Late 1993, biologist Tony Whitten collected some unusual crabs in two adjacent caves on the limestone karst island of Nusa Penida, which nestles to the Southeast of Bali, Indonesia. These and some specimens collected soon after were described as a new species, namely Sesarmoides emdi, by Peter Ng and Tony Whitten. Peter Ng later realized that the collection represented two, not one, species and he named the second species Sesarmoides balicus in 2002. Five years later the genus name for these two species changed to Karstama.
Image: Goa Giri Putri (Tony Whitten, 2015)
Karstama (Sesarmoides balicus/emdi) features
The crabs have relatively long legs suggesting they have evolved in darkness, a common morphological adaptation to life in caves. The carapaces of the males measured about 20x18 mm. A body of only about 15-24 mm across and 12–20 mm from front to back; long legs relative to its body; orange-coloured pincers. It never carries a shell on its back.
Habitat: Goa Giri Putri & Gua Paon
The main cave is known as Gua (Cave) Giri Putri in Karangsari hamlet, Suana Village, on the central eastern coast of Nusa Penida. This cave is relatively simple and is 276 metres long. The other cave is Gua Paon just a few hundred metres away and it is assumed the two caves are somehow connected. Gua Giri Putri has a number of Hindu temples within it, some of which are especially sacred.
Goa Giri Putri has been visited for a few hours every few years by the discoverer when other duties have allowed, but the crabs have never been as apparently common (or at least as easy to see) as on the first occasion in 1993. Subsequent visits have never found the cave environment to be as wet as on the first visit.
In order to assess whether the two crabs are indeed restricted to the two adjacent caves, a pamphlet was distributed by a local conservation NGO, FNPF, across all of Nusa Penida's villages in 2012. It asked villagers to contact FNPF to report any other sites, there were no reports. It is thus reasonable to assume that the known sites of the two species are the only two sites. In late 2013 a letter was sent to the leader of the temple committee asking him to cooperate with producing an action plan for the crabs, and pointing out that Hindu teachings teach the wisdom of conservation.
Images: Karstama (Sesarmoides balicus/emdi) by Tony Whitten, 2015
The crabs are found on the muddy floor and lower walls of the cave. It shelters both in the little (~7 cm diameter) drip-pools in the floor when these are wet, or in clefts in the wall. The cave floor is the path used by pilgrims and worshippers at the cave's temples.
Threats & conservation
The primary threat to this crab is likely the increasing use of the cave by pilgrims and worshippers at the main cave of Giri Putri. In 1993, when the crab was discovered, the infrastructure was minimal, but now there are walkways and seating across some of the cave floor, and electric lights along the entire length of the cave. Six visits since the original discovery have noted an apparent decline in the abundance of the crabs, although some of that may have been caused by visits being undertaken in relatively dry periods when free water on the floor of the cave was less available. Roosting bats (source of guano) also seem to be less common in the caves than before. Conversely, it is possible that the crabs benefit from food offerings left and spilled on the ground but this is not yet confirmed. There are no conservation actions in place. The most effective, feasible preliminary action is to gain the cooperation of the temple authorities to allow a degree of precautionary management and some basic survey and monitoring of crab numbers and ecology and behaviour.
- Davie, P.J.F. and Ng, P.K.L. (2007) - A new genus for cave-dwelling crabs previously assigned to Sesarmoides (Crustacea: Decapoda: Brachyura: Sesarmidae). Raffles Bull. Zool. Suppl. 16: 227-231, http://lkcnhm.nus.edu.sg/rbz/biblio/s16/s16rbz227-231.pdf
- Ng, P. K. L. & A. J. Whitten (1995) - On a new cave-dwelling Sesarmoides (Crustacea: Decapoda: Brachyura: Grapsidae) from Nusa Penida, Bali, lndonesia. Tropical Biodiversity, 2: 369-376
- Ng, P.K.L. (2002) - New species of caverniculous crabs of the genus Sesarmoides from the Western Pacific with a key to the genus (Crustacea: Decapoda: Brachyura: Sesarmidae). Raffles Bull. Zool. 50: 419-435
- Whitten, Tony (2015) - Understanding the Conservation Needs of Two Crab Species Endemic to the Holy Caves of Goa Giri Putri, Nusa Penida, Bali, Indonesia (personal correspondence, July 2015)