Arcotheres placunae ( Hornell & Southwell, 1909 )

Trivedi, Jigneshkumar N., Vachhrajani, Kauresh D. & Ng, Peter K. L., 2018, Redescription of Arcotheres placunae (Hornell & Southwell, 1909) (Crustacea: Decapoda: Brachyura: Pinnotheridae) from India and Pakistan, Zootaxa 4433 (1) : -

Arcotheres placunae ( Hornell & Southwell, 1909)

( Figs. 1–4 )

Pinnotheres placunae Hornell & Southwell 1909: 99 , 100, pl. 1, figs. 1–10; Tesch 1918: 250, 251 (list); Chhapgar 1957a: 503, 504, pl. 12, figs. a–k; Chhapgar 1957b: 41, 42, 66, 74; Chhapgar 1959: 37; Silas & Algarswami 1967: 1179, 1207 (discussion & list); Devi & Rao 1986: 455 –459 (discussion); Schmitt et al. 1973: 81 (discussion); Yeragi & Yeragi 1997: 189, fig. 1; Subba Rao & Sastry 2005: 52 (list).

Arcotheres placunae, Ahyong & Ng 2007: 191 , 205 (discussion); Ng et al. 2008: 264 (list); Pati et al. 2012: 384 (list); Ng et al. 2017: 1092, 1094 (discussion); Naderloo & Türkay 2012: 7, 55 (discussion).

Material examined. India: Neotype (here designated), female (CW 11.0 mm, CL 8.1 mm) ( ZRC 2017.1293 ) Balapur Bay (22°27′08″N, 69°08′09″E), muddy shore, in Placuna placenta Linnaeus, 1758 , Beyt Dwarka Island, Gulf of Kachchh, Gujarat, coll. J. Trivedi, 6 January 2015; 2 males (CW 6.4 mm, CL 5.8 mm; CW 6.8 mm, CL 5.9 mm) , 5 females (CW 6.5–10.6 mm, CL 5.4–7.7 mm) (ZL–AR–CR–100), same data as neotype ; 1 male (CW 7.0 mm, CL 5.8 mm) ( ZRC 2017.1065 ), same data as neotype . Pakistan: 10 females (CW 9.2–13.0 mm, CL 6.9–9.7 mm) ( ZRC 2017.1008 ), Korangi Creek , Karachi (24°45′48″N, 67°10′29″E), in Placuna placenta , coll. Q. Kazmi et al., 4 November 1998 – 25 October 1999.

Type locality. Balapur Bay (22°27′08″ N, 69°08′09″ E), Beyt Dwarka , Gulf of Kachchh (= Rann of Kutch), Gujarat, India.

Description. Female: carapace subhexagonal ( Figs.1A, C, F , 2A ), slightly wider than long, regions clearly defined with prominent shallow cervical and cardiac grooves, anterior margin flat, clearly defined, front not projecting but demarcated by emargination above each orbit, posterior margin concave. Eyes small, pigmented, not visible in dorsal view; orbits oval. Antennules folded obliquely ( Fig. 2B ), antennular fossa slightly larger than orbit; antennae with 6 articles, basal article with large round urinal article, fused with epistome, distal articles setose, reaching beyond inner edge of orbit. Epistome and endostome separated, margin of first sternite emarginated. MXP3 obliquely placed in buccal cavity, outer surface with few scattered setae on mesial margin ( Fig. 2F ), inner margin with long scattered setae ( Fig. 2G ); propodus about 2.2 times as long as high, stout, longer than carpus, dorsal margin with fringe of long setae, anterior margin rounded with long setae; dactylus digitiform distally setose, inserted medially in notch on ventral margin of propodus, distally not overreaching tip of propodus but overreaching anteromesial angle of merus; ischiomerus completely fused, 2.6 times as long as wide, lateral margin convex without setae, mesial margin slightly angular at the widest point forming antero mesial angle, inner margin with long setae. Exopod slightly longer than half length of ischiomerus, with shallow median furrow, flagellum with 2 articles before fine setae ( Fig. 2F, G ).

