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Access Type

Open Access Thesis

Document Type


Degree Program

Environmental Conservation

Degree Type

Master of Science (M.S.)

Year Degree Awarded


Month Degree Awarded



Atlantic horseshoe crabs (Limulus polyphemus) have endured decades of intense harvest pressure. Genetics studies have shown evidence of distinct sub-groups spanning the coast, although few fine-scale studies have been done to delineate these groups on a local level. Massachusetts lies directly between two of these sub-groups. With documented differences in prosomal widths of horseshoe crabs from either side of Cape Cod, it is possible that Cape Cod is a barrier to gene flow and that there are two distinct genetic groups within Massachusetts. Regulations currently consider all horseshoe crabs to be of one stock. I examined 6 microsatellite loci from 193 horseshoe crabs collected from 7 locations across Massachusetts between 5 May and 24 June 2010. I also analyzed the prosomal widths of 324 horseshoe crabs from 8 locations across Massachusetts. Data analysis revealed low divergence with a G′ST of 0.005 (95% CI −0.004–0.013) and a G″ST of 0.015 (95% CI −0.014–0.045). Wellfleet Bay showed evidence of divergence from all other sites except Buzzards Bay. Isolation by distance is apparent via the Atlantic Ocean. Phenotypic variation in the prosomal widths of horseshoe crabs shows greater divergence among sites than neutral markers and indicates the presence of additive genetic effects. Low divergence and high heterozygosity indicate that although documented population declines have occurred, effective population size (Ne) is still large enough to maintain allele frequencies. With isolation by distance, divergence is likely to increase over time if populations remain low. Phenotypic divergence shows the possibility of local adaptation and that the implementation of management units (MUs) to the north and south of Cape Cod would be recommended as a conservative measure.


First Advisor

Andrew Whiteley

Second Advisor

Francis Juanes