Arcotheres tivelae ( Gordon, 1936 )

Ng, Peter K. L., Clark, Paul F. & Naderloo, Reza, 2022, Redescription of Arcotheres tivelae (Gordon, 1936), a pea crab endemic to the Persian Gulf and Gulf of Oman (Crustacea: Decapoda: Brachyura: Pinnotheridae), Zootaxa 5141 (3), pp. 277-286 : 278-284

Arcotheres tivelae ( Gordon, 1936)

( Figs. 1–5 )

Pinnotheres tivelae Gordon, 1936: 167 , 174, fig. 4; Silas & Alagarswami 1967: 1211; Schmitt et al. 1973: 89; Saeedi & Ardalan 2010: 355.

Arcotheres tivelae — Ng et al. 2008: 248; Naderloo 2017: 421, figs. 38.1, 38.2, 38.5a, 38.6; De Gier & Becker 2020: 21; Ng & Ahyong 2022: 143 .

Arcotheres placunae —Naderloo & Türkay 2012: 54; Naderloo et al. 2013: 450, tab. 1; Ng & Ahyong 2022: tab. 1. (Not Pinnoteres placunae Hornell & Southwell, 1909 ).

Type material examined. Holotype: female (13.6 × 11.5 mm) ( NHM 1936.6.19.9), from intertidal clam, Tivela ponderosa (Knock, in Philippi, 1844) ( Veneridae ), Muscat, Oman, present by R. Winckworth . Paratypes: 1 female with damaged carapace, about same size as holotype, 1 female (9.0 × 7.8 mm) ( NHM 1936.6.19.10–11), same data as holotype.

Non-type material examined: Iran: 1 male, 1 female ( ZRC 2006.51), Hormozgan Province, Persian Gulf, coll. H. H. Sahafi, 26 March 2004; 1 male, 1 ovigerous female, 6 females (largest 11.2 × 9.8 mm) ( ZRC 2009.831), in Callista umbonella (Lamarck, 1818) (Veneridae) , Bandar-Abbas, Persian Gulf, coll. H. Saeedi, January 2009; 1 female (10.1 × 9.1 mm) ( ZRC 2016.162), Qeshm Island, coll. M. Safaei, May 2010; 2 males (5.7 × 5.6 mm, 5.6 × 5.6 mm), 2 ovigerous females (8.6 × 7.4 mm, 10.7 × 9.7 mm) ( ZRC 2009.155), Gulf of Oman, coll. E. Kamrani, 2011; 1 male, 1 female ( SMF 38541), Golshahr, Bandar-Abbas, Persian Gulf, coll. M. Ebrahimi, 19 April 2004; 1 male, 17 females ( SMF 38542), Kolahi, Strait of Hormoz, 2702′N, 5651′E, coll. R. Naderloo, A. Kazemi & A. Keykhosravi, 24 April 2008; 3 males, 13 females ( SMF 38543), Kolahi, Strait of Hormoz, 2702′N, 5651′E, coll. R. Naderloo, A. Kazemi & A. Keykhosravi, 24 April 2008; 5 females (2 ovigerous, 2 juveniles) ( ZUTC 5931), east of Bandar-Abbas, Persian Gulf, 2711′N, 5621′E, muddy-sand flat, coll. R. Naderloo, A. Kazemi & A. Keykhosravi, 23 May 2008; 4 ovigerous females ( ZUTC 5204), Tiab, Strait of Hormoz, 2702′50.4′′N, 5161′03.1′′E, coll. 25 January 2008; 4 females ( ZUTC 6923), Kolahi, Strait of Hormoz, 2702′51.8′′N, 5151′04.6′′E, coll. H. Saelhi & A. Kazemi, 29 November 2008; 5 females ( ZUTC 6924), Tis, Chabahar, Gulf of Oman, 2525′23.33′′N, 6035′15.59′′E, coll. H. Salehi & A. Kazemi, 29 November 2008. Kuwait: 1 female ( ZRC 2021.0478), station SDG 54, unknown host, Al-Nuweeseb, coll. S. De Grave, 4 March 2014; 1 female ( ZRC 2021.0479), station SDG 59, unknown host, Ras Kadma, Kuwait, S. De Grave, 17 November 2014.

Description. Female: Carapace and pereopods weakly chitinised, relatively soft. Carapace subcircular, wider than long; dorsal and lateral surfaces smooth, glabrous; domed in frontal view; front slightly projecting anteriorly beyond orbits, margin gently convex to straight; anterolateral margin subparallel with frontal margin or gently sloping posteriorly, forming rounded angle with posterolateral margin ( Fig. 1A, D ). Posterior carapace margin gently convex ( Fig. 1A, D ). Eyes small, not or just visible in dorsal view, mobile, completely filling orbit ( Fig. 1A, D, E ). Epistome with median part wide, triangular, lateral margins concave ( Fig. 1E ).

