First Record of Baseodiscus hemprichii (Nemertea: Baseodiscidae) on Easter Island (Rapa Nui) and a New Eastern Distribution Boundary for the Species

Abstract

A single specimen of the nemertean Baseodiscus hemprichii (Ehrenberg, 1831) was collected from Easter Island during August 1999. This represents the ®rst record of the species on Easter Island, the ®rst identi®ed nemertean from that island, and extends the eastern boundary of the species' range by approximately 4800 km. The small South Paci®c landmass of Easter Island, or Rapa Nui, is situated nearly 3800 km from the South American mainland and over 2200 km from Pitcairn Island, its nearest neighbor to the west. All of the marine fauna shows af®nities to the Indo-Paci®c, although the water temperatures of 17.5± 24 C are relatively cool (DiSalvo et al. 1988). All available data indicate that the marine invertebrate fauna of Easter Island is depauperate compared with other islands of the tropical Paci®c (Rehder 1980, Massin 1996), but it has a high percentage of endemic taxa (e.g., 36% for Mollusca, 39% for Porifera [unpubl. data]). It is therefore considered its own biogeographic province along with tiny Sala y GoÂmez, 415 km to the east (Briggs 1974). On Easter Island, however, specimens of only eight invertebrate phyla have been identi®ed to the species level to date. A single specimen of nemertean was collected during August 1999 as part of an invertebrate survey of Easter Island. It was subsequently readily identi®ed as Baseodiscus hemprichii (Ehrenberg, 1831) on the basis of its distinctive color pattern. This represents not only the ®rst record of the species on Easter Island, but extends the eastern boundary of the species' range by approximately 4800 km. Although nemerteans have been recorded as occurring on Easter Island (DiSalvo et al. 1988), this is the ®rst identi®cation of any specimen to the species level. The specimen is deposited in the American Museum of Natural History, New York (amnh). Family Baseodiscidae Bu .. rger, 1907 Genus Baseodiscus Diesing, 1850 Baseodiscus hemprichii (Ehrenberg, 1831) Figure 1 Nemertes Hemprichii Ehrenberg, 1831:12±13. Baseodiscus hemprichii: Gibson, 1979:146±153 (synonymy). material examined: Under lava boulder, intertidal zone, Anakena, Easter Island (Rapa Nui), Chile, 30 August 1999, coll. C. B. Boyko and S. Reanier: 1 specimen, 25 cm length (amnh 1512). distribution: This species has a broad Indo-Paci®c distribution and is known from the Red Sea to the Hawaiian Islands. This range is now extended southeastward to Easter Island. remarks: The Easter Island specimen had a white ground color with a single median stripe of purple along the dorsal margin 41 1 Funding for the Invertebrate Survey of Easter Island was provided by the U.S. National Park Service, Gateway National Recreation Area, Division of Natural Resources, as part of a Science Museum of Long Island/Explorers Club 5-yr research expedition to explore the impacts of El NinA o events on World Heritage Sites. Manuscript accepted 25 April 2000. 2 Division of Invertebrate Zoology, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, and Department of Biological Sciences, University of Rhode Island, Kingston, Rhode Island 02881 (E-mail: cboyko@amnh.org). Paci®c Science (2001), vol. 55, no. 1:41±42 : 2001 by University of Hawai`i Press