Arcotheres obesus ( Dana, 1852 ) Komai & Kawai & Ng, 2020

Komai, Tomoyuki, Kawai, Kei & Ng, Peter K. L., 2020, On the identity of the poorly known pea crab, Pinnothera obesa Dana, 1852, and description of a new species of Arcotheres Manning, 1993 from the Southwest Pacific (Decapoda: Brachyura: Pinnotheridae), Zootaxa 4822 (2), pp. 221-247 : 223-230

publication ID	https://doi.org/ 10.11646/zootaxa.4822.2.5
publication LSID	lsid:zoobank.org:pub:6FC281F4-CC48-41E4-9C73-80B57A2A2D17
DOI	https://doi.org/10.5281/zenodo.4452496
persistent identifier	https://treatment.plazi.org/id/131587D9-950C-4264-73A2-FF50FDEFC518
treatment provided by	Plazi
scientific name	Arcotheres obesus ( Dana, 1852 )
status	comb. nov.

Treatment

Arcotheres obesus ( Dana, 1852) View in CoL n. comb.

( Figs. 1–6 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 )

Pinnothera obesa Dana, 1852: 380 View in CoL .— Dana 1855: pl. 24 fig. 3.— Woodward 1886: 177 (list).

Pinnotheres obesus .— Silas & Alagarswami 1967: 1204 (list).— Schmitt et al. 1973: 60 (part) (list and discussion).

Pinnotheres obesus . — Ng et al. 2008: 250 (list).

Type material. Neotype: CBM-ZC 15945 , female of post-hard stage V (4.4× 5.5 mm), Kumi Village , Viti Levu, Fiji, intertidal, associated with Gafrarium pectinatum , 22 February 2019, coll. K. Kawai.

Other material examined. Fiji. Females are all in the post-hard stage V. CBM-ZC 15946 , 3 females (4.2×5.3– 5.1× 6.1 mm), 1 ovigerous female (4.8× 5.4 mm), 2 males (3.6×ca 4.2 mm, damaged), Kumi Village , Viti Levu, intertidal, associated with Gafrarium pectinatum , 16 August 2010, coll. K. Kawai ; CBM-ZC 15947 , 15948 , 2 females (4.0×5.3, 7.2× 9.2 mm), ZRC 2019.1876 View Materials , 2 females (6.2×8.8, 6.9× 8.9 mm), 2 juvenile males (gonopods not differentiated) (1.7×1.9, 2.2× 2.6 mm), Waiganake Village , Viti Levu, intertidal, associated with Gafrarium pectinatum , 22 August 2011, coll. K. Kawai ; CBM-ZC 15949–15951 , 3 ovigerous females (5.4×6.7, 5.6×6.6, 4.6× 5.9 mm), same data as neotype, associated with G. tumidum ; ZRC 2020.0006 View Materials , 1 View Materials ovigerous female (4.9× 6.2 mm), associated with Gafrarium pectinatum, Telau , Viti Levu, 22 February 2019, coll. K. Kawai.

Peninsular Malaysia. ZRC 2000.2281 View Materials , 2 females (5.4×7.2, 4.7× 5.7 mm), Pulau Tinggi , Johor, associated with Gafrarium dispar ( Holten, 1802) , 7 May 1969 .

Redescription. Neotype (stage V female). Carapace ( Figs. 2 View FIGURE 2 A–D, 3A) roundly hexagonal, 1.25 times wider than long; dorsal surface gently convex, smooth, glabrous; lateral regions smooth, glabrous; front truncate, weakly projecting anteriorly beyond orbits, margin nearly straight.

Eyes ( Figs. 2 View FIGURE 2 A–D, 3A, B) small, not visible in dorsal view, completely filling orbit.

Antennular peduncle ( Fig. 3B View FIGURE 3 ) folded obliquely into antennular fossae. Antenna ( Fig. 3B View FIGURE 3 ) extending along lateral margin of front; article 2 subtrapezoidal, slightly narrowing distally.

Maxilliped 3 ( Fig. 3C View FIGURE 3 , E–H) outer surface glabrous; propodus about 2.8 times as long as high, moderately stout, distinctly longer than carpus; dactylus short, digitiform, inserted submedially on flexor margin of propodus, tip not reaching propodal apex; ischiomerus completely fused, relatively long, distinctly longer than wide, mesial margin rounded at widest point; exopod slender, about half length of ischiomerus (not illustrated for neotype, but illustrated for non-type female, Fig. 3G View FIGURE 3 ).

