Intertidal microhabitat and selection at Mpi: Interlocus contrasts in the northern acorn barnacle, Semibalanus balanoides

Barnacles were sampled from various microhabitats in the rocky intertidal a t multiple sites in two years. At sites in which there were large differenc es among microhabitats in temperature profiles, Mpi genotype frequencies we re consistently and significantly different. Genotype frequencies for anoth er allozyme locus (Gpi) as well as a DNA marker shown to be neutral (the mt DNA control region) were statistically homogeneous among thermal microhabit ats at all sites in both years. The data indicate that temperature and/or d esiccation mediated selection is operating at Mpi or a linked locus and tha t Mpi genotypes experience differential mortality in the various habitat ty pes. If the relative fitness of genotypes is dependent on habitat type, the Mpi polymorphism may be actively maintained by a Levene model of balancing selection (Levene 1953). Because barnacle larvae are produced in abundance each year and spend five to eight weeks dispersing in the water column, th ere is little opportunity for the accumulation of adaptive divergence over the environmental grain size relevant in intertidal habitats. The Mpi polym orphism may be an important component of a suite of traits involved in the adaptation of barnacles to heterogeneous environments. Due to the relativel y high concentration of mannose in a variety of algal groups, the metabolis m of mannose may substantially affect individual performance and fitness in this and other species that feed on algae and phytoplankton. Because the M pi locus is one of the most strongly polymorphic in marine organisms, these findings may be relevant for a diversity of other such species.