Land Snails of Dåno′ (Cocos Island), Mariana Islands1
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Land Snails of Dåno′ (Cocos Island), Mariana Islands1 Alexander M. Kerr 2,3,5 and G. Curt Fiedler 4 Abstract: We performed the first survey of terrestrial gastropods on Dåno′ (Cocos Island). Dåno′ is a 0.33 km2 uninhabited atoll island located 2.5 km southwest of Guam, the largest island in the Mariana archipelago, western Micronesia. In 4 days of examining vegetation, soil, and leaf litter, we found 14 species of mollusks arrayed in 12 genera and 11 taxonomic families. The fauna is a subset of that found on Guam. Five species are native to the Mariana Islands, two of which are endemic to the archipelago, and the remaining seven are likely introduced there, perhaps prehistorically. Ground-inhabiting species found only as abundant worn shells in soil suggest relatively recent turnover of these forms, perhaps from seawater inundation of the low-lying island during typhoons. There was no evidence of Mariana endemic snails in the family Partulidae, recently afforded legal protection under the U.S. Endangered Species Act. How- ever, neither did we encounter the invasive flatwormPlatydemus manokwari De Beauchamp, currently decimating partulids elsewhere in the Marianas. Hence, we suggest that if further surveys confirm the predator’s absence, Dåno′ might serve as a novel sanctuary for transplanted populations of Guam’s most en- dangered snails. Keywords: atoll, Micronesia, dispersal, species turnover, Omphalotropis, Trunca tella, Palaina Dåno′ (Cocos Island) is a small atoll island been investigated scientifically, perhaps be- in the Mariana archipelago, western Microne- cause they are expected to be composed of an sia, and is geologically (Tracey et al. 1964:88), abbreviated set of species inhabiting its larger as well as politically, part of the much larger neighbor. adjacent island of Guam, an unincorporated Nevertheless, Dåno′ possesses several in- U.S. Territory. Because of Dåno′’s small size, teresting biological features rare or absent limited habitat diversity, and proximity to elsewhere in the archipelago. Its vegetation Guam, the island’s flora and fauna have rarely was documented by Neubauer and Neubauer (1981), and a representative collection is curated at the Guam Herbarium ( Kerr et al. 1 Funded in part by a U.S. Forest Service McIntire- 2015). Eldredge (2003) recorded the strand- Stennis Grant to G.C.F. This is a contribution of the ing of a rare leatherback sea turtle, Dermo University of Guam Marine Laboratory. Manuscript accepted 1 September 2017. chelys coriacea ( Vandelli). Lizards were sur- 2 Marine Laboratory, University of Guam, Mangilao, veyed by McCoid (1996) before and after two Guam 96923, USA. severe typhoons. Wiles et al. (1993) provided 3 Department of Invertebrate Zoology, Florida Mu- an account of new bird records and nesting seum of Natural History, Gainesville, Florida 32611. species. Recent interest in the island as a ref- 4 Division of Natural Sciences, Biology, University of Guam, Mangilao, Guam 96923, USA. uge for Guam’s native vertebrate fauna ( Wiles 5 Corresponding author (e-mail: alexander.kerr@aya et al. 2003) has been motivated by its lack of .yale.edu). the invasive snake Boiga irregularis (Merrem) that has eradicated most native lizards, birds, and bats on Guam (Savidge 1987). As a re- Pacific Science (2018), vol. 72, no. 2:263 – 270 ′ doi:10.2984/72.2.8 sult, Dåno now hosts one of two remaining © 2018 by University of Hawai‘i Press wild populations of the endemic, flightless All rights reserved Guam rail, Hypotaenidia (= Gallirallus) owstoni 263 (CS6) UHP (7”×10”) Janson Text J-3125 Pacific Science, 72:2 pp. 263–270 PS_72-2_08 (p. 263) PMU: (idp) 12/12/2017 5 January 2018 9:40 AM 264 PACIFIC SCIENCE · April 2018 ( W. Rothschild), as well as the Mariana en- parently in 1888 by a naturalist collecting in demic scincid lizard Emoia slevini Brown & the Marianas for the Paris Museum, Antoine- Falanruw, now extinct over most of its former Alfred Marche (1891:24) (Cheng 1982:25), range (Allison et al. 2013). who noted that during his short visit to As best as we can determine, there are no “Danao” he found “some interesting terres- accounts of the land snails of Dåno′. The ear- trial molluscs . .”. liest naturalist to visit the Mariana Islands, In this article, we provide the first account Antonio de Pineda y Ramírez with the Span- of land snails from the island of Dåno′. For ish Malaspina Expedition, mentioned Dåno′ 4 days beginning in late 2015, we surveyed (as “Isla de Cocos”), but he did not visit the is- all major habitat types across the island with land (Pineda 1792, unpubl., in Mallada and emphasis on searching for species recently as- Driver 1990:27). Later, a colonial governor of signed protection under the U.S. Endangered the Marianas, Luis de Ibáñez y García (1887, Species Act ( U.S. Fish and Wildlife Service unpubl., in Driver 1992:88), came ashore 2015). briefly, describing Daneono“ ” as an uninhab- ited sand and limestone island with few trees. Study Site Other major nineteenth- and early twentieth- century accounts of Mariana