Prolonged swimming performance within the threespine stickleback (Gasterosteus aculeatus) adaptive radiation and the effect of dietary restriction

Background: Organisms can be conceptualized as systems in which input energy is allocated to competing needs and on which natural selection acts to maximize one particular need reproductive output. In large part, this is achieved by minimizing the energy required by other needs. Among the more important competing needs in most fish is the need to swim, and fish musculature, physiology, and shape have evolved to minimize swimming energy use in an environment- and lifestyle-dependent manner. Question: How have growth and swimming performance evolved during adaptation to different freshwater habitats in the threespine stickleback? Does variation in early life food rations affect performance? Hypotheses: Based on presumed lifestyles, we predict that oceanic stickleback will show the highest growth rate and swimming performance, and of the freshwater populations, we expect those representing the limnetic ecotype to show stronger swimming performance than populations of the benthic ecotype, but not stronger growth performance. Month-long periods of nutrient deprivation early in life will negatively affect all three ecotypes, with deprivation later in the first growing season (closer to the overwintering period) having the largest effect on both performance variables. Organism: Eighteen allopatric populations of threespine stickleback, including six ancestral oceanic, six benthic, and six limnetic populations. Methods: We used an acute swimming assay performed on sub-adult (9-month-old) fish that measures prolonged swimming ability as an indicator of aerobic muscle capacity. We also measured size achieved, because size itself affects swimming performance. We subjected juvenile stickleback to reduced rations (one-half normal) during either their second, third, or fourth month of life to simulate diet variability, which is known to affect growth trajectories and muscle development. Results: Oceanic stickleback grew faster than freshwater stickleback under control conditions, but comparative growth was more variable when dietary reductions were considered. Diet restriction late in the first fall produced the most significant change in growth. Variations in growth across populations within ecotypes were also observed. Swimming stamina showed strong variation across ecotypes, and also modest population-level variation. Dietary restriction had less effect on swimming stamina than on size.