FRESHWATER FISHES MAINTAIN MULTI-TRAIT PHENOTYPIC STABILITY ACROSS AN ENVIRONMENTAL GRADIENT IN AQUEOUS CALCIUM.

Reductions in a limiting nutrient might be expected to necessitate compromises in the functional traits that depend on that nutrient; yet populations existing in locations with low levels of such nutrients often do not show the expected degradation of functional traits. Indeed, we previously found that fish of three species, Logperch (Percina caprodes), Pumpkinseed Sunfish (Lepomis gibbosus), and Yellow Perch (Perca flavescens) residing in low-calcium water in the Upper St. Lawrence River nevertheless maintained levels of scale calcium comparable to those of conspecific populations in high-calcium water. However, it remains possible that the maintenance of one functional trait under nutrient-limited conditions could come at the expense of maintaining other functional traits that depend on that same nutrient. Hence, we here extend prior work by examining other calcium-dependent traits: skeletal element sizes and bone densities in the same fish species in the same area. Using radiographs of 101 fish from the three species across four locations (two in high-calcium water and two in low-calcium water), we document multi-trait "homeostasis" along the gradient of water calcium. That is, we did not detect any effect of calcium regime (low-calcium versus high-calcium) on any of the measured variables. Further, effect sizes for the skeletal traits were very low - lower even than effect sizes previously documented for scale calcium. Our results thus show that native fishes maintain phenotype stability across a suite of functional traits linked to calcium regulation, perhaps pointing to an "organismal-level homeostasis" scenario rather than a "trait-level homeostasis" scenario.