Abstract
BackgroundOver the last 300 years, interactions between alewives and zooplankton communities in several lakes in the U.S. have caused the alewives’ morphology to transition rapidly from anadromous to landlocked. Lakes with landlocked alewives contain smaller-bodied zooplankton than those without alewives. Landlocked adult alewives display smaller body sizes, narrower gapes, smaller inter-gill-raker spacings, reach maturity at an earlier age, and are less fecund than anadromous alewives. Additionally, landlocked alewives consume pelagic prey exclusively throughout their lives whereas anadromous alewives make an ontogenetic transition from pelagic to littoral prey. These rapid, well-documented changes in the alewives’ morphology provide important insights into the morphological evolution of fish.Predicting the morphological evolution of fish is crucial for fisheries and ecosystem management, but the involvement of multiple trophic interactions make predictions difficult. To obtain an improved understanding of rapid morphological change in fish, we developed an individual-based model that simulated rapid changes in the body size and gill-raker count of a fish species in a hypothetical, size-structured prey community. Model parameter values were based mainly on data from empirical studies on alewives. We adopted a functional trait approach; consequently, the model explicitly describes the relationships between prey body size, alewife body size, and alewife gill-raker count. We sought to answer two questions: (1) How does the impact of alewife populations on prey feed back to impact alewife size and gill raker number under several alternative scenarios? (2) Will the trajectory of the landlocked alewives’ morphological evolution change after 150–300 years in freshwater?ResultsOver the first 250 years, the alewives’ numbers of gill-rakers only increased when reductions in their body size substantially improved their ability to forage for small prey. Additionally, alewives’ gill-raker counts increased more rapidly as the adverse effects of narrow gill-raker spacings on foraging for large prey were made less severe. For the first 150–250 years, alewives’ growth decreased monotonically, and their gill-raker number increased monotonically. After the first 150–250 years, however, the alewives exhibited multiple evolutionary morphological trajectories in different trophic settings. In several of these settings, their evolutionary trajectories even reversed after the first 150–250 years.ConclusionsAlewives affected the abundance and morphology of their prey, which in turn changed the abundance and morphology of the alewives. Complex low-trophic-level interactions can alter the abundance and characteristics of alewives. This study suggests that the current morphology of recently (∼300 years)-landlocked alewives may not represent an evolutionarily stable state.
Highlights
Over the last 300 years, interactions between alewives and zooplankton communities in several lakes in the U.S have caused the alewives’ morphology to transition rapidly from anadromous to landlocked
Modeling rapid morphological changes of alewives To improve our understanding of rapid morphological changes in fish, we developed an individual-based model that simulates rapid changes in the body size and gill raker count of a fish species in a hypothetical, size-structured prey community
Alewife morphological changes before and after the first 250 years For the first 150–250 years after the alewives were introduced into the ecosystems, their growth (P(A)) decreased monotonically and their gill-raker counts (P(B)) increased monotonically under most combinations of trophic structure configuration (2 or 3 prey clusters), within-trophic competition level, and trophic interactions
Summary
Over the last 300 years, interactions between alewives and zooplankton communities in several lakes in the U.S have caused the alewives’ morphology to transition rapidly from anadromous to landlocked. Landlocked alewives consume pelagic prey exclusively throughout their lives whereas anadromous alewives make an ontogenetic transition from pelagic to littoral prey These rapid, well-documented changes in the alewives’ morphology provide important insights into the morphological evolution of fish. To obtain an improved understanding of rapid morphological change in fish, we developed an individual-based model that simulated rapid changes in the body size and gill-raker count of a fish species in a hypothetical, size-structured prey community. Functional trait approaches for studying rapid morphological evolution in fish Predator-prey interactions affect contemporary evolutionary processes such as the rapid morphological changes observed in several fish species [1,2,3,4]. Entire communities can be analyzed by studying the functional abilities of both the assemblage as a whole and the component species individually [5,6,7]
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