Marc Pépino, Pierre Magnan, and Raphaël Proulx
Individual living organisms have the ability to modify their morphology, physiology, or behaviour in response to environmental changes, referred to as phenotypic plasticity. Over the longer term, populations of the same species inhabiting different environments could evolve toward distinct forms, a process called resource polymorphism. For example, brook charr, a cold-water species inhabiting many lakes and rivers of the Canadian Shield, exhibit such a polymorphism: individuals found in the littoral habitat (the shallow near-shore zone) are stouter with longer fins while those found in the pelagic habitat (open-water zone) are more fusiform and have shorter fins. This should make the littoral individuals more efficient at feeding on bottom prey (through increased manoeuvrability) and pelagic individuals more efficient at feeding on zooplankton (through decreased drag for sustained swimming). Having this adaptability should benefit individuals, for example, by improving growth or reproductive success. However, very few studies have explored how fast individuals can adapt to changing environments in natural systems.
We used brook charr to determine if it has the ability to modify its morphology when transferred from the littoral to the pelagic habitat (and vice versa), if these changes are in the expected direction, and if they increase individual growth. We transferred four-month-old individuals of both types, raised in the laboratory, into littoral or pelagic lake enclosures for a period of 12 weeks. We found that growth was higher in the pelagic than in the littoral habitat. The body shape of most individuals shifted toward the morphology expected in the new environment. This plastic response in body shape was related to growth, but only in the littoral habitat, where conditions were presumably more severe (warmer water temperatures, lower food supply). This study provides field evidence that young brook charr have the ability to adapt to a new environment and that these changes can increase their growth. These rapid functional responses (i.e., within a generation) could subsequently hasten the process of natural selection, but only in an environment where growth is more severely constrained.
Image caption: Research assistant installing a pelagic enclosure. Photo credit: P. Magnan.