Armor reduction and pelvic girdle loss in a population of threespine stickleback (Gasterosteus aculeatus ) from western Newfoundland, Canada

Abstract We describe the antipredator armor of a unique population of threespine stickleback ( Gasterosteus aculeatus ) from Narrows Pond in western Newfoundland and compare traits for this population to nearby populations from marine and freshwater systems. After standardizing for length, Narrows Pond stickleback are shallower bodied and have shorter dorsal spines than stickleback from the other populations. Also, though the number of armor plates for Narrows Pond stickleback is greater than for typical low-plate morphs, the size of the lateral plates for Narrows Pond stickleback is much smaller. Finally, most (nearly 75% of sampled individuals) Narrows Pond stickleback do not have a pelvic structure (bilateral pelvic plate, ascending process, and ventral spine) and the remaining individuals have greatly reduced pelvic girdle whereas all individuals from the other populations possessed complete pelvic structures.