MXP3 outer surface with scattered short setae; ischiomerus completely fused, sub-rhomboidal, ca. 2.2 times as long as wide, inner margin rounded at widest point; carpus short; propodus about 2 times as long as high, subspatulate, longer than carpus, tip rounded; dactylus slender, inserted slightly proximal to mid-length of propodus, tip not reaching propodal apex; exopod relatively stout, about half length of ischiomerus, flagellum 2 articles ( Figs. 1G , 2A ).

Adult chela relatively elongate, dactylus about half palm length; palm relatively slender, proximally narrower than distally; outer surfaces of palm, fingers (except for distal part) almost glabrous, with only scattered short setae; ventral margin of palm gently convex; dactylus occlusal margin with large subproximal tooth; pollex occlusal margin with 1 lower proximal tooth, 1 submedian tooth and minute denticles; tips of fingers sharp, hooked ( Fig. 1C, H ).

P2–P5 dorsally, ventrally unarmed; outer surface covered with scattered, short setae or glabrous; ventral margins of propodus and dactylus setose; merus relatively longer, more slender, relative lengths of meri P4>P3>P5>P2; usually right P4 distinctly longest; P4 and P5 propodi lined with setae along flexor margin; P2 and P3 dactyli short, subequal, tip distinctly hooked, half to one-third propodus length; longer P4 dactylus elongate, broadly falciform, distinctly longer than half propodus length, slightly shorter than P5 dactylus; shorter P4 dactylus almost twice length of P3 dactylus; longer female P4 merus ca. 1.7 times longer than P5 merus; P5 merus 4.4–4.7 times longer than wide; P5 dactylus shorter than propodus extensor margin, dorsodistal half glabrous, setae denser on ventral margin, distoflexor margin without rows of spinules ( Figs. 1I , 2B–I ).

Pleon extending to buccal region, covering bases of P2–P5; telson recessed into concave distal margin of somite 6 ( Fig. 1C, F ).

Male: Carapace and pereiopods well chitinised, firm. Carapace almost circular, slightly wider than long or subequal; dorsal surface almost smooth, gently inflated, lateral surfaces with scattered setae; front projecting anteriorly, margin gently sinuous to almost straight ( Fig. 3A, B ). Eyes distinctly visible in dorsal view ( Fig. 3A, B ).

MXP3 as in female ( Figs. 3C , 4A ). Anterior thoracic sternum wide, sternites 1, 2 fused, partially sunken into buccal cavity; suture between sternites 2 and 3 shallow; sternites 3, 4 immovably fused, demarcated only by shallow groove.

Chela relatively stout, proportionally shorter than in female ( Fig. 3F ).

P2–P5 dorsally, ventrally unarmed; outer surface covered with short setae; P3 and P4 carpus and propodus with long natatory setae; left and right meri equal, relative lengths of meri P4>P3>P2>P5; dactyli of P2–P4 progressively longer; P5 dactylus shorter than those of P3 and P4, covered with setae ( Fig. 4B–E ).

Pleon triangular, widest at somite 3, lateral margins of somite 4 gently concave to almost straight; somite 6 trapezoidal; telson semicircular, wider than long ( Fig. 3E ). G1 relatively stout, arcuate, curved outwards, without subdistal dorsal projection, distal part tapering to rounded tip ( Figs. 3H, I , 4F–I ); distal openings of ejaculatory canal oriented slightly towards lateral sides of body. G2 short, with spatuliform tip; exopod slightly longer than endopod length ( Figs. 3J , 4J ).

Variation. The male specimens do not show any obvious leg asymmetry. The MXP3 shows slight variations in the length of dactylus in females, with small specimens (CL<4 mm) usually possessing a relatively shorter dactylus, hardly reaching to the distal two-thirds length of the propodus.

Colour in life. Females: relatively translucent, with large orange gonads internally ( Fig. 5B–D ; see also Naderloo 2017: fig. 38.1b). Males: pale, nearly white to ivory, well calcified, not translucent.

Remarks. The poorly described Pinnoteres (sic) placunae by Hornell & Southwell (1909) from Placuna (Placunidae) in Gujarat state in India and Pakistan has caused some uncertainty about its identity and, in addition the types are lost (cf. Schmitt et al. 1970). Similarities and suggestions that Pinnotheres tivelae Gordon, 1936 , and P. placunae are synonyms (Naderloo & Türkay 2012; Naderloo et al. 2013) may simply be due to the inaccuracies in the descriptions and figures of Hornell & Southwell (1909). Naderloo (2017) later recognised the species of Gordon as a valid Arcotheres , but noted that no males of A. tivelae were available for more detailed comparisons.

