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Abstract
Widely dispersing species experience a variety of abiotic or biotic variables that can affect their distribution and their population genetic differentiation. These variables can be subtle such as differences in microhabitat or community assemblage or more obvious such as continental drift or glaciation. The interaction of these variables and their ability to constrain or facilitate gene flow between populations are addressed along the North American coastline with the marine isopod, Idotea balthica. This species shows latitudinal population genetic differentiation in the gene, mitochondrial cytochrome oxidase I (mtCOI). The mechanisms that created and currently maintain this pattern in population genetic diversity in this species are unknown. This study addressed whether North American populations of I. balthica are defined by distinct feeding behaviors and whether feeding preference could be a diversity maintaining mechanism in this species. Feeding assays showed that feeding preferences did differ between North American populations. Southern populations consume Zostera marina at a higher rate than northern populations while northern populations consume more Fucus vesiculosus than southern populations. PCR-based gut content analyses revealed that northern populations tend to be more selective in their food choices than those in southern populations. Partial reproductive barriers were found from inter-population crosses where fecundity and offspring survivorship were reduced from intra-population crosses. This finding suggests that reproductive barriers may be maintaining genetic diversity in this species and that North American populations of I. balthica may actually be incipient species. The historical dispersal patterns of this species between North America and Europe post-glaciation are also addressed but remain inconclusive.