Chelipeds ( Figs. 3A View FIGURE 3 , 4 View FIGURE 4 A–E) short, symmetrical. Dactylus terminating in hooked, acute tip; upper margin rounded, gently convex; occlusal margin nearly straight, with 1 prominent, proximally directed, triangular tooth proximally and row of minute spiniform setae in distal 0.3. Palm slightly widened distally, 1.6 times as long as high; outer surface glabrous; inner surface almost glabrous, but with longitudinal row of short setae adjacent to lower margin, extending onto fixed finger, becoming multiple rows. Fixed finger terminating in hooked, acute tip, bearing short longitudinal row of stiff setae on inner surface distally ( Fig. 2D View FIGURE 2 ); occlusal margin nearly straight, with 1 small tooth proximally, hiatus proximal to occlusal tooth accommodating occlusal tooth on dactylus, and with comb-like row of minute spiniform setae distal to tooth. Carpus short, subcylindrical, glabrous. Merus slightly curved inwards, slightly compressed laterally, outer surface convex, glabrous; inner surface with scattered short setae.

P2, P3 and P5 symmetrical, P4 asymmetrical; propodus to merus subcylindrical; pereopod 4 longest; relative length of merus P 4>P3>P2>P5. P2 ( Figs. 3A View FIGURE 3 , 4F, G View FIGURE 4 ) moderately long, slender, entirely glabrous; dactylus gently curved, 0.6 times as long as propodus; carpus curved at base; merus almost straight. Pereopod 3 ( Figs. 2A View FIGURE 2 , 3H, I View FIGURE 3 ) moderately long, slender, entirely glabrous, slightly longer than P2, but otherwise similar to it; dactylus slightly curved, about half-length of propodus; merus faintly curved. Shorter P4 ( Figs. 3A View FIGURE 3 , 4J, K View FIGURE 4 ) dactylus about half-length of propodus, glabrous except for sparse short setae on flexor margin; propodus almost straight, glabrous except for row of sparse setae on flexor margin (shorter than those on right P4); carpus and merus glabrous, merus slightly curved. Longer P4 ( Figs. 3A View FIGURE 3 , 4L, M View FIGURE 4 ) about 1.2 times as long as shorter P4; dactylus about half-length of propodus, with row of sparse short setae on extensor margin, scattered very short setae on outer surface and row of mixture of short and long setae on flexor margin; propodus faintly curved, glabrous except for row of sparse long setae on flexor margin; carpus glabrous; merus slightly curved. P5 ( Figs. 3A View FIGURE 3 , 4N, O View FIGURE 4 ) dactylus longest, 0.8 times as long as propodus, approximately 6 times as long as wide, generally gently arcuate but with substantial curvature near base; extensor margin and upper and lower surfaces glabrous, but flexor margin with row of moderately long setae in distal 0.6, more closely spaced distally, not extending to tip, no comb-like rows of minute spiniform setae; propodus almost glabrous except for few setae on flexor margin; carpus glabrous, substantially curved; merus slightly narrowing proximally, glabrous.

Pleon ( Fig. 3A, D View FIGURE 3 ) extending to buccal region, covering bases of ambulatory legs; telson ( Fig. 3D View FIGURE 3 ) very broad, partially recessed into distal margin of pleomere 6.

Adult females. Similar to neotype. Carapace 1.2–1.3 times as long as wide.

Male post-hard stage. Cuticle of body and appendages not strongly calcified but well chitinised. Carapace ( Fig. 5A View FIGURE 5 ) roundly suboctagonal, 1.1 times wider than long; dorsal surface and lateral regions mostly smooth, glabrous; front weakly projecting anteriorly beyond orbits, margin nearly straight in dorsal view.

Eyes ( Fig. 5A View FIGURE 5 ) moderately small, clearly visible in dorsal view, completely filling orbit.

Maxilliped 3 ( Fig. 3B View FIGURE 3 ) similar to that of females.

Chelipeds ( Figs. 5A View FIGURE 5 , 6 View FIGURE 6 A–D) symmetrical, proportionately larger compared with females. Dactylus 0.6 times as long as palm, terminating in hooked, acute tip; upper margin rounded, gently convex; occlusal margin with 1 conspicuous triangular tooth proximally but otherwise apparently smooth. Palm slightly widened distally, 2.2 times as long as high; outer surface glabrous; inner surface almost glabrous, but with longitudinal row of short setae adjacent to lower margin, extending onto fixed finger, becoming multiple rows. Fixed finger terminating in hooked, acute tip; occlusal margin with 1 small tooth proximal to midlength, hiatus proximal to occlusal tooth accommodating occlusal tooth on dactylus, and with comb-like row of minute spiniform setae distal to tooth. Carpus distinctly shorter than palm, widened distally, glabrous. Merus slightly curved inward, slightly compressed laterally; outer surface convex, glabrous, lower distal angle somewhat produced; inner surface with few short setae.

