Discoplax gracilipes

Discoplax gracilipes is a species of cave-dwelling, terrestrial crab found in the Philippines and Japan. The crabs have extremely long, stalky walking legs and are generally purple in colour. They are inhabitants of shallow cave pools which they are known to share with several other crab species, as well as a variety of other animals. The crabs are foragers active during both day and night who live in warm, shallow cave pools and emerge to search for food nearby, and to release their eggs into the sea. They were first reported by scientists in Panglao in the Philippines in the late 20th century but the species was not given a formal scientific description until 2000. In the subsequent decades the crabs were found on other nearby islands in the Philippines, and in 2023 a single crab was discovered living in a cave on a Japanese island near Taiwan, extending its known range by 1400 kilometres. D. gracilipes is believed to be most closely related to Discoplax longipes, and are classified in the Gecarcinidae crab family, in the group Grapsoidea. In Panglao they are called Alikuai by the locals, who catch them and other land crab species.

Discoplax gracilipes
Dorsal and ventral view of a specimen from Hateruma Island, Japan.
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Malacostraca
Order:	Decapoda
Suborder:	Pleocyemata
Infraorder:	Brachyura
Family:	Gecarcinidae
Genus:	Discoplax
Species:	D. gracilipes
Binomial name
Discoplax gracilipes Ng and Guinot, 2001

Classification

Discoplax gracilipes is classified in the family Gecarcinidae, a group of land-dwelling crabs found around the western Indo-Pacific region. A researcher collected from some caves in Bohol small specimens of a Discoplax-genus crab similar to Discoplax longipes. The cave specimens were all either at the juvenile or subadult life-stage and had a smoother texture and proportionally stalkier walking legs than D. longipes. In 1992 these cave specimens were viewed by Peter Kee Lin Ng and Danièle Guinot, who waited until adult specimens of the cave crab were discovered to describe it as a new species. These were uncovered in 2000 by a professor and his students at the University of San Carlos; the next year Ng and Guinot published a description of the cave species in The Raffles Bulletin of Zoology.^{[1]: 311–2} They named the cave crab D. gracilipes, with the holotype specimen being a male collected by townspeople from a local cave in Panglao in 2000.^[1]: 324 In Panglao they are called Alikuai by locals.^[1]: 330 The species' discovery in far southern Japan led to the proposal of a Japanese name for the species, Sūin-oka-gani, derived from sūin, a word for anchihaline tide pools in the local tongue and oka-gani, meaning land crab in Japanese.^[2]: 183

Phylogenetic studies

A 2014 study by Peter Kee Lin Ng and Hsi-Te Shih [wikispecies], also published inThe Raffles Bulletin, used Discoplax gracilipes as an outgroup to determine the relationship among Discoplax longipes, Discoplax hirtipes, Discoplax rotunda, and Discoplax magna.^[3]: 109 Their research showed support a "long-legged" clade composed of D. gracilipes and D. longipes, with the other three species forming a second clade.^[3]: 130 The "long-legged" species are both adapted for cave-dwelling. The three other species would later be siphoned off into a new genus, Tuerkayana, in a 2018 taxonomic revision.^[4]: 562

Description

General appearance

Discoplax gracilipes crabs are smallish animals with oval-shaped carapaces usually somewhat wider than they are long.^[2]:178 In colour they range from "pale grey-purple" to violet, indigo, and deep red The male holotype specimen carapace measured 4.6 by 5.3 cm – actually longer than wide.^[1]:324 Male D. gracilipes crabs have carapace widths ranging from 2.3 to 5.4 cm across; females are slightly more diverse, ranging from 1.7 to 5.5 cm across. On average the carapace width is 115% of the carapace length (CL). The length of the second segment (the meripodite) of the fourth walking leg (pereiopod) averages 101% of the CL (carapace length) in males and 104% in females, with the highest recorded length in proportion in the CL being 136%, in a female. The other leg segments of the fourth walking leg are shorter, with the second segment (the propodus) being the second-longest at on average three quarters the CL.^[2]:182 The third pair of walking legs is the longest; the last, the shortest; and all are hairless and covered with small bumps. The dimensions of the walking legs vary considerably among mature individuals. Other features do not have significant variation. Compared to Discoplax longipes, D. gracilipes has more variation in pereiopod proportions, and both species have longer-legged juveniles.^[1]:325

Discoplax gracilipes have small eyes mounted on short eyestalks, each about 12.5% of the CL The cornea of the eye is spherical and has a diameter of about 11% of the CL.^[2]:182 The eyes are "well-developed" and fill the entire eye socket. The front and reare edges of the carapace are both mildly convex. The chelipeds are slightly bumpy, rough, and unequal in size.^[1]:321–3

Identification

Discoplax gracilipes is remarkable for its "extremely long" walking legs, with the length of the meripodite fourth walking leg sometimes exceeding the width of the crab's carapace. Other diagnostic characteristics are that the eye socket – the orbit – opens out on both sides; there is a file-like structure called the pars stridens on the lower edge of the socket, matched by a projection on the side of the cheliped which may rub the file to produce sound; finally, the upper edge of the socket is distinguished with several horizontal rows of small bumps.^{[2]: 179 & 185 [5]}

Distribution and habitat

Ng and Guinot noted that Discoplax longipes had only been found so far on the Philippine isle of Panglao, which is abundant with anchialine caves and depressions.^[1]: 326 In 2010, the crabs were discovered in a limestone cave on Borocay in the Philippines; three years later they were also found living in Siquijor Island, also in the Philippines; and in 2023 they were found at Kawasan Falls on the island of Cebu.^{[6][7]: 64 [8]: 3} In 2023, a paper published in Species Diversity announced the discovery of a single crab from an anchialine pool in Japan's Hateruma Island, near Taiwan. The crab was collected with a bait trap in a warm and mostly shallow tide pool in a cave. The authors identified the cave crab as D. gracilipes, and the discovery therefore extended its understood range northward by 1400 kilometres.^[2]

The crabs live in warm, shallow cave pools both rocky and muddy.^{[1]: 326–30} D. gracilipes is known to coexist with many other species of crab both aquatic and terrestrial, but also with Eleotris fish, moray eels, small aquatic crustaceans and, in the dry parts of its caves, bats and wood cockroaches.^{[1]: 330 [2]: 181–3} The dry parts of its caves have been recorded to be home to the crabs Gecarcoidea lalandii,[note: cave entrance; common oustide] Geograpsus crinipes, Karstarma boholano, Sesarmoides, and Tuerkayana hirtipes, while the pools are also inhabited by Orcovita miruku, Orcovita fictilia, Tuerkayana rotundum.^[1]: 330

Behaviour

Paratype specimen kept in the French National Museum of Natural History.

In Panglao, the crabs lurk in the darkness of the caves during the daytime and then emerge to forage for food during the night,^[1]: 330 but observations of the crabs on Siquijor showed them active and foraging during the daytime.^[7]: 63 In Panglao the crabs' foraging activities are mostly around the mouth of their caves and in nearby woodland. Land crabs on the island are often caught by locals, with D. gracilipes being no exception. The females have been observed leaving their caves and crossing land to release their eggs in the sea.^[1]: 330 The related species Discoplax longipes behaves similarly.^[1]: 321

Notes