P1 (cheliped) stouter than P2–P5 ( Fig. 1A–D, F ), merus slightly longer than carpus, chela relatively slender ( Fig. 2D, E ), ventral margin straight, outer margin slightly curved, palm about 1.8 times as long as high, dactylus and pollex slightly shorter than dorsal margin of palm, longer than greatest height of palm, palm relatively stout, dactylus dorsal antero distal margin with patch of long setae, cutting edge with 1 large proximal tooth, ends in sharp claw, covered with setae. Pollex proximal margin with patch of long setae and distal margin with row of small setae present medially visible on inner surface ( Fig. 2E ), outer surface smooth ( Fig. 2D ), cutting edge with 2 large proximal teeth followed by undulating margin ending with sharp claw, covered with setae, ventral margin with long setae ( Fig. 2D, E ).

Ambulatory legs almost symmetrical in length except P4 ( Fig. 4A–H ); P2, P3 similar, merus, carpus glabrous, propodus longer than carpus with scattered setae on distodorsal and distoventral margin, dactylus shorter than carpus and propodus, hooked at tip, ventral margin covered with short setae, outer surface with scattered setae, P2 dactylus dorsal margin with few short setae; P4 markedly asymmetrical, longer P4 stouter than P2 and P3, carpus outer margin slightly curved, dactylus sickle-like, slightly smaller than propodus, ends in small curved tip, longer than dactyli of P2 and P3, dorsal, ventral and outer margin with short setae. P5 more slender than P2–P4; segments similar in shape than those of P2 and P3; except dactylus, longer than those of P2–P4, dorsal, ventral and outer margins with short setae, tip with row of small spines, ending in sharp curved spine ( Fig. 4 H'). Relative lengths of ambulatory legs P4>P3>P2>P5 and dactyli of longer ambulatory legs P5>P4>P3>P2.

Pleon wide, ovate ( Figs. 1B , 2C ), with 6 free somites and telson, covering bases of ambulatory legs; pleonal somites increasing in length distally, widening from somite 1–5, decreasing towards telson.

Male: carapace subhexagonal ( Figs. 1D , 3A ), wider than long, regions clearly defined with prominent shallow cervical and cardiac grooves, front broadly triangular, projecting anteriorly, emarginated, with flat anterior margin, eyes visible in dorsal view. MXP3 as in female. Chela slightly stouter than in female ( Figs. 1D, E , 3C ) margin slightly curved, ventral margin straight, outer margin covered with short scattered setae. Dactylus slightly shorter than dorsal margin of palm, few scattered setae present on distodorsal margin, outer surface with short scattered setae, ending in sharp claw, cutting edge denticulate with 1 large tooth proximally, covered with short setae. Pollex cutting edge with 2 large teeth separated by deep notch proximally followed by denticulate ridge ending before reaching distal margin, ending in sharp claw, ventral margin with fringe of short setae ending on mid-ventral margin of palm.

P2–P5 ( Fig. 4I –L ) more slender than cheliped; merus with fringe of short setae on dorsal and ventral margins; carpus with strongly curved outer margin; propodus longer than carpus with curved outer margin; dactylus dorsal and ventral margins with short setae. P3 and P4 with fringe of long swimming setae on outer face of carpus and propodus increasing in size distally; tip of P5 dactylus with row of small spines ending in sharp spine ( Fig. 4 L'). Relative lengths of ambulatory legs P4>P3>P2>P5 and dactyli of longer ambulatory legs P5>P4>P3>P2.

Pleon ( Fig. 3B ) broadly triangular, with six free somites and telson, widest at somite 3, decreasing towards evenly rounded telson, lateral and dorsal margins with short setae. G1( Fig. 3D ) long, curved, with short angular tip, shallow furrow running parallel to dorsal margin, dorsal and ventral surface with long setae; G2 stout with blunt tip, exopod elongate ( Fig. 3E, F ).

Distribution. In India, the species has been recorded from various localities: Gujarat ( Hornell & Southwell 1909; Subba Rao & Sastry 2005), Maharashtra ( Chhapgar 1957a, b; Yeragi & Yeragi 1997) and Andhra Pradesh ( Narasimham1984; Devi & Rao 1986). It is also present in Pakistan (present study). In describing A. placunae, Hornell & Southwell (1909) reported that they had on hand 31 male specimens and three female specimens from Balapur Bay in the Gulf of Kachchh. They, however, also noted that “ Placunae from Ceylon rarely contain this commensal. One large female was, however, taken by one of us some years ago from a large Placuna fished at Tampalakam Bay, proving the geographical range to extend from the Gulf of Kutch to Ceylon.” ( Hornell & Southwell 1909: 101). This record from Sri Lanka (= Ceylon) will need to be confirmed in any case with fresh collections. There is no indication, however, if this specimen was retained or preserved.

