Temperature-related physiological adaptations promote ecological divergence in a sympatric species pair of temperate freshwater fish, Coregonus spp.

1. Understanding the ecological forces that are responsible for the evolution and coexistence of closely related sympatric species is a major interest in evolutionary ecology. In a freshwater system context, flocks or pairs of sympatrically occurring fishes are commonly studied to identify causes and mechanisms of ecological divergence and speciation. 2. Whereas habitat and diet segregation along the pelagic-benthic axis is frequent in many species pairs in temperate freshwaters, two pelagic planktivores coexist in the post-glacial Lake Stechlin, Germany (common vendace, Coregonus albula and the endemic Fontane cisco, C. fontanae). Accordingly, disruptive selection on traits not related to feeding may have contributed to divergence. Since both species differ slightly in their vertical distribution, species-specific physiological adaptations related to temperature are likely. 3. We investigated standard and active metabolic rates (AMR) over the range of environmental temperature, which both species experience naturally (4-15 degrees C). Eighty-four specimens of 7-22 cm length were exposed to five relative flow velocities between 0.5 and 2.0 body lengths per second (BL s(-1)) in swim tunnel respirometers. 4. Fontane cisco showed a reduced standard metabolic rate (SMR) compared to vendace over the entire temperature range. Further, activity metabolism was associated with lower energetic costs in Fontane cisco at 4 degrees C, but higher costs than in vendace at 8 degrees C and 15 degrees C. Total metabolic costs when swimming at 2 BL s(-1) were higher for vendace at < 8 degrees C, but higher for Fontane cisco if temperature exceeds 8 degrees C. 5. The results indicate that species-specific physiological adaptations contribute to ecological divergence, thus preventing competitive exclusion between the pelagic coregonids of Lake Stechlin. The existence of a vertical gradient in water temperature was probably a crucial factor for the evolution of both competing temperature-related physiological strategies.