Trivedi et al. (2018) clarified the taxonomy of A. placunae and corrected the inaccuracies in the figures and description of Hornell & Southwell (1909), confirming that the types are no longer extant and a neotype was selected from the original type locality in the Rann of Kutch (at present Gulf of Kachchh) in Gujarat state in India. Trivedi et al. (2018: 57) elaborated on the differences between the females of the two species, noting that in A. tivelae , the carapace is more subcircular, at 1.2 times wider than long (vs more transversely ovate and 1.4 times wider than long in A. placunae ; Trivedi et al. 2018: figs. 1A, C, F, 2A); the posterior carapace margin is usually gently convex (vs strongly concave in A. placunae ; Trivedi et al. 2018: figs. 1A, C, F, 2A); the MXP3 ischiomerus is 2.2 times as long as wide (vs 2.6 times as long as wide in A. placunae ; Trivedi et al. 2018: figs. 2F, G); the female dactylus is about half the length of the cheliped palm in adults (vs dactylus slightly shorter than the palm in A. placunae ; Trivedi et al. 2018: figs. 2D, E); the longer female P4 merus is 1.7 times longer than the P5 merus (vs 1.3 times in A. placunae ; Trivedi et al. 2018: fig. 4C, D, G, H); the shorter P4 dactylus is almost twice the length of the P3 dactylus (vs 1.2 times in A. placunae ; Trivedi et al. 2018: fig. 4B, C, F, G); the P5 dactylus is about 0.75 times longer than the shorter P4 dactylus (vs 0.84 times longer in A. placunae ; Trivedi et al. 2018: fig. 4C, D, G, H); the dorsodistal half of the P5 dactylus is glabrous (vs covered with short setae in A. placunae ; Trivedi et al. 2018: fig. 4D, H); the P4 and P5 propodi are lined with setae along the flexor margin (vs almost glabrous in A. placunae ; Trivedi et al. 2018: fig. 4A–H); and there are no spinules on the P5 flexor margin (vs with 2 short rows of spinules present on the flexor margin; Trivedi et al. 2018: fig. 4H’, L’). The present female specimens of A. tivelae confirm these differences (cf. Figs. 1A, D, G–I , 2A, D, E, H, I ).

The males of A. tivelae and A. placunae are also distinct. The male carapace of A. tivelae is more circular in shape with the anterolateral margins strongly convex and the dorsal surface inflated; Fig. 3A, B (vs less circular in shape with the dorsal surface less convex and the anterolateral margins gently convex in A. placunae ; Trivedi et al. 2018: figs. 1D, 3A), and the G1 has the distal part bent obliquely inwards towards the sternum; Figs. 3H, I , 4F–I (vs G1 gently curved inwards with a slender tapering distal part in A. placunae ; Ng & Ngo 2022: fig. 9F, G). As noted by Ng & Ngo (2022), the figures of the male pleon and G1 tip of A. placunae are somewhat inaccurate in Trivedi et al. (2018: fig. 3B, D).

