Arachnothelphusa

Arachnothelphusa
Arachnothelphusa rhadamanthysi, Gomantong Caves, Borneo.
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Arthropoda
Clade: Pancrustacea
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
Infraorder: Brachyura
Family: Gecarcinucidae
Genus: Arachnothelphusa
Ng, 1991

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]

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 morphology of various Arachnothelphusa species, Jongkar Grinang & Peter K. L. Ng (2021)

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]

Gonopod morphology of various Arachnothelphusa species, Jongkar Grinang & Peter K. L. Ng (2021)

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]

Illustration of Arachnothelphusa merarapensis, new species, holotype male (22.5 × 16.8 mm) (ZRC), emerging at night from its filled-water tree-hole in primary dipterocarp forest, Merarap Hot Spring, Lawas, Sarawak.

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]

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Ng, Peter K. L. (1991). "Bornean freshwater crabs of the genus Arachnothelphusa gen. nov. (Crustacea: Decapoda: Brachyura: Gecarcinucidae)". Zoologische Mededelingen. 65 (1): 1–12.
  2. 1 2 Ng, Peter K. L.; Ng, Paul Y. C. (2018). "The freshwater crabs of Danum Valley Conservation Area in Sabah, East Malaysia, with a description of a new species of Thelphusula Bott, 1969 (Crustacea, Brachyura, Gecarcinucidae, Potamidae, Sesarmidae)". ZooKeys (760): 89–112. Bibcode:2018ZooK..760...89N. doi:10.3897/zookeys.760.24787. PMID 29872365.
  3. 1 2 3 4 5 6 7 Grinang, Jongkar; Ng, Peter K. L. (2021). "A new species of the genus Arachnothelphusa Ng, 1991 (Crustacea: Decapoda: Gecarcinucidae) from a limestone cave in Sarawak (Malaysian Borneo)". Raffles Bulletin of Zoology. 69 (452): 1–7. doi:10.26107/RBZ-2021-0001.
  4. 1 2 3 4 5 Ng, Peter K. L. (2021). "On two new species of arboreal crabs from phytotelms in Sarawak, Borneo (Crustacea: Brachyura: Gecarcinucidae: Arachnothelphusa)". Zootaxa. 5016 (3): 407–418. doi:10.11646/zootaxa.5016.3.6. PMID 34810440.
  5. ↑ Borradaile, L. A. (1900). "On a small collection of decapod crustaceans from freshwater in North Borneo". Proceedings of the Zoological Society of London. 1900 (2): 93–95. doi:10.1111/j.1096-3642.1890.tb01706.x.
  6. ↑ De Man, J. G. (1899). "Zoological Results of the Dutch Expedition to Central Borneo. The Crustacea. Part II, Brachyura". Notes from the Leyden Museum. 21: 53–144.
  7. 1 2 3 4 5 6 Grinang, Jongkar; Pui, Yong Min; Ng, Peter K. L. (2015). "A new species of tree-hole dwelling freshwater crab of the genus Arachnothelphusa Ng, 1991 (Crustacea: Decapoda: Brachyura: Gecarcinucidae) from northern Sarawak, Malaysia, Borneo". Raffles Bulletin of Zoology. 63: 454–460.
  8. 1 2 3 4 Ng, Peter K. L.; Goh, R. (1987). "Cavernicolous freshwater crabs (Crustacea Decapoda, Brachyura) from Sabah, Borneo". Stygologia. 3 (4): 313–330.
  9. ↑ Esser, C.; Cumberlidge, N. (2008). "Arachnothelphusa kadamaiana". The IUCN Red List of Threatened Species. Retrieved 7 April 2026.
  10. ↑ Esser, C.; Cumberlidge, N. (2008). "Arachnothelphusa rhadamanthysi". The IUCN Red List of Threatened Species. Retrieved 7 April 2026.
  11. ↑ Esser, C.; Cumberlidge, N. (2008). "Arachnothelphusa terrapes". The IUCN Red List of Threatened Species. Retrieved 7 April 2026.
  12. ↑ Esser, C.; Cumberlidge, N. (2008). "Arachnothelphusa melanippe". The IUCN Red List of Threatened Species. Retrieved 7 April 2026.
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