Date of Award

Summer 8-19-2016

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)


Marine Biology


Laurie Connell

Second Committee Member

Brian Beal

Third Committee Member

Neal Pettigrew

Additional Committee Members

Paul Rawson

Rebecca Van Beneden


The softshell clam, Mya arenaria, is a commercially important bivalve species that is found in soft-bottom intertidal habitats throughout the Gulf of Maine, USA. This species is subjected to seasonal blooms of the toxic algae Alexandrium spp., and acts as a vector for paralytic shellfish poisoning (PSP) during harmful algal bloom (HAB) events. Some clams possess a naturally occurring genetic mutation of their voltage-gated sodium channels that grants them a resistance to the paralytic effects of saxitoxin (STX) produced by Alexandrium spp. The mutation allows these individuals to continue feeding during HABs, and greatly increases their tissue toxicity through bioaccumulation. This work describes the distribution of the resistant mutation in wild clam populations in the Gulf of Maine, and explores the population structure of M. arenaria with regard to the mutation, as well as neutral genetic markers. Analysis of neutral markers revealed no significant population structure within the Gulf of Maine, however M. arenaria does exhibit strong localized structure at the STX-resistant mutation locus. This structure is sustained by differential selective pressure exerted by Alexandrium spp. blooms, despite freely occurring gene flow among clam populations. In Penobscot Bay, one area where the prevalence of the resistant mutation did not match the strength of selective pressure, it is likely that the resistant allele is maintained by gene flow through larval transport from other regions, rather than by seeding of hatchery stock carrying the mutation. This work can aid PSP monitoring efforts by identifying areas where risk is greatest to humans due to high numbers of resistant clams. In addition, distinguishing areas where one genotype is clearly favored over the others may be of interest to seeding programs trying to ensure that their stock is well suited for the location to which they will be transplanted.