snail in a 0.5 L glass container with 300ml carbon-filtered H 2 O.…”