Small-scale morphological differentiation in three brooding sea star species with limited dispersal

For species with limited dispersal potential, variation in morphological traits on a small geographic scale might be due to some combination of phenotypic plasticity, natural selection or random genetic drift. We analyzed morphological variation for eight characters in three closely-related species of sea stars in the genus Leptasterias, subgenus Hexasterias, each with an obligate brood-protecting mode of reproduction. The four sampling locations were closely spaced (distances between locations were less than 20 km), and each species was present at three or four of the locations. For two species (Leptasterias aequalis species B and L. hexactis), the amount of morphological variation among the four locations was not significantly different from zero. The remaining species (L. aequalis species A) exhibited morphological variation among the same locations that was significantly greater than zero. The different patterns of microgeographic variation in morphology for the three species could be the result of either natural selection in a heterogeneous environment or phenotypic plasticity, but cannot be attributed to some other non-selective explanations, such as genetic drift, hybridization or undetected cryptic species.