Genetic structure and phylogeography of four eastern Pacific estuarine species, with an emphasis on eelgrass dependent taxa

Estuarine and eelgrass habitats are in decline worldwide. Many animals utilize these habitats for a part of their lifecycle, only a few animals are dependent on this habitat for their entire life cycle. This dissertation examined the population structure of three eastern Pacific eelgrass-dependent species and a species that uses estuarine habitat for only part of its lifecycle using mitochondrial DNA sequence data.; Genetic structure of the eelgrass dependent fish, Syngnathus leptorhynchus (eastern Pacific bay pipefish) was examined from Alaska to Baja California, Mexico. A strong genetic break was found at Puget Sound (&phis;CT = 0.306). Genetic diversity and divergence levels and tests of demographic change indicate a post-glacial expansion.; A second eelgrass species (Taylor's seahare, Phyllaplysia taylori ) was examined from British Columbia to southern California. Significant genetic structure consistent with isolation by distance was found, with the strongest genetic break in the Southern California Bight. Locations in Central California exhibited the highest haplotype diversity and was an admixture zone. A population expansion most likely occurred from Central California.; The third eelgrass species examined was the eastern Pacific eelgrass limpet Tectura depicta. Extremely strong genetic structure was detected between Central and Southern California locations (&phis;CT = 0.953; P = 0.028), approximately at the Southern California Bight and despite about a week-long pelagic larval phase. This break corresponded to shell morphology.; Lastly, the population structure of the Pacific staghorn sculpin ( Leptocottus armatus), a euryhaline fish that utilizes estuaries only during its juvenile phase, was examined in comparison to the three eelgrass-dependent taxa. Significant genetic structure was detected, which corresponded to boundaries between previously proposed subspecies, rather than to biogeographic breaks observed in other eastern Pacific fishes. Reanalysis of the morphological data with which the subspecies were described, revealed a break further north of one detected by the genetic data. This discrepancy could be the result of an intergradation zone of the northern and southern groups or changes to the range termini of the subspecies due to climate change.