Subterranean fauna of Christmas Island: habitats and salient features

Abstract

A review is presented of the history of subterranean fauna exploration, the nature of the subterranean environments of Christmas Island and the biogeographical significance of members of the subterranean and springdependent species. The island, which has probably been emergent since the mid-Miocene, has a basalt core largely covered by Tertiary carbonates which have undergone intense karstification owing to the high rainfall on tropical forest, and through mixing corrosion as relative sea level changed and marine terraces formed as a result of tectonic uplift of the island. As a result, surface water is largely absent and drainage is largely as submarine groundwater discharge, however, there is a perched underground stream tapped for water on the plateau and some spring discharge on lower terraces where basalt intercepts the carbonate surface. Together these provide a range of subterranean habitats including terrestrial, freshwater streams and anchialine systems, in addition to springs and spring rills. Terrestrial caves shelter glossy swiftlets that support a rich guano-based community. Sediment banks and the tree root mats spreading over them support a community, thus far not rich, but containing troglobitic species, including the cockroach Metanocticola (Blattodea), scorpion (Ischnuridae), pseudoscorpions (Chthoniidae), microwhipscorpion (Hubbardiidae), spiders (from families Symphytognathidae; Trochanteriidae; Ochyroceratidae; Oonopidae; ?Desidae) and a variety of oniscoid isopods. As expected on an isolated seamount, it supports a procaridiid-type anchialine fauna characterised by an endemic species of Procaris (Procarididae). Unexpectedly, it is the only seamount supporting, in addition, representatives of a remipede-type anchialine fauna, namely Humphreysella (formerly Danielopolina) (Myodocopa: Thaumatocyprididae) and Halosbaena (Thermosbaenacea). In addition, the anchialine system supports the only known extant population of Microceratina (Podocopida: Cytheruridae). The freshwater subterranean stream and spring outlets support an extraordinary diversity of Darwinulidae (Ostracoda). A number of invasive species of fish have been introduced to freshwater areas and if established in springs could have a detrimental impact of stygiofauna and potentially invade anchialine waters. Direct anthropogenic impacts on caves is low owing to minimal caving activity on the island but the most sensitive to trampling are root mats and sediment banks in some caves, and especially guano piles, the disturbance to which could extend to glossy swiftlet colonies. Indirect impacts on fauna may occur owing to changes to the spring discharges as a result of water abstraction which could also impinge on the anchialine system by changing the hydrodynamic relationships between freshwater outflow and seawater, and through contaminants arising from land or sea.