Background: Anchialine caves contain tidal, sea level pools of fresh to fully marine waters that are inhabited by a diverse community of obligate, cave-adapted (stygobitic) fauna dominated by crustaceans (Iliffe, 2000; Iliffe & Kornicker, 2009). A variety of anchialine taxa stygobites are found in caves on opposite sides of the Atlantic as well as from the Indian Ocean and have been referred to as Tethyan relicts. Anchialine Tethyan relict fauna from the Indian Ocean tend to have a similar distribution (Caribbean, Canary Island and Western Australia) and include members of the crustacean class Remipedia, thermosbaenacean genus Halosbaena, and thaumatocyprid genus Humphreysella. Further investigations of anchialine caves from the Indian Ocean promise to yield discoveries of new fauna and localities that can shed light on the origin, timing and modes of dispersal of these unique organisms.

Christmas Island, a territory of Australia, lies in the eastern Indian Ocean at 10o 30’ S, or about 350 km southwest of Java and separated from it by the Java Trench. The island, which is the tip of a submerged seamount, rises 4.5 km from the ocean floor. It consists of late Cretaceous to early Tertiary age volcanic rocks that are capped by Tertiary limestones, which in turn are partially buried by phosphate- rich soil with pinnacled karst. The limestones tend to be relatively thin (20–30 m) in the center of the island, but then thicken towards the coast above the steep-dipping volcanic surface where they reach up to 250 m in thickness. The steeply cliffs at the coastline rise via a series of terraces to the phosphate-blanketed interior plateau. Despite the high rainfall, little surface water is present since karstic subterranean drainage occurs through numerous caves. Groundwater flow follows the limestone/volcanic contact to emerge at major conduit springs, located at or below sea level. Uplifted caves occur along the present coast. Cave development takes place at the mixing zones between fresh and sea water in the coastal zone, and between vadose and phreatic waters beneath the plateau. Cave locations and forms are controlled by the rock structure (especially jointing), the location of the volcanic contact, and the combination of uplift with present and past sea levels - which controls the location of the mixing zone (Grimes, 2001).

Diving exploration: Considering that two distinctly different types of caves were explored and investigated during the expedition, specialized techniques are required for each. Inland anchialine caves required side-mounted tanks that facilitate transport of tanks through dry caves and allowed divers to penetrate low profile restrictions. Search for ocean caves required boats for access and closed circuit, mixed gas rebreathers due to depths of 80 m or more.  All divers participating in the project were highly experienced at these types of dives meeting recognized cave diving safety standards.

Biological methodology: Inland caves contained haloclines at 2-3 m depths, with saltwater below. The focus of the expedition was to examine these deeper waters for stygobitic fauna. Since undisturbed waters were exceptionally clear, initial collections focused on visual observation and collection. As visibility deteriorated due to being disturbed, a plankton net was used to capture smaller organisms. Finally, baited plastic bottle traps were left at depths below the halocline for approximately 24 hours to attract shrimp and other scavengers. In the lab, still and video photography were used to document coloration and swimming behavior. After photographic documentation, specimens were preserved in ethanol, RNA later, or suitable preservatives depending on their use for taxonomic, molecular or electron microscopic purposes.  All material was curated for deposition in the collections of the Western Australia Museum.

Photo/video documentation: Still and HD video photography allowed for thorough documentation of the caves and for the procedures and personnel used to explore them.  The three UW photographers in our team were assigned appropriate tasks to ensure complete coverage.

Results: We anticipate that several new species will be described as a result of this expedition.  The geology (volcanic seamount capped with limestone), types of caves (dry limestone caves with anchialine pools in their interior), and geologic history (mid-ocean basaltic seamount of Cretaceous to Tertiary age) of Christmas Island in the Indian Ocean compares closely with that of Bermuda in the Atlantic. The rich anchialine fauna of Bermuda (>80 cave-adapted species) suggests that numerous additional discoveries await on Christmas Island.  Considering that very little information exists on anchialine fauna from the Indian Ocean, our data should be of considerable biogeographic and evolutionary importance. Furthermore, our diving exploration of the underwater caves of Christmas Island provides significant data relevant to the mode and timing of cave formation in relation to sea level history and island uplift.

Although Christmas Island is world famous for its abundant populations of land crabs, it is also home to an Australian national park containing verdant tropical forests, coastal limestone cliffs and terraces, deserted beaches and pristine coral reefs. This unique tropical island ecosystem includes the prodigious land crabs and rare sea bird colonies, but also now as a result of this expedition, new cave-adapted organisms with close relatives from anchialine caves around the world have been added. The island’s natural history offers a fascinating glimpse into a biologically diverse and unspoiled tropical paradise.