Arcotheres pernicola ( Bürger, 1895 )

Trivedi, Jigneshkumar N., Gosavi, Swapnil, Vachhrajani, Kauresh D. & Ng, Peter K. L., 2019, Redescription of Arcotheres pernicola (Bürger, 1895) (Crustacea: Decapoda Brachyura: Pinnotheridae) from the oyster Magallana gryphoides (Schlotheim 1820) in India, Zootaxa 4706 (4), pp. 587-593 : 588-590

Arcotheres pernicola ( Bürger, 1895)

( Figs. 1–3 )

Pinnotheres pernicola Bürger 1895: 375 , 376, pl. 9 fig. 17, pl. 10 fig. 16; Nobili 1899: 264 (discussion); Laurie 1915: 415 (list);

Schmitt et al. 1973: 66 (discussion). Pinnoteres pernicola — Nobili 1906: 303 , 304. Pinnotheres pernicolus — Tesch 1918: 250 , 254, 287 (list, key); Silas & Algarswami 1967: 1206, 1216, 1222 (list) Pinnoteres pernicolus — Guinot 1967: 279 (list) Arcotheres pernicola — Ahyong & Ng 2007: 202 , 203, fig. 10; Ng et al. 2008: 248 (list); Ng et al. 2017: 1094 (discussion);

Trivedi et al. 2018a: 198 (discussion).

Material examined. 8 females (CW 4.1 mm, CL 3.1 mm to CW 7.3 mm, CL 5.9 mm) (ZL-AR-CR-128), in Magallana gryphoides (Schlotheim, 1820) (Ostreidae) , from Alibaugh, Maharashtra, India, 18°38’11.5”N 72°51’52.2”E, coll. S. Gosavi, 8 November 2016. 1 damaged female (ca. CW 11.7 mm, CL 8.5 mm) (NHM 1955.6.22.55), coast of Kenya, east Africa, coll. S. H. Copley, 1955.

Description. Female: carapace subelliptical ( Figs. 1 , 2A ), wider than long, regions poorly defined, antero-lateral margin bluntly rounded, front curved, posterior margin concave ( Figs. 1 , 2A ). Eyes small, pigmented, not visible in dorsal view; orbits ovate. Antennules folded obliquely, antennular fossa slightly larger than ocular cavities. Antennae with 6 articles; basal article with large elliptical tubercle, fused with epistome, distal segment setose, scarcely overreaching orbito-antennular fossae ( Fig. 2B ). margin of first sternite of thoracic sternum emarginated. MXP3 ( Fig. 2F ) obliquely placed in buccal cavity, outer surface with scattered setae on mesial margin, inner margin with long scattered setae; propodus about 1.7 times as long as wide, stout, longer than carpus, anterior margin rounded with long setae; digitiform dactylus 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, 1.7 times as long as wide, lateral margin convex with short scattered setae, mesial margin angular at the widest point forming antero mesial angle, inner margin almost straight with long slender setae. Exopod longer than half length of ischiomerus, flagellum with 2 articles, distally with fine setae ( Fig. 2F ).

P1 (cheliped) stout, merus slightly longer than carpus; chela ( Fig. 2D, E ) slightly globose, ventral margin gently convex, outer margin slightly curved, palm about 1.8 times as long as high; dactylus and pollex shorter than dorsal margin of palm, longer than greatest width of palm, palm relatively stout; dactylus cutting edge with one large tooth proximally, ending with sharp claw. Pollex inner surface with patch of long setae, outer surface smooth, cutting edge with 3 large proximal teeth separated by notch with denticulate margin ( Fig. 2D,E ).

P2–P5 subequal ( Fig. 3 A–H), those on right side longer than left side. P2, P3 ( Fig. 3 A–D) similar in shape; merus, carpus, propodus glabrous, propodus longer than carpus, slightly curved on both margins; dactylus conical, shorter than carpus and propodus, ventral margin with short fine setae, scattered setae on outer margin. P4 ( Fig. 3E, F ) markedly asymmetrical, distinctly longer than P2, P3, P5; articles (except dactylus) similar in shape to those of P2, P3. Longer P4 ( Fig. 3E ) more slender than P2, P3, carpus outer margin slightly curved; dactylus sickle-like, slightly shorter than propodus, ending in small curved tip, longer than dactyli of P2, P3, P5, ventral margin of P4 dactylus with long fine setae, few short setae on outer margin. P5 ( Fig. 3G, H ) more slender than P2–P4; articles (except dactylus) similar in shape to those of P2, P3; dactylus longer, sickle-like, much longer than those of P2, P3 but shorter than that of P4; ventral margin of P5 dactylus with long fine setae, 2 rows of scattered setae present on outer margin, short setae present on dorsal side in distal half, dorsal and ventral margins of tip with short setae; distal part with 2 rows of short, 6–9 just visible very short spinules ( Fig. 3I ). Relative length of ambulatory legs, P4>P3>P5>P2, and dactyli, P4>P5>P3>P2.

