Contemporary phenotypic change in correlated characters in the African cyprinid, Rastrineobola argentea

Abstract

Phenotypes reflect a complex interplay between the direct and indirect effects of multiple selective pressures; adaptive responses in individual traits may therefore be constrained by architectonic considerations and correlations with other traits. We explored whether and how three suites of neighbouring structures (respiratory, trophic and cranial) have changed over time in Rastrineobola argentea, a small pelagic cyprinid fish that is endemic to the Lake Victoria Basin in East Africa. We compared museum specimens from 1966 to modern (2010) specimens, spanning a period of almost 50 years of strong anthropogenic change in the lake, including increased frequency and extent of hypoxia and anoxia, and a dramatic restructuring of the lacustrine fish community and food-web. We found that modern R. argentea had significantly larger gills, wider heads, and shorter and more tightly packed gill rakers than their historical counterparts. Larger gill size probably represents a direct, adaptive response to more frequent bouts of hypoxia over time; the increase in head width and decrease in raker spacing may reflect correlated (and potentially maladaptive) responses to the increase in gill size. Integrative studies such as this are key to unravelling complex interactions between phenotypes and the environment.