Comparative Study of Sun Compass Orientation in Migrating Anadromous versus Resident Freshwater Threespine Sticklebacks (Gasterosteus aculeatus).

In the northern coastal hemisphere, different ecotypes of the threespine stickleback (Gasterosteus aculeatus) can be distinguished phenotypically by a different number of lateral bone plates and by their body shape and size. We focused on (1) anadromous sticklebacks, which migrate from the sea to rivers to spawn and (2) freshwater sticklebacks, which live in rivers all year round. Migration behavior is a key feature in the evolution of ecotypes, but the underlying mechanisms of migration are poorly understood. To learn more about possible orientation mechanisms that could lead to goal-directed migration, we tested anadromous sticklebacks for their sun compass orientation and compared their orientation behavior with that of the freshwater ecotype. Behavioral experiments revealed ecotype-dependent differences, whereby the ability to orient is consistently present in the anadromous ecotype, whereas the orientation in the freshwater ecotype corresponds to a random directional distribution.