Arachnothelphusa
Genus of crabs
From Wikipedia, the free encyclopedia
Arachnothelphusa is a genus of highly specialised, semi-terrestrial freshwater crabs in the family Gecarcinucidae.[1] Established by Peter K. L. Ng in 1991, their name translates roughly to "spider-crab," referring to their unusually long, slender ambulatory legs.[1] Strictly endemic to the island of Borneo, their distribution spans across the Malaysian states of Sabah and Sarawak, as well as the Indonesian province of Kalimantan.[1] They have largely abandoned rivers for extreme micro-habitats; several species are exclusively arboreal and live in phytotelms (water-filled tree holes) high in the rainforest canopy,[2] while others are cavernicolous (cave-dwelling), living deep within pitch-black limestone karst caves.[3]
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| Arachnothelphusa | |
|---|---|
| |
| Arachnothelphusa rhadamanthysi, Gomantong Caves, Borneo. | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Clade: | Pancrustacea |
| Class: | Malacostraca |
| Order: | Decapoda |
| Suborder: | Pleocyemata |
| Infraorder: | Brachyura |
| Family: | Gecarcinucidae |
| Genus: | Arachnothelphusa Ng, 1991 |
Taxonomy
Taxonomic history
Prior to 1991, species currently assigned to Arachnothelphusa were classified under the broader genus Thelphusa.[1] However, researchers had previously noted that Thelphusa contained distinctly different morphological groupings, suggesting the genus was heterogeneous.[1] One specific group of these Bornean crabs stood out due to their exceptionally long walking legs, convex carapace edges and the distinct structure of their gonopods (male reproductive appendages).[1] To address this taxonomic inconsistency, Peter K. L. Ng formally erected the new genus Arachnothelphusa in 1991 to accommodate these cursorial species, designating Potamon melanippe as the type species.[1]
Etymology
The generic name is a combination of the Greek word Arachne (meaning "spider") and thelphusa.[1] This name refers to the unusually long, spider-like ambulatory legs characteristic of the genus.[1]
Species
The genus Arachnothelphusa currently contains eight recognized species:
- Arachnothelphusa bako Ng, 2021[4]
- Arachnothelphusa kadamaiana (Borradaile, 1900)[5]
- Arachnothelphusa melanippe (De Man, 1899)[6]
- Arachnothelphusa merarapensis Grinang, Pui & Ng, 2015[7]
- Arachnothelphusa rhadamanthysi Ng & Goh, 1987[8]
- Arachnothelphusa rimba Ng, 2021[4]
- Arachnothelphusa sarang Grinang & Ng, 2021[3]
- Arachnothelphusa terrapes Ng, 1991[1]
Morphology
Species within Arachnothelphusa are relatively small crabs, with the largest recorded male (A. terrapes) measuring 25.7 by 18.6 mm.[1] They possess several distinct morphological traits adapted for their semi-terrestrial, cursorial (running and climbing) lifestyle:[1]
Carapace
The carapace is transversely ovate, with a surface that is slightly to distinctly convex and often rugose (wrinkled or granular).[1] The anterolateral margins are convex and depending on the species, the epibranchial tooth can range from acutely triangular to low and indistinct.[1]
Legs
Their most defining feature is their exceptionally long, spider-like ambulatory legs.[1] The merus (upper leg segment) is subequal to the length of the entire carapace.[1] The meri of their chelipeds (claws), however, are not distinctly elongated.[1]
Gonopods
In males, the first gonopod (G1) is slender and sinuous, featuring a cylindrical, tapering terminal segment that is about one-third to one-quarter the length of the subterminal segment.[1] The second gonopod (G2) has a short distal segment.[1]
Coloration
Live coloration varies drastically depending on the species' habitat. Epigeal (surface-dwelling) and arboreal species are often vibrantly colored, such as the bright purple A. merarapensis[7] or the dark reddish-brown to purple A. terrapes.[1] Conversely, cavernicolous species like A. rhadamanthysi[8] and A. sarang exhibit pale body colouration ranging from straw-yellow to light purplish-brown.[3]
Habitat and ecology
While some early specimens like A. melanippe were collected from small forest streams, the genus is best known for its members' adaptations to extreme, isolated micro-habitats.[1]
Arboreal and phytotelm behaviour
Several species, including A. terrapes, A. merarapensis and A. rimba, are true phytotelm crabs.[1][7][4] They are predominantly arboreal, living exclusively inside water-filled tree holes or crevices within tree buttresses in primary dipterocarp forests.[7] Their unusually long ambulatory legs allow them to easily scramble up tree trunks, branches and twigs to reach these arboreal pools.[4] They are nocturnal and highly sensitive to light; when disturbed, they will swiftly retreat and submerge themselves into the murky water of their tree holes to avoid predation.[7] These crabs are completely reliant on these micro-habitats for their life cycle, with females observed carrying newly hatched young directly inside the tree holes.[2] Because they live high up in trees or on hill inclines away from typical water bodies, this adaptation may help them avoid competition with other river-dwelling freshwater crabs.[4]
Cavernicolous behaviour
Other species in the genus, specifically A. rhadamanthysi and A. sarang, have evolved to live inside limestone karst cave systems.[8][3] A. sarang inhabits water pools with fine substrates deep within the interior of cave systems,[3] while A. rhadamanthysi has been found on stalagmite walls in more exposed areas near cave entrances.[8] Despite their pale colouration and cave-dwelling nature, they are not considered true stygobitic or troglobitic species.[3] They are classified as troglophilic, as their eyes are not reduced and their corneas remain fully pigmented and functional.[3]
Conservation status
Within the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, the conservation statuses of assessed Arachnothelphusa species vary. A. kadamaiana,[9] A. rhadamanthysi[10] and A. terrapes[11] are currently classified as Least Concern. Conversely, A. melanippe is classified as Vulnerable.[12]
Many of the more recently discovered species currently lack official IUCN assessments (effectively unassessed or Data Deficient). However, researchers have strongly recommended that newer discoveries, such as the phytotelm-dwelling A. merarapensis, be officially classified as Vulnerable. This is due to their highly localized, micro-endemic distributions and the looming threat of habitat loss from local logging activities.[7]