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Page 2 THE FESTIVUS Vol. XXXIX: Supplement Figure 2. The five tropical eastern Pacific oceanic island groups. Map by D.L. Geiger. nature of insular biota relative to immigration and distributions, Newman (1996) gave a historical review and insight into oceanic islands. The five tropical eastern Pacific oceanic groups are: Islas Revillagigedo, México (18/N); Île Clipperton, French possession (10/N); Isla del Coco, Costa Rica (5/N); Isla de Malpelo, Colombia (3/N), and Islas Galápagos, Ecuador (0/) (Figure 2). They are all separated from the mainland by abyssal depths. Malpelo is closest at 435 km and Clipperton farthest at 1,280 km. The molluscan fauna has been intensely studied in the Galápagos (Finet, 1994; Kaiser, 1993, 1997). Kaiser & Bryce (2001) published an extensive monograph on the mollusks of Isla de Malpelo. Two major papers have been published on the Islas Revillagigedo (Emerson, 1995; Reyes-Bonilla, 1999). A number of notable expeditions have visited Isla del Coco in the 1980s and 1990s, but only preliminary reports have appeared (Montoya, 1983; Shasky, 1983a, 1989a; Montoya & Kaiser, 1988; Chaney, 1992; Mulliner, 1993; Kaiser, 1998; J. Hertz & Kaiser, 1998a,b; Kaiser, 2001; Kaiser & C. M. Hertz, 2001). A French possession, Clipperton is characterized by its extreme isolation, small size, and most importantly, low habitat diversity. These physical parameters have contributed to its greatly impoverished molluscan fauna in comparison to other TEP oceanic islands. First discovered in 1711 as “Isla de la Pasión” (Skinner & Schmieder, 1996), the only atoll of the tropical eastern Pacific, which is much nearer to the American mainland than to the nearest Pacific islands to the west, has long intrigued biogeographers. Collections of marine fauna including fishes (Allen, 1995, Robertson & Allen, 1996; crustaceans (Poupin et al., in press); corals (Glynn et al., 1996; Carricart-Ganivet & Reyes- Bonilla, 1999; Flot & Adjeroud, in press); echinoderms (Solis-Marin & Laguarda Figueras, in press) have revealed that both tropical eastern Pacific (Panamic Province) and substantial Indo-Pacific elements are represented. In that Clipperton is the easternmost atoll of the vast area known as the East Pacific Barrier and the westernmost island of the tropical eastern Pacific, the high percentage of Indo-Pacific fauna (33.5%) is due to two factors. Firstly, the atoll is unique in that it is situated in an overlap zone of the Pacific Ocean and,
Vol. XXXIX: Supplement THE FESTIVUS Page 3 therefore, is the first land mass to be intermittently exposed to the North Equatorial Countercurrent from the west which brings source elements including planktotrophic larvae across the Barrier. The nearest atoll Pukapuka, in the Tuamotu Archipelago, is 4,280 km to the southwest and the closest island is Ua Huka of the Marquesas and is at a distance of 3,965 km. Secondly, low habitat diversity of the atoll, which is mainly prolific coral cover, makes suitable habitats for these planktonic links from the west. Those source elements from the east on the North Equatorial Current encounter a lack of the coarse intertidal and subtidal environments of the coastal diversity. Clipperton lacks the rocky shores, mud and sand bottoms, mangroves, and such food sources as marine algae and other plants that are necessary to establish a hold and maintain viable populations for many west American mainland (TEP) taxa. Physical Environment Île Clipperton located at the nexus of the Panamic Province (10/18'N and 109/13'W), is somewhat characteristic of Pacific atolls having volcanic foundations. Where the top of the highest peak of the submarine Clipperton Ridge, described by Menard & Fisher (1958), reaches the surface of the sea, an eggshaped coral reef supports a narrow, low, uninterrupted land strip of limestone debris with a single cavernous mass of altered trachyte rising to approximately 20 m (Clipperton rock) on the southeastern border. Clipperton is a “near atoll,” since a true atoll has no volcanic outcroppings. The small ring is 11.8 km in circumference, with only 1.7 square km of exposed land encircled by a healthy fringing coral reef (Figure 1). The lagoon, which makes up 85% of the atoll, is surrounded by a ribbon of land variable in width from 25 m to 320 m (pers. comm., John Munch, 2007). Beyond the wavewashed outer ring of the atoll are reef-flats lined by coral-sand beaches or cobble strands. From there, white ridges of unconsolidated limestone fragments (coral bits) reach a maximum elevation of four meters from where the land mass slopes down and encloses the sizeable (7.2 km square) central lagoon. The lagoon has sediment of fine organic debris and hordes of minute biting isopods (Cirolana sp.) Poupin, pers. comm, 2007). Clipperton has a landing strip built in 1944 on the northwest side of the widest part of the atoll. The strip consists of coral pieces that are stained dark gray or black by microscopic algae and is not recommended for use today. The following eight habitats are represented at Clipperton and environs: (1) large deposits of storm-generated coral, with one extensive volcanic outcropping, bordering an intolerably low salinity lagoon (Figure 3), Figure 3. Aerial view taken from the south of Clipperton Rock located on the edge of the lagoon, 2005. Photo: C. Fresser. (2) a shallow, often tidally exposed reef front with low coral cover and abundant crustose coralline algae (Figure 4), Figure 4. Tidally exposed reef front on west side of Clipperton, 2005. Photo: C. Fresser.
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