Patterns of Morphological Diversity among and within Arcid Bivalve Species Pairs Separated by the Isthmus of Panama

Geminate species are morphologically similar sister-species found on either side of the Isthmus of Panama. The existence of all geminates in the tropical Eastern Pacific ocean and the Caribbean Sea is most often explained by vicariance: closure of the Central American Seaway 3.1 to 3.5 Ma simultaneously isolated populations of species with amphi-American distributions. In this paper, we test the potential of morphological measurements for discriminating between Recent geminate species pairs from three genera (Area, Arcopsis, and Barbatia) in the bivalve family Arcidae and examine the prospects for distinguishing nominal species in the fossil record. Fourteen morphological variables were used to characterize shell shape and multivariate methods were used to discriminate between five Recent species pairs. Collection sites were also used as a priori groups for discrimination to describe patterns of intraspecific morphological variation and to evaluate differences among samples from different geographic regions. On average, 84 percent of specimens within geminate pairs are classified correctly following five separate discriminant analyses with nominal species as the grouping variable. Although all but one arcid species pair are discriminated with high statistical significance, some collection sites within species are highly morphologically distinct. Overall, a large proportion of specimens from each collection locality (79 percent on average) can be classified correctly to site although no single site possessed a multivariate centroid that was significantly different from all other conspecific centroids. The distinctiveness of some collection sites, however, raises the possibility that some nominal species may harbor cryptic species, indicating the need for wider geographic surveys of both molecular and morphological variation within geminate species pairs. The eigenvalue coefficients derived from the Recent samples of one geminate pair (Area mutabilis and A. imbricata) were used to assess the potential for identifying arcid species in the fossil record. Discriminant analyses of fossil Area indicate that the forms that characterize Recent A. mutabilis and A. imbricata are present in the fossil record as far back as the Late Early Miocene, in the Cantaure Formation of Venezuela. Because a deep water connection between the Eastern Pacific and Western Atlantic existed until the Middle Miocene, the morphological differences associated with Recent A. mutabilis and A. imbricata likely existed well before the rising Isthmus affected ocean circulation patterns in tropical America. Therefore, despite great overall morphological similarity, these putative geminate species likely have a time of divergence that is at least four times older than final seaway closure. The geographic distribution of fossils also suggests that morphological forms associated with each Recent species had amphi-American distributions both before and after isthmus formation but are now geographically restricted to either side of the isthmus in the Recent fauna. INTRODUCTION seaway (Mayr, 1954; Jones, 1972; Humphries and Parenti, 1986). UNDERSTANDING THE relative timing of environmental change Because the timing of final seaway closure is so well characterand species diversification presents a challenge for devel'zed in the geological record, geminate species provide both a oping hypotheses regarding the processes that generate biological useful and unusual paleobiological system for evolutionary study, diversity in the world's oceans. The rise of the Isthmus of Panama Complete cessation of water exchange between the WA and EP and the environmental changes associated with the closure of the occurred 3.5 to 3.1 Ma (Keigwin, 1982; Duque-Caro, 1990), setCentral American seaway provide a focal point for recent studies ting a lower limit on the amount of time that geminate pairs have characterizing patterns of speciation, extinction, and morphologbeen physically isolated. ical evolution in the context of tectonic, climatic, and océanoAlthough final seaway closure is well constrained in the geographic change over the last 20 Ma (e.g., Günther, 1868; Rosenlogical record, the divergence times of living geminates are typblatt, 1963; Vermeij, 1978; Stanley, 1986; Lessios, 1981, 1998; ically assumed to fall within the range of dates ascribed to final Cronin, 1985; Vermeij and Petuch, 1986; Duque-Caro, 1990; Lesseaway closure (for a review see T. M. Collins, 1996). Recent sios and Cunningham, 1990; Knowlton et al., 1993; Cronin and molecular and paleontological data, however, strongly suggest Dowsett, 1996; Budd et al., 1996; Jackson and Cheetham, 1999; that the isolation of the faunas of the EP and WA was a protracted Jackson et al., 1996, 1999; Roopnarine, 1996a; Knowlton and rather than abrupt event (Coates et al., 1992; Jackson et al., 1993, Weigt, 1998; CoUins and Coates, 1999; Jackson and Johnson, 1996; Knowlton et al, 1993; Coates and Obando, 1996; L. S. 2000). "Geminate" species pairs (Jordan, 1908), in which one Collins et al., 1996; T. M. Collins, 1996; Knowlton and Weigt, species inhabits the eastern Pacific (EP) and the other the Western 1998). These data call into question the assumption that morphoAtlantic (WA), have served as pivotal examples of the role that logically similar taxa found on either side of the isthmus were geographic barriers and the disruption of gene flow play in the necessarily isolated at the time of final seaway closure. Most noprocess of allopatric speciation (Mayr, 1954, 1963; Woodring, tably, varying amounts of DNA sequence and protein divergence 1966; Olsson, 1972; Vermeij, 1978; Knowlton et al., 1993; Beramong putative geminate pairs suggest that some pairs were mingham et al, 1997; Knowlton and Weigt, 1998; Lessios, 1998). formed before water flow between the WA and EP was disrupted Geminate pairs are hypothesized to have descended from single by the rising Isthmus (for reviews see T M. Collins, 1996; Lesbiological species that were broadly distributed throughout the gjog^ jççg) Although molecular sequences demonstrate that caltropical EP and WA prior to the closure of the Central American ¡bration of the molecular clock with geminate species may result in overestimates of the rate of molecular evolution (e.g., Knowl' Author to whom correspondence should be addressed, current address: ton and Weigt, 1998) other hypotheses can potentially explain Natural History Museum of Los Angeles County, 900 Exposition Blvd., why molecular divergence varies among putative geminate pairs Los Angeles, CA 90007, (T. M. Collins, 1996). Even though some of these hypotheses can