Brachyuran Crabs of Cocos Island (Isla Del Coco), Costa Rica: Leucosiidae, Calappidae, and Parthenopidae, With Descriptions of Two New Species

Abstract

Recent additions from Cocos Island, Costa Rica, to the collections of the Natural History Museum of Los Angeles County have increased the number of species in the brachyuran crab families Leucosiidae, Calappidae, and Parthenopidae known from the island from 4 to 10. Two of these are described: Thyrolambrus verrucibrachium, new species, and Osachila kaiserae, new species. Review of all material in the LACM collections for these species has resulted in range extensions and increases in the known maximum size for several of the species. The increase in species richness suggested by these collections may make Cocos the most speciose single island in the eastern Pacific, after the remaining crab families are studied. The zoogeographic relatedness of the oceanic islands of the eastern Pacific is supported, and the link between Cocos and the Revillagigedo Islands is strengthened. Cocos Island, which is now one of the national parks of Costa Rica, sits on the Cocos Ridge at 5032'57N, 86059'17W, 494 km west of Cabo Blanco, Costa Rica, 673 km northeast of La Pinta (Abingdon) Island in the Galapagos, and 2,375 km southeast of Clipperton Island. Cocos is volcanic in origin, with lavas dated at around two million years old (Bellon et al., 1983; Castillo et al., 1988), and is covered by wet tropical forest. Much of the 23.3 km of coastline is bounded by steep cliffs, and only two areas afford protected anchorage: Chatham Bay and Wafer Bay. The island is situated in the path of the eastward flowing North Equatorial Countercurrent for much of the year, but from December-April westerly currents reach the island from the mainland when the counter current shifts to the south (Wyrtki, 1965). The brachyuran fauna of Cocos Island has been somewhat undercollected, when compared to the other oceanic islands of the tropical eastern Pacific (Revillagigedo, Clipperton, and the Galapagos Islands), although numerous collecting expeditions have visited Cocos Island in the past. Among those cruises reporting decapods were the Albatross in 1891 (Faxon, 1895), the Arcturus in 1925 (Boone, 1927), the Velero III in 1933 and 1938 (Fraser, 1943), and the Presidential Cruise of 1938 (U.S.S. Houston) (Schmitt, 1939). None of these expeditions stayed at the island for more than a few days, and only 31 species of Brachyura were listed by Hertlein (1963), who reported on what was known of the fauna of the island at the time. A review of the literature by the authors has increased