PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 113(2):561-571. 2000. Porites arnaudi, a new species of stony coral (Anthozoa: Scleractinia: Poritidae) from oceanic islands of the eastern Pacific Ocean Hector Reyes-Bonilla and Juan P. Carricart-Ganivet (HRB) Universidad Autonoma de Baja California Sur, Departamento de Biologia Marina, Apdo. Postal 19-B, La Paz, B.C.S. 23000, Mexico; (JPCG) Departamento de Ecologia Acuatica, ECOSUR, Apdo. Postal 424, Chetumal, Q. Roo, 77000, Mexico Abstract. — A new species of Porites (P. arnaudi) was found at depths of 7 to 37 m at Clipperton Atoll, France, and the Revillagigedo Islands, Mexico, oceanic islands located west of the American mainland (eastern Pacific). Col-onies are uncommon but very distinctive because of the presence of tiered plates, or laminae, expanding from a single base, smooth and concave surfaces, and a lack of living tissue on the inferior part of the plates, except on its growing edge. Calices are 0.8 to 1.4 mm in diameter, with wide walls. Cor-allites have a free triplet with all three septa about the same size. Lateral septa are fused in pairs and better developed than the rest. There are six to eight pali, and one or two denticles per septum, and the color of tissue is greenish-gray to dark brown. With this new species, the current total number of species in the genus Porites in the eastern Pacific reaches nine, with most species restricted to oceanic islands of the region. In the last 20 years, taxonomic studies on the scleractinian zooxanthellate fauna of the eastern Pacific have noticeably increased in number and quality (Wells 1983, Guzman & Cortes 1993, Hodgson 1995) and as a consequence, there is now fairly good agreement on coral identities since some key taxonomic problems have been re-solved (Squires 1959, Wells 1983, Veron 1986). New research also has shown that the actual richness of coral species from the eastern Pacific is much higher than once thought [e.g., originally less than 20 species according to Veron (1993) and Paulay (1997)]. For example, Mexico, Costa Rica and Panama have 20 to 25 species each and almost 40 are known to inhabit western America (Hoist & Guzman 1993, Cortes & Guzman 1998, Reyes-Bonilla & Lopez-Perez 1998). This perceived increase in richness comes from new records for par-ticular localities or areas (e.g., Reyes-Bon-illa 1992, Cortes & Guzman 1998) and by descriptions of new species (Budd & Guz-man 1994, Glynn 1999). Members of the genus Porites Link, 1807 are widely distributed in the eastern Pacific, and they are one of the dominant corals in this region, both in abundance and species richness (Glynn 1997). To date, eight species have been reported: P. aus-traliensis Vaughan, 1918, P. baueri Squires, 1959, P. lichen Dana, 1846, P. lob-ata Dana, 1846, P. lutea Milne Edwards, 1860, P. panamensis Verrill, 1866, P. rus Forskaal, 1775, and P. sverdrupi Durham, 1947 (Guzman & Cortes 1993, Glynn 1997, Reyes-Bonilla 1999). Both P. panamensis and P. lobata are present in coral commu-nities from the Galapagos Islands, or the Ecuadorian mainland, to Mexico (Reyes-Bonilla 1993, Glynn 1997), while P. sver-drupi and P. baueri are endemic to the Gulf of California and the Marias Islands, Mex-ico, respectively (Squires 1959, Reyes-Bon-illa & Lopez-Perez 1998). Porites rus is an
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