GENETIC-STRUCTURE OF BRACKISH-WATER CLAMS (RANGIA SPP)

Two congeneric species of brackish water clams, Rangia coneata (Sowerb y, 1831) and Rangia flexuosa (Conrad, 1839), that are sympatric in the northern Gulf of Mexico, were analyzed for electrophoretic variation at 19 allozyme loci. Rangia cuneata also was analyzed for restriction- site and (partial) nucleotide sequence variation for a mitochondrial g ene, cytochrome oxidase I (COI). Rangia cuneata has greatly increased its abundance along the mid-Atlantic coast of the U.S.A. within the la st 100 years, due either to colonization from southern populations or to expansion of indigenous populations. Rangia cuneata populations fro m Virginia to southern Florida formed a discrete and homogeneous group based on allozyme allele frequencies, as did populations from Mississ ippi to Texas (Nei's unbiased genetic distance between the two groups was 0.03). Atlantic coast populations of R. cuneata also had uniformly lower expected heterozygosities and lower numbers of alleles per locu s than Gulf populations. The COI gene was largely monomorphic and most ly uninformative about possible genetic differences between Atlantic a nd Gulf coast R. cuneata, but a polymorphic Mboll restriction site in the Gulf was monomorphic in all examined Atlantic populations. These d ata are more consistent with the indigenous-expansion model than the s outhern-expansion model. Nei's genetic distance between R. cuneata and R. flexuosa was large (1.45), but there was no evidence for the exist ence of a complex of sibling or semi-species within R. cuneata.