Achaeus pholcus Ng & Forges & Lee 2023, n. sp.
Creators: name: Ng, Peter K. L.
name: Forges, Bertrand Richer De
name: Lee, Bee Yan
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Inachidae
Achaeus
Achaeus pholcus

description: Achaeus pholcus n. sp.
(Fig. 1C, 10–13)
Type material. Holotype: male (cl 8.1 mm, pcl 7.3 mm, cw 5.8 mm) (MNHN-IU-2014-8034), stn CP 4423, New Ireland, Papua New Guinea, 02°20’S 150°38’E, 550–649 m, coll. N.O. Alis, KAVIENG 2014 expedition, 28 August 2014.
Diagnosis. Carapace subpyriform, regions clearly demarcated; branchial and gastric regions appear almost globular, wide and long (Fig. 10A, B); frontal and epigastric regions short, not elongate (Fig. 10B, C, E); gastric regions not elongate, gently convex, without spine or sharp tubercles (Fig. 10B, C, E); cardiac region prominently swollen with sharp tubercle (Fig. 10B, E, F); intestinal region unarmed (Fig. 10B, E); pseudorostrum relatively short, with 2 subtruncate lobes, separated by deep U-shaped cleft, margin of each lobe with 3 low tubercles (Figs. 10B–D, 13A); supraorbital eave lined with small granules, without spines, antorbital angle visible but not dentiform or spiniform (Figs. 10B, C, 13A); hepatic region prominently swollen, round (Fig. 10B, C, E, F); surface and margin of carapace with numerous small rounded granules (Fig. 10B, C, E); epistome quadrate (Fig. 10D); basal antennal article long, slender, distal margin with 3 longer sharp spines, outer and inner margins each with row of low spines, antennal article 3 two-thirds length of basal antennal article, antennal article 4 ca. half length of article 3; antennular fossa long, basal antennular article with sharp anterior spine (Fig. 10 BC, D); interantennular spine large, triangular, lobiform (Fig. 10D, E). Inner margin of third maxilliped merus with 2 sharp spines, ischium with 2 rows of sharp tubercles (Figs. 12C, 13B). Chelae slender, dorsal and ventral margins with short spines, fingers as long as palm (Fig. 12A, B). P2–P5 long, slender, merus, carpus and propodus unarmed, P2 merus 7.8 times pcl, dactylus gently curved, without subterminal spine, ventral margin with evenly spaced 10 or 11 spinules (Figs. 10A, 13C–H). Male thoracic sternites 3 and 4 fused; sternite 3 with 2 median depressions; sternite 4 arched ventrally to form median transverse convex crest anterior to sternopleonal cavity, crest medially clefted, serrate (Fig. 11A, C). Male pleon short, triangular; outer surfaces and margins of somites 1–4 with sharp granules; somite 4 trapezoidal; somite 5 subrectangular, as wide as pleotelson; somite 6 and telson completely fused with only lateral suture just visible, median part swollen to form subrectangular convexity (Fig. 11B, D, E). G1 slender, broadly C-shaped, distal part with broadly rounded tip, small dorsal fold marking subterminal aperture (Figs. 12D–F, 13I–N).
Colour. The overall colour is yellowish-white to white (Fig. 1C).
Etymology. The species name is derived from the Greek “pholkos” for bow-legged; alluding to its resemblance to arachnids of the genus Pholcus, better known as “daddy-long-legs”. The name is used as a noun.