P2, P3 and P5 symmetrical, P4 asymmetrical (left longer than right), P4 longest; relative length of merus P4>P3>P2>P5. P2 ( Figs. 5A View FIGURE 5 , 6E, F View FIGURE 6 ) moderately long, stout, entirely glabrous; dactylus curved distally, 0.6 times as long as propodus; merus almost straight. P3 ( Figs. 5A View FIGURE 5 , 6G, H View FIGURE 6 ) entirely glabrous; dactylus curved distally, about half-length of propodus; merus faintly curved. Shorter (right) P4 ( Figs. 5A View FIGURE 5 , 6K, L View FIGURE 6 ) dactylus about half-length of propodus, glabrous except for 4 widely spaced, long setae on flexor margin and some very short setae on extensor margin; propodus slightly arcuate, almost glabrous except for 2 long setae on distal part of flexor margin. Longer (left) P4 ( Figs. 5A View FIGURE 5 , 6I, J View FIGURE 6 ) about 1.2 times as long as shorter (right) P4; dactylus about half-length of propodus, with row of sparse short setae on extensor margin and 3 long and few short setae on flexor margin; propodus faintly curved, almost glabrous except for 3 long setae on distal 0.3 of flexor margin; carpus and glabrous, merus nearly straight. P5 ( Figs. 5A View FIGURE 5 , 6M, N View FIGURE 6 ) shortest; dactylus distinctly longer than dactyli of P2–4, arcuate, 0.7 times as long as propodus, approximately 4 times as long as wide; extensor margin and upper and lower surfaces glabrous, but flexor margin with row of well-spaced, moderately long setae over entire length; propodus almost glabrous except for several setae on flexor margin; carpus glabrous, nearly straight; merus subcylindrical, straight, also glabrous.

Thoracic sternum ( Fig. 5C View FIGURE 5 ) broad. Sternites 3 and 4 fused, only slit-like incision still discernible in broad concavity at base of maxilliped 3; anteromedian part produced into broadly rounded lobe, anterolateral angle also produced to rounded lobe; lateral margins concave, accommodating coxae of chelipeds; anterior surface shallowly sulcate. Exposed parts of sternites 5–7 nearly smooth; sternite 5 posterolaterally with distinct episternite; sternites 6 and 7 each with shallow notch on lateral margin; sternite 8 clearly visible in lateral part, with concave lateral margin. Sterno-pleonal cavity deep, defined by sharply delimited margin, narrowing anteriorly, reaching anteriorly to midlength of fused sternites 3/4; rounded press button present on sternite 5 inside of cavity.

Pleon ( Fig. 5D View FIGURE 5 ) moderately narrow, tapering distally from pleomere 4 to telson. Pleomeres 1 and 2 as wide, longitudinally very short. Pleomere 3 oblong. Pleomeres 4–6 all subtrapezoidal. Telson semicircular, 1.7 times as wide as long.

G1 ( Fig. 5E, F View FIGURE 5 ) noticeably curved at midlength, gradually tapering, with row of sparse short setae laterally; distal part subtruncate, tip subacutely pointed. G2 ( Fig. 5G, H View FIGURE 5 ) endopod somewhat flattened, tapering distally; exopod about half-length of endopod, not tapering, terminal margin rounded.

Color in life. Unavailable.

Size. Largest male in post-hard stage 2.2× 2.6 mm; ovigerous females 4.6×5.9–5.4× 6.7 mm; largest female 7.2× 9.2 mm.

Distribution. Presently known from Fiji and Peninsular Malaysia; intertidal; associated with Gafrarium pectinatum , G. tumidum and G. dispar .

