Population genetics and systematics of the Leptasterias hexactis (Echinodermata: Asteroidea) species complex

Morphological characters and 14 enzyme loci were examined for 1 040 sea stars, currently recognized as forms of Leptasterias hexactis, from Lynn Canal, Alaska, and Puget Sound, Washington, USA, between March 1988 and April 1989. Three morphologically and two genetically distinct Leptasterias forms were identified. The most common form found at both localities was L. epichlora (Brandt)sensu Verrill. L. hexactis (Stimpson)sensu Verrill co-occurred with L. epichlora at all study sites and apparently hybridizes extensively with L. epichlora in the Puget Sound region, but rarely, if at all, in Alaska. The presumptive product of this hybridization morphologically resembled L. aequalis (Stimpson)sensu Fisher, and was conspicuously absent from Alaskan samples. Considerable genetic distance existed between L. epichlora and L. hexactis (Nei's D=0.19±0.01) and moderate genetic differentiation occurred between populations of each species from Alaska and Washington (Weir and Cockerham's F_RT=0.29±0.04 for L. epichlora and 0.21±0.15 for L. hexactis). A significant (p<0.05) deficiency in the proportion of heterozygous individuals was found compared to Hardy-Weinberg expectations (Wright's fixation index, F_ID=0.12±0.04 and 0.31±0.08 for L. epichlora and L. hexactis, respectively). However, mean observed heterozygosity for each species (0.09±0.03, 0.14±0.04 and 0.14±0.04 for L. epichlora, L. hexactis and L. aequalis, respectively) fell within the range of reported values for other asteroid species (ca. 0.04 to 0.37). The results of this study indicate that considerable genetic integrity is maintained between L. epichlora and L. hexactis, which warrants their recognition as distinct species despite their apparent hybridization in the Puget Sound region.