Body shape variation within and among three species of darters (Perciformes: Percidae)

Abstract –  Using geometric morphometric (GM) techniques, we quantified intra and interspecific variation in female body shape using five collections each of Etheostoma caeruleum Storer, Etheostoma nigrum Rafinesque, and Etheostoma stigmaeum (Jordan). Nested multivariate analysis of variance showed significant variation in body shape among populations (Wilks' Λ = 0.0687, F _144,7585.1 = 19.35, P  < 0.0001), as well as among species (Wilks' Λ = 8.7 × 10⁻⁶, F _24,2 = 28.18, P  = 0.0348). Etheostoma caeruleum displayed greater body depth relative to both E. nigrum and E. stigmaeum , whereas E. nigrum displayed a compressed arrangement of mid-body landmarks relative to both E. stigmaeum and E. caeruleum . The broader implications of these findings highlight the value of geometric morphometrics as both an exploratory and analytical approach. Conclusions drawn from comparisons among moderately differentiated darter species in future studies of body size and shape are likely robust to intraspecific variation within species, and will permit more rigorous investigations into the ecomorphology of these benthic stream fishes.     

Body shape variation in two species of darters (Etheostoma,Percidae) and its relation to the environment

Environmental variation can shape phenotypic variation in organisms, but most evidence for trait differentiation comes from analyses of dichotomous habitat types that differ in only one or few key environmental factors. In reality, environmental variation is often more subtle, gradual and multifarious. Here, we investigated geographic variation in body shape of two darter species (Etheostoma spectabile and Etheostoma flabellare; Percidae) that occur along river gradients. This study addressed three specific questions: Is there intraspecific geographic variation in the two species across different sites in the Ozark Highlands of Oklahoma (USA)? Is phenotypic variation across sites correlated with abiotic environmental conditions? Do the two species share site‐specific (i.e. convergent) phenotypic variation in areas where they occur together? Our results indicated significant body shape variation in both species. Population differences in body shape were particularly correlated with variation in substrate composition. The combined analysis of both species indicated a small but significant effect of convergence on body shape wherever they are sympatric; shared variation, however, was not related to any environmental variables included in the analysis. While it remains unclear whether phenotypic variation in these species is due to heritable differentiation or environmentally induced plasticity, our results indicate that even subtle and gradual environmental variation can induce substantial variation in phenotypes on a relatively small spatial scale.     

Seasonal and interannual components of intrapopulation variation in clutch size and egg size of a darter

Abstract –  Previous studies have offered suggestive but sometimes conflicting glimpses at the range and patterns of seasonal and interannual variation in female darter life-history traits. The present study examined clutch-size and egg-size variation from a single Mississippi population of brighteye darters (Etheostoma lynceum ), collected at regular intervals over four sequential spawning seasons. Both life-history traits were significantly correlated with body size, and they both showed significant variation within and among breeding seasons. The overall intraseasonal trends in these traits involved an increase in clutch size (CS) and a decrease in egg size from early to late periods of the spawning season. Clutch size and egg size showed a weak but significant negative correlation, which was indicative of a trade-off between the two clutch parameters. Seasonal changes in temperature appear to be related to differing patterns of seasonal variation in egg size reported in the literature for darters. Inverse seasonal shifts in egg size and CS in the brighteye darter may represent adaptive phenotypic plasticity that allows females to produce larger, competitively superior offspring early in the reproductive season when there is low supply of food for them (parental investment hypothesis) or to produce larger eggs early in the reproductive season to ensure that each egg is adequately provisioned in the face of uncertainty (bet-hedging hypothesis) if the food available for the young is unpredictable early in the season. Consistent with theoretical predictions, egg size showed less phenotypic variability than CS.     

A descriptive and experimental study of microhabitat use by young-of-the-year benthic stream fishes

Abstract– We studied microhabitat use of young-of-the-year (YOY) benthic stream fishes ( Cottus, Etheostoma, Percina ) using underwater surveys conducted in the Little River of Tennessee, United States. At the upstream site, only Cottus was present and its density was low. Further downstream all genera were present, and YOY density was high and concentrated in deep water habitats. Microhabitat use by the species differed from microhabitat availability, with most species using deep, slow-moving water with silt-free, coarse substrates that lacked riverweed, Podostemum ceratophyllum. Despite differences in microhabitat use, there was substantial overlap among species. Because YOY were positively associated with coarse substrates and coarse substrates were uncommon in deep water, we tested their importance by adding cobbles to 3 deep water habitats. We found that YOY benthic fishes colonized areas with cobbles but not sand. After 2 months, however, many of the cobble areas were covered by sediment. These data suggest that YOY fish are substrate-limited in deep water habitats and that the hydrological properties of pools preclude establishment of permanent instream cover. It is unclear why YOY occur in areas with little instream cover.