Editorial by Dr. Thomas Iliffe
& Dr. William Humphreys

Photography by Ross Anderson,
Jill Heinerth & Brian Kakuk


Thomas M. Iliffe
   Department of Marine Biology
   Texas A&M  University at Galveston
   Galveston, TX 77553-1675, USA
William F. Humphreys
   Western Australian Museum
   University of Western Australia
   Nedlands, Western Australia, Australia



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).

Title Photo Above: Brian takes survey notes while mapping a submerged cave on Christmas Island.
Left Photo: Checking his gear one last time Tom prepares for a dive in Runaway Cave. Underwater, the cave consists of massive boulders wedged between two bedrock walls – like diving through spaces between giant marbles in a goldfish bowl.


Expedition objectives and accomplishments:

Explore anchialine and submarine caves, focusing on known caves with highest biodiversity as well as previously unstudied caves
Collect and photo-document cave-adapted aquatic fauna for use in taxonomic and molecular genetic studies
Photo and video surveys of the caves to highlight unique features and habitats
Assemble data to justify protection and preservation of the island’s unique caves

Expedition plan:

A two-week expedition to Christmas Island sponsored by the National Geographic Society took place from March 18 – April 1, 2013.

Team members:


An eight person team consisting of world renowned marine biologists, technical cave and rebreather divers, and underwater photographers from the US and Australia participated this expedition included:

Prof. Thomas Iliffe (Texas A&M University) expert cave and rebreather diver who has led numerous expeditions, studying anchialine caves world-wide and discovering more than 300 new species.

Dr. Bill Humphreys (Western Australia Museum) expert on Australian anchialine ecology and leader of the 1998 biospeleological expedition to Christmas Island.

Jill Heinerth (Into the Planet, US) pioneering underwater cave explorer and award-winning filmmaker, specializing in HD still and video photography and media management.

Brian Kakuk (Bahamas Caves Research Foundation) former US Navy diver and foremost expert on diving in the Blue Holes of the Bahamas.

Ken Smith (Australia) veteran cave diver having explored most of Australia’s caves as well as numerous ones overseas; developed low frequency pinger used in precision cave mapping.

Craig Challen (Australia) one of Australia's leading technical divers with record setting dives of distance in Cocklebiddy Cave, Australia, and depth in Pearse Resurgence, New Zealand.

John Dalla-Zaunna (Australia) mapping coordinator of the CDAA (Cave Divers Association of Australia) specializing in 3D & virtual cave mapping & design.


Photo: The margins of pools in of some of the more inland caves
are well decorated with stalactites and stalagmites.



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.

Left Photo: The robber crab, a terrestrial hermit crab weighing up to 4 kg and having a length of 1 m, is the world’s largest land-dwelling arthropod. It is a protected species on Christmas Island.
Below Photo: Many cave pools on Christmas Island are floored with huge boulders clearly illustrating the collapse origin of these caves.


Biological discoveries: Exciting and unanticipated animals, likely representing new species, were collected, primarily from inland caves close to the coast. The thermosbaenacean Halosbaena is one of the iconic marker taxa of anchialine systems from tropical and subtropical waters. Caves with this species characteristically contain an assemblage of other anchialine restricted crustaceans. The occurrence of Halosbaena on Christmas Island was unexpected, although the thaumatocyprid ostracod Humphreysella baltanasi (previously Danielopolina baltanasi; see Humphreys et al., 2009 and photo at right). Specimens of H. baltanasi collected during the expedition will be used for molecular genetic comparison with other closely related species. Until recently this type of anchialine fauna was restricted to areas bounded by shallow continental waters such as the Caribbean, Canary Island and NW Australia.  The occurrence of such fauna on isolated seamounts such as Christmas Island and also Minami Daito-jima (island) in the Okinawans (Pacific) suggests that the ancestors of these animals have been able to disperse across the ocean within the time of emergence of such islands; in the case of Christmas Island, since the mid-Miocene or possibly Eocene. A swimming polychaete worm from the family Spionidae looks very much like a member of the genus Prionospio (see photo at left). Another undescribed swimming species from this genus occurs in a submarine lava tube in Lanzarote, Canary Islands – nearly on the opposite side of the world! DNA sequencing can determine how closely related they actually are. A cave adapted amphipod appears to be new species within the family Anamixidae and the genus Anamixis (see photo at right). Finally, a number of copepods were collected from several caves and are currently being identified.

Left Photo: Bill displays his battle wounds and mud from a trip into Whip Cave.


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.

Left Photo: John and Craig prepare for a rebreather dive in a search of submerged cave springs off Christmas Island delivering fresh water from the inland aquifer.

  Photo: John passes dive tanks down through the narrow entrance to Whip Cave.


Grimes, K.G. (2001). Karst features of Christmas Island (Indian Ocean). Helectite, 37(2):41-58.
Hobbs, J.A., Darren J. Coker, P.T. Green, D.J. James, W.F. Humphreys, I.A.W. McAllan, S.J. Newman, M.S. Pratchett, T.M. Staeudle, & S.D. Whiting. (2014). "An annotated bibliography of the research on marine organisms and environments at Christmas Island and the Cocos (Keeling) Islands." Raffles Bulletin of Zoology, Supplement 30.

Humphreys, W.F., Kornicker, L.S. & Danielopol, D.L. (2009). On the origin of Danielopolina baltanasi sp. n (Ostracoda Thaumatocypridoidea) from three anchialine caves on Christmas Island, a seamount in the Indian Ocean. Crustaceana, 82(9):1177-1203.

Humphreys, W.F. (2014). Subterranean fauna of Christmas Island: habitats and salient features. Raffles Bulletin of Zoology, Supplement, 30.

Iliffe, T.M. (2000). Anchialine cave ecology. Pages 59-76 in: Ecosystems of the World. 30. Subterranean Ecosystems, H. Wilkens, D.C. Culver, & W.F. Humphreys (eds.), Elsevier Science, Amsterdam.

Iliffe, T.M. and L.S. Kornicker (2009). Worldwide diving discoveries of living fossil animals from the depths of anchialine and marine caves. Smithsonian Contributions to Marine Sciences, 38:269-280.