Remarks. The most distinctive character of A. pholcus n. sp. is the long and wide branchial, cardiac, and intestinal regions, together with the distinctly inflated hepatic regions, which give the carapace a subglobular appearance; with the orbital and frontal regions appearing disproportionately short (Fig. 10A–C). The cardiac region is also prominently raised and capped by a sharp tubercle (Fig. 10D, E). All other species of Achaeus have a more pyriform and slender carapace, or carapace that appears subquadrate; most species have a less prominent and sharp cardiac region. Following the key of Griffin & Tranter (1986: 4), A. pholcus n. sp. belongs to a group of species in which the supraorbital eave has only small spinules lining the margin, the dorsal surface of the carapace does not have a transverse ridge or any large spines or tubercles (except for the cardiac region), the hepatic region is prominently produced with the epistome well-developed: A. lacertosus Stimpson, 1857, A. villosus Rathbun, 1916, and A. brevidactylus Sakai, 1938. These three species can easily be distinguished from A. pholcus n. sp. by the latter’s longer and more inflated carapace (Fig. 10B) (cf. Sakai 1938: pl. 21 fig. 4). Achaeus lacertosus has the antennular fossa and basal antennal article relatively short, the ambulatory dactylus semilunate with strong teeth along the ventral margin, and the male pleon is prominently broader, with somite 5 and the pleotelson very wide (Griffin 1970: fig. 5a, c, d) (versus the antennular fossa and basal antennal article distinctly longer, the ambulatory dactylus is gently curved with scattered low spinules along the ventral margin, and the male pleon is narrower with somite 5 subquadrate and the pleotelson less wide in A. pholcus n. sp.; Figs. 10A, D, 11D, 13F, H). Achaeus villosus differs in having a more entire frontal margin with only a low concavity separating the two lobes, there are many small spinules along the surfaces of the hepatic and branchial regions, the chela is stouter and shorter with the margins smooth, the ambulatory dactylus is relatively shorter, and the male pleotelson is more triangular (Griffin & Tranter 1986: fig. 4b–d) (versus two large frontal lobes distinctly separated by a deep U-shaped cleft, the hepatic and branchial regions covered with granules, the chela is more slender and longer with the margins lined with spinules, the ambulatory dactylus is relatively longer, and the male pleotelson is relatively wider in A. pholcus n. sp.; Figs. 11A–D, 13A). Achaeus brevidactylus can be separated by its distinctive P2 and P3 dactyli, which have the distal part dilated and spade-like (Sakai 1938: text-fig. 8a, b), and its male pleotelson is distinctly triangular in shape with almost straight lateral margins (Sakai 1938: text-fig. 8f) (versus P2 and P3 dactyli tapering to a sharp tip with the male pleotelson possessing strongly sinuous lateral margins in A. pholcus n. sp.; Fig. 9D). The G1 of A. brevidactylus is not known but those of the other two species are quite different: A. villosus has a relatively shorter and more sinuous G1 with the tip wide and appearing bilobed (Griffin & Tranter 1986: fig. 5g, h) while that of A. lacertosus is shorter and weakly C-shaped with the distal part subconical (Griffin 1970: fig. 14a, b). The G1 of A. pholcus n. sp. is relatively long, slender, C-shaped with the distal part wide (Fig. 12D–F, 13I–N).
Achaeus villosus is known from the southern Philippines (Sulu islands), northern Moluccas, Ambon, Timor and Sunda Strait (Indonesia) (Rathbun 1916; Griffin 1976; Griffin & Tranter 1986). Achaeus lacertosus has a wide range across the Indo-West Pacific, ranging from South Africa and Iranian Gulf to southern India, Java, Kai Islands in the Moluccas, Aru islands, northern Australia, and Japan (Stimpson, 1857, 1907, Sakai 1938, 1976; Griffin 1970; Griffin & Tranter 1986). Achaeus brevidactylus is known only from Japan (Sakai 1938, 1976). In addition, it is noteworthy that A. pholcus is a deep-water species (550–649 m); the other three species occur in depths of 200 m and shallower.
description: Published as part of Ng, Peter K. L., Forges, Bertrand Richer De & Lee, Bee Yan, 2023, New and rare deep-sea majoid crabs (Crustacea: Brachyura) from Papua New Guinea, pp. 1-44 in Zootaxa 5318 (1) on pages 14-17, DOI: 10.11646/zootaxa.5318.1.1, http://zenodo.org/record/8158079