Remarks. The identity of Pinnotheres obesus (as Pinnothera [sic] obesa ) has long been uncertain and this has been compounded by the fact that the type material of Dana (1852) is no longer extant (see Evans 1967; Schmitt et al. 1973: 60; Deiss & Manning 1981; Manning 1993 a; Manning & Reed 2006). In establishing Pinnotheres obesus, Dana (1852: 380) noted that he had at least one male and two females (on the basis of the captions for his plate) obtained from an unknown host or hosts from Fiji. His description, however, is too brief by modern standards, and details of the ambulatory leg structure and proportions, which are specifically diagnostic in pinnotherids, were not stated. Dana (1852) noted that P. obesa was close to Pinnotheres globosus Hombron & Jacquinot, 1846 , but he did not elaborate in which aspects the two taxa were similar. Dana (1852: pl. 24 fig. 3a) provided an overall rather schematic figure of a specimen he believed was a male ( Fig. 1A View FIGURE 1 ), as well as its frontal region and pleon ( Dana 1852: pl. 24 fig. 3h; present Fig. 1D, H View FIGURE 1 ). Figures of the carapace of two female specimens ( Dana 1852: pl. 24 fig. 3b, c; present Fig. 1B, C View FIGURE 1 ) and their maxilliped 3 ( Dana 1852: pl. 24 fig. 3e, f; present Fig. 1E, F View FIGURE 1 ) were also included. A chela was figured ( Dana 1852: pl. 24 fig. 3i; present Fig. 1G View FIGURE 1 ) but it was not specified which specimen it was from. No type was designated. No host was mentioned and we are even unsure if the three specimens are conspecific. As noted earlier, all of Dana’s material is no longer extant, and as such, none of these uncertainties can now be resolved objectively.

The adult female specimens in our series generally match the figure of Dana (1852) in the carapace shape and the maxilliped 3 structure (in particular, the short dactylus not reaching to the propodal apex). On the other hand, the matured male specimen in our material is substantially different from the “male” illustrated by Dana (1852), particularly in the shape of the P2 and P3 dactyli (these dactyli illustrated by Dana are much more elongate and slender than in our male specimen). Nevertheless, such slender, elongate dactyli of the P2 and P 3 in males are not seen in any known species of Arcotheres . Considering the close similarities in females and the locality of the type material, we believe that the specimen regarded as a male by Dana (1852) was likely a young female (early post-hard stage) instead. Dana’s (1852) taxon, if the figures are taken at face value, can thus be assigned to Arcotheres on account of the P4 and P5 being longer than the P2 and P3, the P4 and P5 dactyli being distinctly longer than those of the P2 and P3, the asymmetrical P4, and the spatulate dactylus of the maxilliped 3. Considering the many inadequacies of Dana’s description and figures, it can even be argued that Pinnotheres obesus is actually conspecific with what is described here as Arcotheres ocularius n. sp., although the available evidence seems to suggest otherwise. Both species occur sympatrically in Fiji, albeit from different hosts. In any case, as Dana (1852) did not specify the host of Pinnotheres obesus , we are not even sure if all the material came from the same bivalve species. In the interests of nomenclatural stability and with consideration of the complex state of the pinnotherid taxonomy, we hereby designate a neotype (female of the stage V of the post-hard stage) for Arcotheres obesus n. comb. (CBM-ZC 15945). The designated neotype agrees with the original material relatively well. Their general carapace shape, structure of the chelipeds and legs are similar. The pleon of the specimen Dana illustrated as male (1855: pl. 24, fig. 3H; here reproduced as Fig. 1H View FIGURE 1 ) is of the same general shape and proportions of the present male specimen, although the telson of our male specimen appears relatively shorter ( Figs. 1H View FIGURE 1 versus Fig. 5D View FIGURE 5 ).

With regards to Dana’s (1852: 380) comment that Pinnotheres obesus was close to Pinnotheres globosus Hombron & Jacquinot, 1846 , he had noted that his species possessed a weaker tooth on the cutting edge of the dactylus of the chela and the form of the maxilliped 3 was different. The two species, however, are actually morphologically different. Pinnotheres globosus was described from Singapore very briefly (for precise authorship of this species, see Clark & Crosnier 2000; Holthuis 2002; Ng et al. 2008) and Dana (1852) had based his observations entirely on the very schematic and inaccurate figures in Hombron & Jacquinot (1846: figs. 22, 23). The identity of what is true P. globosus is being treated by one of the authors (PKLN) with S. T. Ahyong when a neotype will be designated. Suffice it to say, P. globosus is actually very different from P. obesus , not only being much larger in adult size but also with less asymmetrical P4 and occupation of fan shells of the family Pinnidae (see Ng & Corbari 2019). In view of this, we agree with Tesch (1918) that A. globosus and A. obesus are not synonyms, as was supposed by A. Milne-Edwards (1873: 318–319).