Despite previous comparisons with A. placunae , the morphology of A. tivelae is actually most similar to A. exiguus ( Bürger, 1895) and A. rayi Ahyong & Ng, 2007 , two species that also inhabit venerid bivalves. Ng & Ahyong (2022) revised the taxonomy of these two species, showing that Pinnotheres winckworthi Gordon, 1936 , P. vicajii Chhapgar, 1957 , P. casta Antony & Kutyamma, 1971 , and P. obscuridentata Dai & Song, 1986 , were all junior synonyms of A. exiguus (see also Trivedi et al. 2020). Arcotheres exiguus has a wide distribution in the Indo-West Pacific, occurring from the western Indian Ocean to southern China, while A. rayi is known thus far only from the Philippines and Peninsular Malaysia ( Ng & Ahyong 2022). Females of A. tivelae can be distinguished from those of A. exiguus and A. rayi by the P4 dactylus being slightly shorter than that of P5 (vs slightly or much longer in A. exiguus and A. rayi ). The carapace of female A. exiguus and A. rayi (e.g., Ng & Ahyong 2022: figs. 37A, C, 39A, D, 46A, 47A, C) is generally wider than that of A. tivelae ( Fig. 1A, D ); but as Ng & Ahyong (2022: 5) argued, carapace shape is not always reliable, and there are some specimens of A. exiguus that also have a more rounded carapace (e.g., Ng & Ahyong 2022: fig. 38A, B). The carapace of A. tivelae is also more inflated and firmer than that of A. exiguus and A. rayi , which are flatter and more membranous; unfortunately, these aspects are difficult to quantify and best observed by direct comparison of specimens of each species. The inner margin of the ischiomerus of the MXP3 at the widest point is always rounded in all the specimens of A. tivelae examined ( Figs. 2A , 4A ) but in A. exiguus , it is usually angular ( Ng & Ahyong 2022: figs. 41B, 42A, 45A) but this character is unreliable as in some specimens, it is also rounded ( Ng & Ahyong 2022: figs. 41Q, 42L). In A. rayi , the angle of the inner angle of the ischiomerus is rounded ( Ng & Ahyong 2022: figs. 49B, 52A); but it can easily be separated from A. tivelae by its proportionately shorter P5 dactylus ( Ng & Ahyong 2022: figs. 48D, E, 49I, M). The most unambiguous character that separates the species is actually the form of the G1. Compared to A. exiguus and A. rayi , the G1 of A. tivelae is relatively less curved with the distal part gently tapering and without a subdistal projection; Figs. 3H, I , 4F–I (vs distinctly C-shaped in A. exiguus with a dorsal subdistal projection and the tip is rounded; Ng & Ahyong 2022: figs. 44K, L, 45H, I, 52N, O). Arcotheres tivelae is also superficially similar to another congener also found in venerid bivalves, A. obesus ( Dana, 1852) , which is known only from Gafrarium from Fiji and Malaysia ( Ng & Ahyong 2022). The carapace of A. obesus , however, is soft and poorly chitinised, the dactylus of MXP3 is relatively longer, reaching the tip of the propodus, and P2–P5 are proportionately much longer; and in addition, the G1 has a low dorsal subdistal subtruncate angle before the sharp tip (cf. Komai et al. 2020: figs. 2A–C, 3E–H, 4F, H, J, L, N; Ng & Ahyong 2022: figs. 55A, B, 56B–I, K, L).

Host. Arcotheres tivelae was described from the venerid clam Tivela ponderosa (Koch, in Philippi, 1844); currently Tivela stefaninii (Nardini, 1933) . Recent specimens from Iran are all from the venerid clam Callista umbonella (Lamarck, 1818) . Saeedi & Ardalan (2010: 657–658) studied the ecology of A. tivelae and observed that the infestation rate was at about 9% for the Callista specimens examined, and of the 89 specimens found, 10% were males. The authors also suggested that the crab had negative growth impacts on the clam ( Saeedi & Ardalan 2010: 659). The bivalve is mostly found in the muddy–sand flats along the Persian Gulf and Gulf of Oman.

Figures

FIGURE 1. Arcotheres tivelae (Gordon, 1936). A–C, holotype female (13.6 × 11.5 mm) (NHM 1936.6.19.9), Muscat; D–F, G, I, A–D, F, female (11.2 × 9.8 mm) (ZRC 2009.0831), Iran; H, female (10.1 × 9.1 mm) (ZRC 2016.162), Iran. A, D, overall dorsal view; B, E, frontal view of cephalothorax; C, F, ventral view of cephalothorax; G, right third maxilliped; H, right chela; I, right P5 (ventral view). Scales: 1.0 mm.

FIGURE 2. Arcotheres tivelae (Gordon, 1936), female (11.2 × 9.8 mm) (ZRC 2009.0831), Iran.A, right MXP3 (denuded); B–E, right P2–P5, respectively; F–I, left P2–P5, respectively. Scales: A = 0.5 mm; B–I = 1.0 mm.

FIGURE 3. Arcotheres tivelae (Gordon, 1936), male (5.7 × 5.6 mm) (ZRC 2011.0155), Iran. A, overall dorsal view; B, dorsal view of carapace; C, left third maxilliped; D, frontal view of cephalothorax; E, pleon; F, left chela; G, left P5 (ventral view); H, right G1 (ventral view); I, right G1 (dorsal view).

FIGURE 4. Arcotheres tivelae (Gordon, 1936), male (5.7 × 5.6 mm) (ZRC 2011.0155), Iran.A, left MXP3 (denuded); B–E, left P2–P5, respectively; F, right G1 (ventral view); G, right G1 (dorsal view); H, distal part of right G1 (denuded, ventral view); I, distal part of right G1 (denuded, dorsal view); J, left G2. Scales: A, F, G, J = 0.2 mm; B–E = 0.5 mm; H, I = 0.1 mm.

FIGURE 5. Colour in life. Arcotheres tivelae (Gordon, 1936).A, host clam, Callista umbonella; B, female (part of ZUTC 6924); C, D, female (8.4 × 7.7 mm) (ZUTC 6923). B, in situ in Callista umbonella; C, overall dorsal view; D, overall ventral view. All from Iran.