Metabolic rate is commonly thought to scale with body mass to the 3/4 power as a result of universal body design constraints. However, recent comparative work has shown that the metabolic‐scaling slope may vary significantly among species and higher taxa, apparently in response to different lifestyles and ecological conditions, though the precise mechanisms involved are not well understood. To better understand these underappreciated ecological effects and their causes, it is important to control for extraneous phylogenetic and environmental influences. We demonstrate how this may be done by comparing the ontogenetic scaling of resting metabolic rate among populations of the same species (the amphipod Gammarus minus) in mid‐Appalachian freshwater springs with similar, relatively constant environmental conditions, except for the varying presence of the predatory fish Cottus cognatus. We found that populations of G. minus exhibit significantly lower metabolic‐scaling slopes (0.54–0.62) in three freshwater springs with C. cognatus than in two springs without these fish (0.76–0.77). We tested multiple hypothetical causes for these population differences. Our results best supported the hypothesis that metabolic scaling was influenced by the effects of size‐selective predation on the ontogeny of growth, a metabolically expensive process. The body size scaling of growth is significantly less steep in the populations inhabiting springs with vs. without fish, thus paralleling the interpopulation differences in metabolic scaling. Prematurational growth of G. minus is as high or higher in the fish springs, whereas postmaturational growth is significantly lower, often approaching zero. Similarly, the amphipods in the fish springs tend to have higher metabolic rates at small sizes, but lower metabolic rates at large sizes, compared to those in the fishless springs. Our results do not support other hypothetical causes of the interpopulation variation in metabolic scaling, including differential scaling of cell size or low‐metabolism body components (fat and mineralized exoskeleton), or possible effects of other environmental factors associated with the presence of fish. However, fish‐induced population differences in adult behavioral activity may influence metabolic scaling in G. minus, a possibility under current study. We conclude that ecological factors may significantly influence metabolic scaling, contrary to common belief.

Abstract. Intraspecific variation in eye size in relation to ecological factors has not been well studied. Here, for the first time, we show that larger eyes in a freshwater crustacean may be associated with the presence of predators. In central Pennsylvania (USA), individuals of the amphipod crustacean Gammarus minus have significantly larger eyes in two freshwater springs with numerous fish predators (Cottus cognatus) than in three springs with few or no fish predators. Although we do not know the precise causes of these differences, this study and previous work on cave populations of G. minus suggest that eye size is an evolutionarily malleable trait that may respond to multiple selection pressures, either directly or indirectly. Three plausible explanations for the eye‐size variation observed among our study populations include (1) larger eyes may enable amphipods to better detect and avoid fish predators, (2) fish predation favors nocturnal or shallow interstitial activity that is facilitated by larger, more light‐sensitive eyes, or (3) the presence of fishes is associated with other environmental factors that may favor relatively large eyes. Available evidence suggests that the first hypothesis is the most viable explanation, but further study is required.

We infer the body-size scaling slope of metabolic rate in a trilobite by applying a cell-size model that has been proposed to explain metabolic scaling in living organisms. This application is especially tractable in fossil arthropods with well-preserved compound eyes because the number and size of eye facets appear to be useful proxies for the relative number and size of cells in the body. As a case study, we examined the ontogenetic scaling of facet size and number in a ∼390-Myr-old local assemblage of the trilobite Eldredgeops rana, which has well-preserved compound eyes and a wide body-size range. Growth in total eye lens area resulted from increases in both facet area and number in relatively small (presumably young) specimens, but only from increases in facet area in large (presumably more mature) specimens. These results suggest that early growth in E. rana involved both cell multiplication and enlargement, whereas later growth involved only cell enlargement. If the cell-size model is correct, then metabolic rate scaled allometrically in E. rana, and the scaling slope of log metabolic rate versus log body mass decreased from ∼0.85 to 0.63 as these animals grew. This inferred age-specific change in metabolic scaling is consistent with similar changes frequently observed in living animals. Additional preliminary analyses of literature data on other trilobites also suggest that the metabolic scaling slope was <1 in benthic species, but ∼1 in pelagic species, as has also been observed in living invertebrates. The eye-facet size (EFS) method featured here opens up new possibilities for examining the bioenergetic allometry of extinct arthropods.

Islands of high genomic divergence that contain genes of evolutionary significance may form between diverging species. The gopher rockfish, Sebastes carnatus, and black-and-yellow rockfish, S. chrysomelas, are sympatrically distributed temperate marine species inhabiting rocky reefs and kelp forests on the west coast of the United States. Prior studies documented low levels of genetic divergence between the two species, except at a single microsatellite locus that displayed high divergence, Sra.7-2. To better characterize genome wide divergence, we scored 25 additional microsatellite loci. Mean neutral divergence between species (F(ST) = 0.01) changed little from prior estimates. Sra.7-2 continued to be an extreme divergence outlier. Five novel microsatellites within ± 15 kb of Sra.7-2 were characterized. High divergence, consistently low diversity in S. chrysomelas, and linkage disequilibrium were detected at these loci, suggesting the influence of recent selection. However, coalescent modelling of divergence at neutral and Sra.7-2 regions showed that initial divergence at Sra.7-2 was ancient, likely predating divergence at neutral regions. It is therefore unlikely that Sra.7-2 divergence represents solely recent ecological divergence within one species and may represent the action of recurrent selection. Introgressive gene flow (2N(E) m) was much higher (>>1) at neutral than Sra.7-2 regions (<<1) despite evidence that two S. carnatus individuals have recently mixed ancestry at the Sra.7-2 region. The Sra.7-2 genomic region is likely one of several regions containing genes involved in initiating and maintaining species integrity. Completion of the final stages of speciation appears to be a slow and ongoing process for these species.