Microbiological Evidence for Clipperton Island as a Stepping-Stone for Migration Across the Eastern Pacific Barrier.

Clipperton Island, located in the tropical eastern Pacific Ocean (10.2833°N, 109.2167°W), is an egg-shaped ring of consolidated coral rubble ~2.7 km in diameter and 11.8 km in circumference, with only 1.7 square km of exposed land varying in width from 25-320 m. The ring surrounds a lagoon that makes up the majority (85%) of the island. The island is of biogeographical significance because of its extreme isolation, situated ~1,000-1,300 km from the Revillagigedo Islands and the mainland of Mexico and Central America than to the nearest Pacific islands to the west (Line Islands, 5,700 km away). Clipperton Island lies just east of the vast 5000-km Eastern Pacific Barrier separating the tropical Indo-Pacific and the tropical Eastern Pacific. The foraminiferal fauna and associated microbiota were investigated at 20 sites in the intertidal zone around the perimeter of the island and from the edge of the inner brackish lagoon. Due to the island's geographic location in a low productivity zone, the lack of habitat diversity on and surrounding the island, and the destructive effect of heavy surf, a depauperate fauna of mixed biogeographic affinities was recovered. The shallow-water foraminiferal assemblage includes 29 benthic taxa, dominated by species with tropical Indo-Pacific and eastern Pacific (Panamic) affinities, as well as one of Caribbean affinity. No endemic species were present. The most abundant taxa are Sorites spp. and Quinqueloculina spp. Noticeably absent are any species of Amphistegina, even though they are considered ubiquitous in the tropical Pacific. Two species of planktic foraminifera were also collected. The seven ostracod species include endemics found restricted to brackish Clipperton Island lagoon, as well as the marine Indo-Pacific and Panamic provinces. The 15 molluscan taxa include Clipperton Island endemics, a tropical Pacific/Inter-Island endemic, and species with tropical eastern Pacific oceanic islands/Panamic Molluscan affinities. The microbiota are thought to disperse to Clipperton Island by means of broadcast spawning in the North Equatorial Current and Countercurrent, by rafting on natural and anthropogenic objects, and on birds' feet and feathers, suggesting that the island is indeed a stepping-stone for migration both east and west across the Eastern Pacific Barrier.