Phylogeography of the Christmas Island blue crab, Discoplax celeste (Decapoda: Gecarcinidae) on Christmas Island, Indian Ocean lucy m. turner 1,2 , j. paul hallas 3 , michael j. smith 4 and stephen morris 1† 1 School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK, 2 Marine Biology and Ecology Research Centre, School of Marine Science and Engineering, University of Plymouth, Drake Circus, Plymouth, Devon PL4 8AA, UK, 3 School of Applied Sciences, University of Glamorgan, Upper Glyntaff, Pontypridd CF37 4AT, UK, 4 Department of the Environment, Water, Heritage and the Arts, Christmas Island National Park, PO Box 867, Christmas Island, Indian Ocean 6798, Australia, † Professor Morris died on 11 August 2009 before this work was completed. This paper is dedicated to his memory. The land crab, Discoplax celeste (Gecarcinidae) is endemic to Christmas Island in the Indian Ocean. Due to a freshwater- dependant life history, in which the megalopae migrate from the ocean up freshwater streams to their adult terrestrial/ freshwater habitat, D. celeste inhabits only a few isolated locations on the island. This restricted distribution is one of a number of factors which has previously highlighted the vulnerability of this species to outside threats. A number of anthro- pogenic factors including the introduction of multiple invasive species and habitat destruction have led to drastic ecosystem change on Christmas Island. The aim of this study was to investigate whether the restricted geographical distributions of D. celeste populations contribute to significant genetic structuring across Christmas Island, with an objective to inform future conservation strategies for this species on Christmas Island. Fragments of the mitochondrial cytochrome oxidase I gene and the control region were sequenced from 95 individuals collected from all five locations on Christmas Island known to be inhabited by D. celeste. Analyses using analysis of molecular variance revealed no evidence of population sub- structuring, indicating that despite any geographical isolation, there is a single population of D. celeste on Christmas Island. This lack of population differentiation is probably explained by the oceanic dispersal of larvae, rather than terrestrial migration of D. celeste. Therefore, based on these results, for conservation purposes, D. celeste on Christmas Island can be considered a single management unit. Keywords: Discoplax celeste, Christmas Island, population genetics, freshwater, COI, control region, endemic Submitted 3 April 2012; accepted 10 April 2012 INTRODUCTION The gecarcinid land crab Discoplax celeste Ng & Davie, 2012 is endemic to the Australian Territory of Christmas Island, Indian Ocean (10828 ′ S 105838 ′ E) (Figure 1A). Previously mis- identified on Christmas Island as Discoplax hirtipes (Dana, 1851), recent taxonomic study has recognized this crab as a distinct new species (Ng & Davie, 2012). Discoplax celeste has a porcelain blue carapace and chelae and it was previously thought that this species on Christmas Island was an endemic colour form of D. hirtipes (Hicks et al., 1990). Distributed throughout the Indo-West Pacific, D. hirtipes has a brown to purplish-brown carapace and chelae (Hicks et al., 1990; Ng & Guinot, 2001). However, there are additional colour forms that have also been identified as D. hirtipes, conse- quently further taxonomic study is required (see Ng & Davie (2012) for a detailed discussion). Recent work on D. celeste (as D. hirtipes or Cardisoma hirtipes (Ng & Guinot (2001) separated Discoplax A. Milne-Edwards, 1867, from Cardisoma Latreille, 1828)) has shown that the need for long term nitrogen, ion and water balance drives this crab into association with freshwater, therefore limiting the local distribution of this species (Greenaway, 1989; Adamczewska & Morris, 1996; Turner, 2010). Thus, unlike the other two most obvious and abundant land crab species on Christmas Island, the Christmas Island red crab, Gecarcoidea natalis (Pocock, 1888) and the robber crab Birgus latro (Linnaeus, 1767), D. celeste is restricted in its distribution during the dry season (April– October) to areas of freshwater seepages and springs, usually well inland (.1 km from and 150 m above the ocean) (Dela-Cruz & Morris, 1997a, b; Morris, 2005). Due to the underlying geology (Barrett, 2001), these areas of freshwater and thus established populations of D. celeste are found at only a few places on the island: the area around the Dales streams in the west and the springs between Waterfall Bay and the Ravine, which includes Ross Hill Gardens, and Dolly Beach on the south-east coast (Hicks et al., 1990; Turner et al., 2011) (Figure 1B). There have also been unconfirmed Corresponding author: L.M. Turner Email: lucy.m.turner@plymouth.ac.uk 1 Journal of the Marine Biological Association of the United Kingdom, page 1 of 12. # Marine Biological Association of the United Kingdom, 2012 doi:10.1017/S0025315412000598