Remarks. Hornell & Southwell (1909: 100) described the female P4 and P5 dactyli of Arcotheres placunae as one and a half times longer than those of the P2 and P3, but this observation does not match with their figures of a female specimen, which show P2–P4 symmetrical and with all the dactyli subequal in length ( Hornell & Southwell 1909: pl. 1 figs. 3–10).

Chhapgar (1957a) also recorded A. placunae from Mumbai, Maharashtra state, India. The figure of the female specimen in Chhapgar (1957a) is imprecise in many ways and also does not match his description. His figure (see Chhapgar 1957a: pl. 12 fig. a) shows the right P2–P4 as longer than those on the left but based on the measurements of P2–P 4 in his description, the left P2–P4 should be longer than those on the right. Chhapgar (1957a) also described the P4 and P5 dactyli being one and a half times longer than those of the P2 and P3, but his figure ( Chhapgar 1957a: pl. 12 fig. a) shows the left P5 dactylus to be shorter than those on the left P2–P4; while on the right side, the P3 dactylus is longer than those of P2, P4 and P5 ( Chhapgar 1957a:pl. 12 fig. a).

The specimens examined in the present study generally agree with the description and figures of the female carapace, male pleon and some pereiopods provided by Hornell & Southwell (1909). Hornell & Southwell (1909) mentioned in the description that the dactyli of the female P4 and P5 are 1.5 times longer than those of P2 and P3 but the artist must have simplified the drawings by making all the dactyli of P2–P5 equal in length. The dactyli of P2–P5 of the female specimens examined in the present study, however, match the description provided by Hornell & Southwell (1909). Arcotheres placunae closely resembles species such as A. latifrons ( Bürger 1895) and A. rayi Ahyong & Ng, 2007 (both from Bohol, Philippines) in having a subtrapezoidal carapace, a flat anterior carapace margin and having the MXP3 dactylus not reaching the apex of the propodus.

Arcotheres placunae differs from A. latifrons in the following features: 1) the female carapace is 1.4 times wider than long ( Figs. 1A, C, F , 2A ) (vs. carapace 1.1 time wider than long in A. latifrons , cf. Ahyong & Ng 2007: fig. 5A); 2) the eyes are present but not visible in dorsal view ( Figs. 1A, C, F , 2A ) (vs. eyes absent in A. latifrons , cf. Ahyong & Ng 2007: fig. 5A); 3) the carapace posterior margin is strongly concave ( Figs. 1A, C, F , 2A ) (vs. moderately concave in A. latifrons , cf. Ahyong & Ng 2007: fig. 5A); 4) the antennae reach beyond the inner edge of the orbit ( Fig. 2B ) (vs. do not reach the inner edge of the orbit in A. latifrons , cf. Ahyong & Ng 2007: fig. 5B); 5) the MXP3 propodus is 2.2 times as long as high ( Fig. 2F, G ) (vs. 3 times as long as high in A. latifrons , cf. Ahyong & Ng 2007: fig. 5D); 6) the anterior margin of the MXP3 propodus is rounded with long setae ( Fig. 2F, G ) (vs. less rounded with short setae in A. latifrons , cf. Ahyong & Ng 2007: fig. 5D); 7) the MXP3 ischiomerus is 2.6 times as long as wide ( Fig. 2F, G ) (vs. less than 2 times as long as wide in A. latifrons , cf. Ahyong & Ng 2007: fig. 5D); 8) the mesial margin of the MXP3 ischiomerus is slightly angular at its widest point ( Fig. 2F, G ) (vs. distinctly angular in A. latifrons , cf. Ahyong & Ng 2007: fig. 5D); 9) the inner and outer margins of the MXP3 exopod are almost straight ( Fig. 2F ) (vs. inner and outer margin convex in A. latifrons , cf. Ahyong & Ng 2007: fig. 5D); 10) the cheliped dactylus is slightly shorter than the dorsal length of the palm ( Fig. 2D, E ) (vs. dactylus about half palm length in A. latifrons , cf. Ahyong & Ng 2007: fig. 5C); 11) the P2 and P3 dactyli have short setae on the dorsal, ventral and lateral margins ( Fig. 4A, B, E, F ) (vs. dactyli glabrous in A. latifrons , cf. Ahyong & Ng 2007: fig. 5A); 12) the P4 dactylus has short setae on the dorsal, ventral and lateral margins ( Fig. 4C, G ) (vs. few setae on distoventral margin in A. latifrons , cf. Ahyong & Ng 2007: fig. 5A); and 13) the P5 dactylus has short setae on the dorsal, ventral and lateral margins ( Fig. 4D, H ) (vs. ventral margin with short setae in A. latifrons , cf. Ahyong & Ng 2007: fig. 5A). Males of A. latifrons are not known.

