Adaptive divergence of lateral plate ultrastructure in threespine stickleback (Gasterosteus aculeatus)

The lateral plates of threespine stickleback (Gasterosteus aculeatus Linnaeus, 1758) are well studied for their adaptive morphological responses to predators, yet it is unknown whether habitat influences plate ultrastructure. We investigated using scanning electron microscopy the lateral plate ultrastructure (tubercles and ridges) of stickleback (N = 61 adult fish) from nine Haida Gwaii (coastal British Columbia, Canada) wild-type populations, two experimental transplants, and two lab-reared cohorts reared from source populations. Tubercle density, but not ridge density, differed significantly across habitat types and populations. Among wild-type fish, tubercle densities were greatest in dystrophic habitats containing predatory fish, and lowest in weakly dystrophic systems featuring bird-invertebrate predation and marine populations with diverse predatory fish. No differences in tubercle density were detected between source and transplant populations, despite major habitat shifts. Lab-reared fish exhibited significantly lower tubercle densities than their source populations (less than one generation). Tubercle density differences across habitat types may reflect adaptation to divergent predation regimes, with tooth-bearing predators selecting for denser tubercles that disperse point forces. Conservation of ridge density across populations suggests an essential function in dispersing forces applied to dorsal spines during predator manipulation. Lateral plate ultrastructure in threespine stickleback thus results from both heritable effects and developmental plasticity.