Pinnotheres obesus has been mentioned or listed by various authors (see synonymy) but few authors have actually examined specimens. The record of “ Pinnotheres obesus ” by Miers (1880: 314, pl. 4 fig. 4) from Malaysia was referred to Pinnotheres latissimus Bürger, 1895 , with doubt by Gordon (1936: 176) and Schmitt et al. (1973: 51) but we are now confident it is actually A. exiguus ( Bürger, 1895) instead (PKLN & S.T. Ahyong, unpublished data). This also applies to the record by Walker (1887: 111) from Malaysia. The records of “ P. obesus ” by Fulton & Grant (1906: 18) (relisted in Schmitt et al. 1973: 60; Davie 2002: 433) from southern Australia are Austrotheres holothuriensis (Baker, 1907) instead (see Ahyong 2018: 550). Hornell & Southwell (1909: 102) recorded “ P. obesus ” from two venerid clams, Meroe Schumacher, 1817 (present day Sunetta Link, 1807 ) and Cytheraea Lamarck, 1805 (present day Meretrix Lamarck, 1799 ), but given the host records, this is more likely what is today A. exiguus . Tesch (1918) recorded a small male specimen (3.1×3.0 mm) as “ P. obesus ” collected from an Arca shell ( Arcidae ) from Ceram in the Moluccas, Indonesia. This is certainly not A. obesus or any of the species of Arcotheres discussed here and the figured maxilliped 3 ( Tesch 1918: fig. 3a) has the dactylus very short and inserted on the propodus rather distally. The other records of “ A. obesus ” from other parts of the Indo-West Pacific (see Schmitt et al. 1973: 60) must all be rechecked to ascertain which species they actually belong to. Schmitt et al. (1973: 60) noted that “In a personal communication Dr. Isabella Gordon says that the only specimens of Pinnotheres obesus in the British Museum are from the Fiji Islands ”. Unfortunately, we were not able to examine this non-type material; they could either be A. obesus as defined here, or the new species (see below). Ng et al. (2008) retained Pinnotheres obesus in Pinnotheres Bosc, 1802 s. str., mainly because the detailed structure of the female ambulatory legs was not known. With the designation of the present neotype, this species clearly belongs to Arcotheres Manning, 1993 , as diagnosed by Campos & Manning (2000) (see also Ahyong & Ng 2007; Ng et al. 2017).

The carapace of A. obesus is superficially similar to A. rotundatus (known only from the holotype from the venerid Circe Schumacher, 1817 ). The maxilliped 3 dactylus of A. obesus is, however, relatively more slender and shorter, never reaching the tip of the propodus ( Fig. 3C View FIGURE 3 , E–H) (versus relatively stouter and longer, reaching beyond the tip of the propodus in A. rotundatus ; cf. Ahyong & Ng 2007: fig. 13I); and the female P4 is proportionately much longer ( Fig. 4L View FIGURE 4 ) (versus proportionately shorter in A. rotundatus ; cf. Ahyong & Ng 2007: fig. 13F).

The general carapace shape, maxilliped 3 and chelipeds of A. obesus are similar to members of the A. exiguus group ( A. exiguus ( Bürger, 1895) , A. winckworthi ( Gordon, 1936) and A. vicajii ( Chhapgar, 1957)) , but the female P2, P3 and P5 meri are proportionately more slender and longer ( Figs. 3A View FIGURE 3 , 4F, H, N View FIGURE 4 ) [versus P2, P3 and P5 meri proportionately shorter and stouter in the A. exiguus group; cf. Ahyong & Ng 2007: 4 A; Gordon, 1936: fig. 7a; Antony & Kuttyamma 1971: text-fig. 2(4); Trivedi et al. 2020: figs. 4A, B, D, E, F, H, 6C, D, F–K]; the longer female P4 has a dactylus that is proportionately shorter ( Fig. 4L View FIGURE 4 ) [versus longer female P4 dactylus proportionately longer in the A. exiguus group; cf. Gordon, 1936: fig. 7a; Antony & Kuttyamma 1971: text-fig. 2(4); Trivedi et al. 2020: figs. 4G, 6E] and the G1 is relatively stouter and gently curved, being a broad C-shaped with a subtruncate tip ( Fig. 5E, F View FIGURE 5 ) [versus G1 relatively more slender, distinctly C-shaped with a short subterminal projection in the A. exiguus group; cf. Antony & Kuttyamma 1971: text-fig. 1(5, 6); Trivedi et al. 2020: figs. 3D, 7H, I].

In addition to the good series of specimens of A. obesus from Fiji, we also examined two specimens from Peninsular Malaysia (ZRC 2000.2281) which are clearly referable to this species. Noteworthy is that both were collected from the reef clam Gafrarium dispar .