Arcotheres placunae differs from A. rayi in following features: 1) the female carapace is 1.4 times wider than long ( Figs. 1A, C, F , 2A ) (vs. carapace 1.3 times wider than long in A. rayi , cf. Ahyong & Ng 2007: fig. 11A); 2) the carapace posterior margin is strongly concave ( Figs. 1A, C, F , 2A ) (vs. slightly concave in A. rayi , cf. Ahyong & Ng 2007: fig. 11A); 3) the antennae clearly reach beyond the inner edge of the orbit ( Fig. 2B ) (vs. slightly reaching beyond the inner edge of the orbit in A. rayi , cf. Ahyong & Ng 2007: fig. 11B); 4) the MXP3 ishchiomerus is 2.6 times as long as wide ( Fig. 2F, G ) (vs. about 2 times as long as wide in A. rayi , cf. Ahyong & Ng 2007: fig. 11E); 5) the MXP3 exopod has the inner and outer margins almost straight with a clear median shallow furrow ( Fig. 2F ) (vs. inner and outer margin convex without median furrow in A. rayi , cf. Ahyong & Ng 2007: fig. 11E); 6) the palm is about 1.8 times as long as high ( Fig. 2D, E ) (vs. about 3 times as long as high in A. rayi , cf. Ahyong & Ng 2007: fig. 11D); 7) the cheliped dactylus is slightly shorter than the dorsal length of the palm with a tuft of short setae on the distodorsal margin ( Fig. 2D, E ) (vs. dactylus about half palm length with the distodorsal margin glabrous in A. rayi , cf. Ahyong & Ng 2007: fig. 11D); 8) the ventral margin of the pollex has short setae ( Fig. 2D, E ) (vs. glabrous in A. rayi , cf. Ahyong & Ng 2007: fig. 11D); 9) the order of relative lengths of the ambulatory dactyli is P5>P4>P3>P2 ( Fig. 4A–H ) (vs. P4>P5>P3=P 2 in A. rayi , cf. Ahyong & Ng 2007: fig. 11A); 10) the fifth pleonal somite is the widest ( Fig. 2C ) (vs. fourth pleonal somite widest in A. rayi , cf. Ahyong & Ng 2007: fig. 11C); and 11) the lower margin of the telson is concave ( Fig. 2C ) (vs. convex in A. rayi , cf. Ahyong & Ng 2007: fig. 11C). Males of A. rayi are not known.

