Abstract The Atlantic surfclam, Spisula solidissima

• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.463.9299; http://www2.dnr.cornell.edu/HareLab/pdf/2005b.pdf

and Cape Hatteras harvests offshore populations of one subspecies, S. s. solidissima. A smaller coastal form, S. s. similis Say (also known as S. s. raveneli Conrad), has a partially sympatric geographic distribution, but differs in several life-history characteristics. DNA sequence variation in mitochondrial cytochrome oxidase I (COI) and in introns at two nuclear calmodulin loci was examined to measure genetic divergence between the two subspecies and to test for population structure among populations of S. s. solidissima. Surfclams were collected from seven localities between 1994 and 2001. Based on both mitochondrial and nuclear DNA variation, the two subspecies of S. solidissima are reciprocally monophy-letic, with a net COI divergence of 13.9%, indicating long-term reproductive isolation. The only significant differentiation among populations of S. s. solidissima (based on an AMOVA analysis of COI sequences) was between the Gulf of St. Lawrence and more southerly populations. A long internal branch in the S. s. soli-dissima genealogy coupled with low haplotype diversity in the northern-most population suggests that popula-tions north and south of Nova Scotia have been isolated from each other in the past, with gene exchange more recently. Populations of S. s. similis from Atlantic and Gulf of Mexico coasts had a net COI divergence of 9.2%. Thus, diversification of Spisula spp. clams in the western North Atlantic involved an early adaptive divergence between coastal and offshore forms, with later barriers to dispersal emerging in the offshore form from north to south and in the coastal form between Atlantic and Gulf of Mexico populations.