Ophiopsila xmasilluminans: A bioluminescent, cave-dwelling Brittle Star from Christmas Island.

Brittle Stars, Ophiuroidea, are five-limbed Echinoderms related to Starfish and having a similar body-plan. However while the five limbs of the Starfish are rigid extensions of the central disk, and movement is achieved by means of tube-feet on the underside of the animal, the limbs of Brittle Stars are fully articulated and used for locomotion. The fossil record of Brittle Stars goes back to at least the Early Ordovician, though these Palaeozoic Brittle Stars have not been extensively studied. Recent studies in the Ryukyu Islands of southwestern Japan have suggested that submarine caves may be an important, and underexplored habitat for Brittle Stars, with explotation teams visiting cave systems on Christmas Island, an Australian terretory to the south of Java, from 2010 to 2012 also reporting seeing Brittle Stars.

In a paper published in the Raffles Bulletin of Zoology on 5 August 2019, Masanori Okanishi of the Misaki Marine Biological Station of the University of Tokyo, Yuichi Oba of the Department of Environmental Biology at Chubu University, and Yoshihisa Fujita of the Okinawa Prefectural University of Arts, describe a new species of bioluminescent, cave-dwelling Brittle Star from Thunderdome Cave on the north coast of Christmas Island.

Thunderdome cave is located on the central part of the north shore of Christmas Island, about 17 m bellow sea level. It has a wide entrance, 10-15 m across, leading to a cave that extends 20-30 m to the left of the entrance, and a few metres to the right. The left hand cave opens into an airpocket.

The location of Thunderdome Cave. Okanishi et al. (2019).

The new species is placed in the genus Ophiopsila, and given the specific name xmasilluminans, implying that it is bioluminescent, and comes from Christmas Island. The examined specimens of the species had cenral disks 5.0 to 6.3 mm across, and arms up to 85 mm in length. The disc surface is entirely covered by thick skin with embedded small and delicate scales and granules. Ophiopsila xmasilluminans was found living burried beneath the sand at the bottom of the cave, with its disk and the inner part of its tentacles covered, and only the outer parts of its tentacles exposed. The tentacles have bands of bioluminescent material on every fourth to sixth tentacle, which flash when the tentacle is touched. Quite how this is useful is unclear, as the cave is also home to numerous sighted Fish and Crustaceans, which might be expected to be attracted to such flashing, and which would be far more capable of consuming a Brittle Star than the other way around.

Ophiopsila xmasilluminans, anesthetised (A) paratype, RUMF-ZE-00152, aboral view (B) with buried disc and extended arms, specimen not collected. (C) an excavated specimen, also not collected. Okanishi et al. (2019).

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Two new species of Cave Crab from Christmas Island.

Cave Crabs of the genus Orcovita are known from marine caves across the West Pacific. They tend to have localized distributions, inhabiting limestone caves on Oceanic Islands.

In a paper published in The Raffles Bulletin of Zoology on 29 February 2012, Peter Davie of the Queensland Museum and Peter Ng of the Tropical Marine Science Institute and Raffles Museum of Biodiversity Research at the Department of Biological Sciences at the National University of Singapore describe two new species of Cave Crabs in the genus, both from Christmas Island in the southern Indian Ocean; the first time such crabs have been described from the Indian Ocean. Both new crabs are described from specimens captured using commercially available baited crab traps.

The first new species described is named Orcovita orchardorum, in honour of Max Orchard, the Head Ranger on Christmas Island for 18 years and subsequently Project Officer on the island, and his wife Beverly, who between them have made an invaluable contribution to conservation on the island. Orcovita orchardorum is a yellowish-brown Crab up to 23 mm in width.

Orcovita orchardorum. (A) Male. (B) Female. Davie & Ng (2012).

The second new species described is Orcovita hicksi, named in honour of John Hicks who was the Australian Government Conservator on Christmas Island from 1979 to 1983, and who wrote a noted book on the Crabs of the island. It is a yellowish-white to orange Crab up to 21 mm in width, though specimens larger than 16 mm tend to be lighter in colour.

Orcovita hicksi. (A) Male. (B) Female. Davie & Ng (2012).

Orcovita hicksi; colour variation. (A) Male, 15.9 mm width. (B) Male, 14.4 mm width. (C). Female, 20.8 mm width. Davie & Ng (2012).

Map showing the distribution of known species in the genus Orcovita. Davie & Ng (2012).

See also Nauplius Larvae frim the Windyfield Chert in Aberdeenshire, Scotland, A new species of Barnacle from Taiwan, The Christmas Island Blue Crab recognized as a distinct species, Two new species of freshwater Isopod Crustaceans from Lake Pedder in Tasmania and Marine Invertebrates on Sciency Thoughts YouTube.

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The Christmas Island Blue Crab recognized as a distinct species.

Christmas Island in the eastern Indian Ocean is noted for its distinctive Crab fauna, most notably the abundant Red Land Crab, Gecarcoidea natalis. Another distinctive form on the island is the Blue Crab, which has generally been regarded as a colour variation on the widespread Discoplax hirtipes, which is found from the Andaman and Nicobar Islands in the west to Hawaii in the east. This lives in burrows beside freshwater pools and streams. 

In a paper published in The Raffles Bulletin of Zoology on 29 February 2012, Peter Ng of the Tropical Marine Science Institute and Raffles Museum of Biodiversity Research at the Department of Biological Sciences at the National University of Singapore, and Peter Davie of the Queensland Museum, formally describe the Christmas Island Blue Crab as a separate species, as a result of a genetic study of the Crabs. 

The Christmas Island Land Crab. Ng & Davie (2012).

The new species is named as Discoplax celeste, celeste implying sky, or heavens, in reference to the sky-blue colour of the Crabs. It is essentially similar to Discoplax hirtipes in form and behavior, and develops its distinctive colouration only as an adult.

Colouration during the growth of Discoplax celeste. (A) juvenile (7.9 × 7.1 mm); (B) female (15.9 × 14.1 mm); (C) female (22.3 × 19.0 mm); (D) female (27.5 × 23.5 mm); (E) female (39.0 × 34.0 mm); (F) male (44.4 × 38.7 mm); (G) female (42.4 × 35.8 mm); (H) female (47.1 × 39.2 mm). Ng & Davie (2012).

Individuals with the purplish-brown with orange claws colouration of Discoplax hirtipes from Sumatra and the Nicobar Islands are also found on Christmas Island. Genetic sampling of these individuals suggested that they were closely related to the Crabs of Sumatra, not greatly surprising as Sumatra is the closest landmass to Christmas Island. Ng & Davie refer to these Crabs as Discoplax aff. hirtipes, on the basis that they suspect that these are likely to be a separate species from the Pacific Crabs.

Discoplax celeste (top) and Discoplax aff. hirtipes (bottom) from Waterfall Bay on Christmas Island. Both about 80 mm in diameter. Ng & Davie (2012).

The recognition of the Blue Crabs as a separate species is important, as these Crabs are increasingly at risk on Christmas Island, due to habitat destruction and invasive species. Recent fieldwork on the island has found very few juvenile specimens, which suggests the Crabs could be in serious difficulties. Recognition of the Blue Crabs as a separate species should (hopefully) ensure better protection for them.

See also A new species of Barnacle from Taiwan, Two new species of freshwater Isopod Crustaceans from Lake Pedder in Tasmania and The Snail that eats Crabs.

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