Comparing specimens of A. tivelae with A. placunae , the following differences are obvious: 1) the female carapace is 1.4 times wider than long ( Figs. 1A, C, F , 2A ) (vs.1.2 times wider than long in A. tivelae , cf. Gordon 1936: fig. 4a; Naderloo 2017: fig. 38.2); 2) the carapace posterior margin is strongly concave ( Figs. 1A, C, F , 2A ) (vs.slightly convex in A. tivelae , cf. Gordon 1936: fig. 4a; Naderloo 2017: fig. 38.2); 3) the MXP3 ischiomerus is 2.6 times as long as wide ( Fig. 2F, G ) (vs. 2.2 two times as long as wide in A. tivelae , cf. Gordon 1936: fig. 4b; Naderloo 2017: fig. 38.5); 4) the cheliped dactylus is slightly shorter than the dorsal length of the palm ( Fig. 2D, E ) (vs. dactylus about half palm length in A. tivelae , cf. Gordon 1936: fig. 4b); 5) the P4 merus is 1.3 times longer than that of P5 ( Fig. 4C, D, G, H ) (vs. P4 merus 1.7 times longer than that of P 5 in A. tivelae , cf. Gordon 1936: fig. 4a); 6) the P4 dactylus is 1.2 times longer than the P3 dactylus ( Fig. 4B, C, F, G ) (vs. P4 dactylus 2 times longer than P3 dactylus in A. tivelae , cf. Gordon 1936: fig. 4d, e); 7) the P5 dactylus is 0.84 times longer than the P4 dactylus ( Fig. 4C, D, G, H ) (vs. P5 dactylus 0.75 times longer than P4 dactylus in A. tivelae , cf. Gordon 1936: fig. 4e, f); 8) the P5 dactylus dorsodistal half is covered with short setae ( Fig. 4D, H ) (vs. glabrous in A. tivelae , cf. Gordon 1936: fig. 4f); and 9) the P2–P5 merus is glabrous ( Fig. 4A–H )(vs. P4 and P5 merus with setae on proximal margin in A. tivelae , cf. Gordon 1936: fig. 4a). Males of A. tivelae are not known. Considering the above mentioned morphological differences between A. placunae and A. tivelae and that the two species have different host (see Saeedi &Ardalan 2010; Naderloo 2017), there is no doubt about the distinctness of both species.

As discussed earlier, the types of A. placunae ( Hornell & Southwell, 1909) are almost certainly lost. In view of the uncertainty of what the species actually was until recently (see above) and the discrepancies in the descriptions and figures by the original authors, it is better to designate one of the recent specimens from the original type locality (Gulf of Kachchh) as the neotype of the species. This will not only stabilize the taxonomy of the species but also be useful in distinguishing other allied species in subsequent taxonomic studies of the Pinnotheridae . We here designate a female specimen (CW 11.0 mm, CL 8.1 mm; ZRC 2017.1293) ( Fig. 1C ) collected from Balapur bay in Beyt Dwarka Island, Gulf of Kachchh, Gujarat, India, as the neotype of Pinnotheres placunae Hornell & Southwell 1909 .

Hornell & Southwell’s (1909) material is unusual because it is composed almost entirely of males—in almost all studies; female specimens invariably outnumber male ones, with males often not even found. Even more unusual was his report that he found two Placuna shells that had 11 and 12 male specimens each—normally, bivalves only have single adult individuals or heterosexual pairs. It suggests that many of their specimens may perhaps be juveniles which may have just only recently metamorphosed in the bivalve hosts.

Figures

FIGURE 1. Arcotheres placunae (Hornell & Southwell, 1909). A, B, female, ZL-AR-CR-100 (CW 10.6 mm, CL 7.7 mm), India; C, neotype female, ZRC 2017.1293 (CW 11.0 mm, CL 8.1 mm), India; D, E, male, ZL-AR-CR-100 (CW 6.8 mm, CL 5.9 mm), India; F, female, ZRC 2017.1008 (CW 11.5 mm, CL 7.9 mm), Pakistan. A, C, D, F, dorsal view; B, E, ventral view.

FIGURE 2. Arcotheres placunae (Hornell & Southwell, 1909), female, ZL-AR-CR-100 (CW 10.6 mm, CL 7.7 mm), India. A, dorsal view; B, frontal view; C, pleon; D, right chela, outer view; E, right chela, inner view; F, left MXP3, inner view; G, left MXP3, outer view.

FIGURE 3. Arcotheres placunae (Hornell & Southwell, 1909). A–E, male, ZL-AR-CR-100 (CW 6.8 mm, CL 5.9 mm), India; F, male, ZRC 2017.1065 (CW 7.0 mm, CL 5.8 mm), India. A, dorsal view; B, pleon; C, right chela, outer view; D, G1; E, G2; F, left G2 with exopod (lateral view, setae denuded).

FIGURE 4. Arcotheres placunae (Hornell & Southwell, 1909). A–H, female, ZL-AR-CR-100 (CW 10.6 mm, CL 7.7 mm), India; I–L, male, ZL-AR-CR-100 (CW 6.8 mm, CL 5.9 mm), India. A–D, female left P2–P5 outer view; E–H, female right P2–P4 outer view; H', tip of dactylus of female right P5; I–L, male left P2–P5, outer view; L', tip of dactylus of male right P5.