Breeding ecology and evidence of reproductive isolation of a widespread stickleback fish (Gasterosteidae) in Nova Scotia, Canada

Abstract

A stickleback with brilliant white dorsal breeding colours is widely distributed in north-eastern Nova Scotia, Canada, where it often breeds sympatrically with the threespine stickleback, Gasterosteus aculeatus. Breeding males are highly conspicuous and visible at distances of 20 m or more whereas sympatric G. aculeatus are cryptic and difficult to detect even at 2 m. The white stickleback nests only above the substrate in filamentous algae, where G. aculeatus nests only on the substrate. The white stickleback is smaller in size and more terete than G. aculeatus, but it is morphologically similar in having a complete row of lateral plates and similar lateral plate and gill raker numbers. The white stickleback occurs only in environments where there are filamentous algae (which appears to be an obligatory nesting substrate) and where the water is clear, saline and relatively still. Female choice tests in the laboratory show that the white stickleback is reproductively isolated from G. aculeatus, and field observations on natural spawning support this conclusion. We suggest that the bright breeding colouration may have evolved through sexual selection and/or to advertise unprofitability to predators.