The reduction of the number of bony lateral plates following the invasion of freshwater habitats by ancestral marine threespine sticklebacks (Gasterosteus aculeatus) became a textbook example of adaptive radiation and parallel evolution. Even the genetic background (variation in the Ectodysplasin gene) of plate number variation has been resolved.
Both abiotic (salinity, ion availability) and biotic (predation) environmental factors were envisioned as the main selective force behind the repeated, independent lateral plate reduction observed in the species. However, experimental tests have started to accumulate only recently. It has been shown at both phenotypic and genetic levels that low number of lateral plates is beneficial in freshwater even in the absence of predation. There is experimental evidence showing that insect predators (highly abundant in freshwater) select against body armour in sticklebacks. However, the hypothesis postulated by Thomas Reimchen in 1992 (Evolution 46:1124-1230), predicting (i) a selective advantage for high plate number in pelagic habitats (where fish cannot hide) due to protection against toothed predators, and (ii) a selective advantage for low plate number in benthic habitats (where refuges are available) due to increased maneuverability and fast-start performance, has never been experimentally tested.
Here, we set up an experiment with laboratory-reared predator-naive threespine sticklebacks and their common predators (pike, Esox lucius) to compare piscine predation induced selection on morphology in the absence / presence of refuge. The results fully supported Reimchen’s hypothesis: pikes selected against low plate number in the absence and against high plate number in the presence of refuge. In addition, we found that irrespective of refuge availability, survival was negatively correlated to caudal peduncle length.
Our results support another selective force behind the decrease in threespine stickleback lateral plate number following freshwater invasions, and provide an example of the context-dependent value of an antipredatory trait.
Leinonen T, Herczeg G, Cano JM, Merilä J 2011 Predation-imposed selection on threespine stickleback (Gasterosteus aculeatus) morphology: a test of the refuge use hypothesis. Evolution, in press.