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Reference: Biol. Bull. 198: 396–403. (June 2000)

Genetic Diversity of Oceanic Island Lasaea (Mollusca: Bivalvia) Lineages Exceeds That of Continental Populations in the Northwestern Atlantic

´ JOONG-KI PARK AND DIARMAID O FOIGHIL*

Museum of Zoology and Department of Biology, University of Michigan, Ann Arbor, Michigan 48109-1079

Abstract. Direct-developing lineages of the intertidal ma- rine bivalve Lasaea have colonized both upstream mainland (southeastern Florida) and downstream oceanic island (Ber- muda) locations in the western North Atlantic. Replicate samples from these two regional populations, separated by about 1500 km of open ocean, were sequenced for a 655- nucleotide portion of the mitochondrial (mt) cytochrome oxidase subunit I gene. Twelve haplotypes (2 Floridian and 10 Bermudan) were obtained that differed by a maximum of four substitutions among pairwise comparisons. Phyloge- netic analysis yielded a parsimony network within which the mainland lineages clustered in one of the terminal branches; a mirror image of a priori expectations based on regional surface-current polarity. It is difficult, however, to envisage a plausible countercurrent dispersal mechanism. This tree topology may stem from divergent demographic processes operating on these two evolutionarily recent re- gional populations. The starlike phylogenetic pattern of Bermudan lineages is consistent with a history of rapid population growth. The restricted genetic repertoire and relative ecological scarcity of Floridian lineages imply ei- ther a recent founder event by unstudied Caribbean source populations or else a history of pronounced bottlenecks in population size. Bermuda’s impoverished Caribbean marine biota may allow western North Atlantic Lasaea lineages to escape severe competitive interactions impacting other parts of their geographic range.

Received 15 September 1999; accepted 17 February 2000.

  • *

    To whom correspondence should be addressed. E-mail: diarmaid@

umich.edu

Introduction

Oceanic islands have never been connected to continental landmasses and represent some of the most isolated envi- ronments on earth (Nunn, 1994). They receive their biotas solely through dispersal from geographically distant source populations and from subsequent in situ diversification (Paulay, 1994). Stretches of deep ocean surrounding such islands represent dispersal filters for benthic shallow-water marine taxa. The stringency of these filters varies according to physical parameters (e.g., distance from potential source populations, current speed and polarity) and to taxon-spe- cific life-history traits including presence of a prolonged pelagic larval ontogeny (Scheltema, 1992; Jaeckle, 1994; Palumbi et al., 1997; Lessios et al., 1998), rafting ability

(Jokiel, 1990; Helmuth et al., 1994; Ingo´lfsson, 1995; ´

  • O

    Foighil et al., 1999), and propensity for anthropogenic

transfer (Carlton, 1989; Carlton and Geller, 1993; Carlton and Hodder, 1995). In general, classical oceanic island biotas exhibit a depauperate and disharmonic composition that may include relict taxa and endemic radiations (Paulay, 1994).

Bermuda is composed of a cluster of small western North Atlantic oceanic islands (32°18 N, 64°46 W) rising from a 65-km2 carbonate platform and supported by a submerged volcanic peak (Nunn, 1994; Sterrer, 1998). It is situated within the warm Sargasso Sea section of the North Atlantic Gyre, about 1000 km east of the coast of North Carolina, and the main flow of the Gulf Stream System arcs well to the west and north of its position (Fig. 1). However, the Bermuda Platform is regularly approached by mesoscale anticyclonic eddies originating from the Gulf Stream Sys- tem (Parker, 1971; Spitzer, 1989). It is generally assumed that Bermuda has remained stable throughout the Quater-

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