The genetic structure and evolutionary history of an endemic anchialine species, the shrimp Halocaridina rubra Holthuis, 1963 (Crustacean: Decapoda: Atyidae), was investigated across its range in the Hawaiian archipelago using mitochondrial (e.g., cytochrome oxidase subunit I and large subunit ribosomal) gene sequences. A survey of 573 individuals collected from 34 sites on the islands of Hawai'i, Maui, and Oahu revealed 13 distinct genetic groups belonging to eight divergent lineages. In general, a Halocaridina genetic group or lineage was restricted to a particular region of a single Hawaiian Island, with no individuals being exchanged between them. This pattern 1 Corresponding author (santos@auburn.edu). 675stems from a combination of intrinsic organismal properties such as large egg size, abbreviated development, restricted larval habitat and larval feeding mode, and extrinsic obstacles to gene flow in the form of a marine barrier and geologic features that compartmentalize the islands' aquifers. The phylogeographic structuring on and between islands suggests that evolutionary diversification in Halocaridina is driven by population fragmentation, isolation, and subsequent diversification in the aquifers of the Hawaiian Islands. Calibration of cytochrome oxidase subunit I sequence divergence between sister Halocaridina lineages to the geologic age of Kilauea volcano on Hawai'i implies that diversification in the genus is proceeding at a short-term rate of 20% per million years. The examined mitochondrial genes were generally inadequate for inferring phylogenetic relationships between the Halocaridina lineages.

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