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

Host. Bürger (1895: 376) recorded the species were from “ Perna ”. Schmitt et al. (1973: 9, 66) noted that Bürger’s “ Perna ” is not the mytilid (Mytilida: Mytilidae ) but was actually Isognomon Lightfoot, 1786 , a pteriid oyster (Ostreida: Pteriidae ), instead. This was probably because at that time, there was confusion in the genus name (Bürger himself recorded other host mytilids as Mytilus ); with Bürger likely to be referring to Perna Bruguière, 1789 , which is now regarded as a junior synonym of Isognomon . True Perna Philipsson, 1788 , is a mytilid (Mytilida) while the modern understanding of Mytilus Linnaeus, 1758 , places it in the Ostreida (see Bouchet et al. 2010). As such, the species name of the pinnotherid, “ pernicola ” is rather unfortunate as it suggests it lives in Perna s. str., when it does not. Nobili (1899: 264) recorded the species from Katau in Papua New Guinea but did not indicate the host. Nobili (1906:303–304) later recorded this species from oysters from Djibouti. The present record from India is from the oyster Magallana gryphoides (Schlotheim, 1820) .

Remarks. Bürger (1895: 376) described Arcotheres pernicola from two ovigerous females measuring CW 7.5 mm, CL 5.5 mm and CW 8.75 mm, CL 6.5 mm. Ahyong & Ng (2007: 202) found only one extant specimen measuring CW 6.0 mm, CL 5.0 mm and designated it as the lectotype of the species. The discrepancy in measurements with the smaller specimen listed by Bürger and the lectotype can easily be explained by the soft and poor condition of the specimen. The present specimens differ from the redescription of A. pernicola given by Ahyong & Ng (2007) in several aspects: the frontal margin is distinctly convex ( Fig. 2A ) (versus curved gently; cf. Ahyong & Ng 2007: fig. 10F); the MXP3 propodus not tapering distally and dactylus does not reach the propodal apex ( Fig. 2F ) (versus propodus tapering distally and dactylus reaching propodal apex; cf. Ahyong & Ng 2007: fig. 10G); the cheliped dactylus is slightly longer than half the length of the palm ( Fig. 2D, E ) (versus dactylus is half the length of the palm cf. Ahyong & Ng 2007: fig. 10A) and the longer P4 dactylus is 1.13 times as long as that on the longer P5 ( Fig. 3E, G ) (versus dactylus of the longer P4 dactylus about twice as long as dactylus of longer P5; cf. Ahyong & Ng 2007: fig. 10E, F).The African specimen (NHM 955.6.22.55), although larger than the present series from India, agrees in all the key characters. It is hard to ascertain the significance of these differences because the lectotype female is in poor condition, and what is observed may merely be an artifact of preservation. As such, we prefer to treat the Indian Ocean material as A. pernicola for the time being. When fresh specimens of A. pernicola become available from or near the type locality in the Philippines, the material from the Indian and Pacific Oceans must be compared again to determine if they are all conspecific.

Following Trivedi et al. (2018a), A. pernicola ( Bürger, 1895) , belongs to a small subgroup of Arcotheres having the longer P4 dactylus longer than that of the P5. There are three other species in this subgroup: A. winckworthi ( Gordon, 1936) , A. rayi Ahyong & Ng, 2007 , and A. shahi Trivedi, Campos & Vachhrajani, 2018 (Campos 2001; Trivedi et al. 2018a). The morphological differences between these species are given in Table 1 .

Figures

FIGURE 1. Arcotheres pernicola (Bürger, 1895), female (CW 6.2 mm, CL 4.8 mm) (ZL-AR-CR-128). Colour in life, dorsal view.

FIGURE 2 Arcotheres pernicola (Bürger, 1895), female (CW 6.2 mm, CL 4.8 mm) (ZL-AR-CR-128).A, carapace; B, frontal view; C, pleon and telson; D, right chela, outer view; E, right chela, inner view; F, left MXP3, inner view.

FIGURE 3. Arcotheres pernicola (Bürger, 1895), A–H, female (CW 6.2 mm, CL 4.8 mm) (ZL-AR-CR-128); I, female (CW 7.3 mm, CL 5.5 mm) (ZL-AR-CR-128).A, C, E, G, right P2–P5 dorsal view; B, D, F, H, left P2–P5 dorsal view; I, right P5 dactylus tip (drawn at an angle, setae not drawn).