biodiversity, in- Dåno′ (0.33 km2) (13.239° N, 144.653° E) is a cluding snails, made no mention of Dåno′ small, sandy island 2.5 km southwest of Guam, (Quoy and Gaimard 1832, Quadras and Mariana Islands, Micronesia (Figure 1). The Mo¨llendorff 1894, Safford 1905, Prowazek island is situated atop an otherwise sub- 1913, Crampton 1925). The earliest observa- merged atoll forming a barrier reef with a tions on the land snails of the island were ap- well-developed lagoon flanking Guam. The Figure 1. Map of Dåno′ with approximate locations of numbered collection sites (map of Guam after www.freemap .jp). (CS6) UHP (7”×10”) Janson Text J-3125 Pacific Science, 72:2 pp. 264–270 PS_72-2_08 (p. 264) (CS6) UHP (7”×10”) Janson Text J-3125 Pacific Science, 72:2 pp. 265–270 PS_72-2_08 (p. 265) PMU: (idp) 12/12/2017 5 January 2018 9:40 AM PMU: (idp) 12/12/2017 5 January 2018 9:40 AM Land Snails of Dåno′, Mariana Islands · Kerr and Fiedler 265 island’s exposure and maximum elevation (see Acknowledgments). Species were catego- of about 2 m render it vulnerable to flooding rized as either having been collected (1) live, by typhoon-generated waves ( Wells 1991). and if not, then (2) as fresh shells, meaning There are no permanent human inhabitants, the shell was still pigmented or translucent, but several hundred tourists per annum visit entire, and polished internally, but if not, a small day resort operating there. Previous then only (3) as bleached, eroded, and some- land use by humans has also considerably times partial shells. Representative specimens altered the environment ( Neubauer and of each species collected were deposited in the Neubauer 1981, McCoid 1996). Neverthe- Bernice Pauahi Bishop Museum Malacology less, Dåno′ is now almost entirely forested: Collection (accession number 2017.089, cata- the northern half with an extensive stand of logue numbers bpbm 283822 – 283834). Casuarina equisetifolia L., and the southern half largely of native back-strand trees, princi- results pally Hernandia sonora L., Cordia subcordata Lam., Guettarda speciosa L., and Cocos nucifera Table 1 lists the species collected on Dåno′. L. Seaward of the forests lies a circumisland The table also indicates the species’ biogeo- skirt of cosmotropical strand vegetation, pri- graphic status: native, indicating that the spe- marily Scaevola taccada (Gaertn.) Roxb. and cies naturally occurs in the Marianas but is Heliotropium foertherianum Diane & Hilger also found elsewhere; endemic, if it is only (= Tournefortia argentea L. fil.), with the sea- reported from the Marianas; and introduced, wardmost karstic exposures along the wind- if it has likely been brought by humans to ward east coast hosting stands of Pemphis the archipelago, including through prehis- acidula J. R. Forst. & G. Forst. Centrally, the toric transport. Approximate collection sites island also supports two diminutive (ca. 0.1 ha are shown in Figure 1. No terrestrial mollusks total) wetlands maintained by a subterranean were found at site 10. lens of freshwater recharged by precipita- Several identifications warrant comment. tion (Ayers and Vacher 1986), the northern Pacificella variabilis (Figure 2A) is distin- one a pond rimmed with Cyperaceae, the guished from the similar Mariana achatinellid southern one often without standing water genus Lamellidea Pilsbry by fewer and more and shaded by giant taro, Alocasia macrorrhizos convex whorls, more obtuse apex, and less- (L.) G. Don. impressed sutures (Cooke and Kondo 1960). The larger Omphalotropis (Figure 2B) most closely matched descriptions of materials and methods O. elongatula, including having a shell of mostly solid colors During the wet season, snails were collected of either tan or orange as seen in our exem- by two persons during the day across Dåno′ plars. The second Omphalotropis was smaller (13 November and 30 December 2015, 24 with stepped whorls. Among the three species November 2016, 2 March 2017). Collections of Mariana Omphalotropis displaying the latter were made by examining the undersides of character ( Kerr and Bauman 2013), only O. leaves, sifting through leaf litter, and digging granum (Figure 2C ) has a combination of through the top 5 – 10 cm of soil at 12 hap- coastal habit, smooth shell, and wide separa- hazardly selected, roughly equidistant sites tion of the basal carina and columella as in our among five distinctive habitat types found on specimens. Our examples, however, seemed the island: Casuarina forest (sites 10, 11 in somewhat smaller, to a maximum ca. 3 mm in Figure 1), strand (1, 12), native back-strand height, sans trace of peripheral angulation on forest (2 – 4, 6), wetland (5, 9), and ornamental the body whorl. Liardetia sculpta (Figure 2G ) vegetation near buildings (7, 8). Snail iden- is the only Mariana species from this genus tifications were based primarily on original lacking spiral striae on the body whorl and descriptions and revisionary monographs of hence the beading pattern that results from Pacific-island taxa as referenced in Kerr and the crisscrossing of the threadlike axial rib- Bauman (2013) or in consult with specialists bing (Baker 1938).