A new species of Polyonyx (Crustacea, Anomura, Porcellanidae) inhabiting polychaete-worm tubes (Annelida, Chaetopteridae) in the Indo-West Pacific Author Werding, Bernd Author Hiller, Alexandra text ZooKeys 2019 818 25 34 http://dx.doi.org/10.3897/zookeys.818.30587 journal article http://dx.doi.org/10.3897/zookeys.818.30587 1313-2970-818-25 95F1F6EC005E4D648FD68730220ECE47 95F1F6EC005E4D648FD68730220ECE47 Polyonyx socialis sp. n. Figs 1, 2 a-g , 3 Material. Holotype: female, SMF 52400, South China Sea, south coast of Vietnam, Nhatrang Bay, Tre Island, Dam Bay, 6-8 m, silty sand, hand collection from tube of Chaetopterus sp. No 66, cohabiting with a pair of pinnotherid Tetrias sp., coll. Britayev and Martin, 15.04. 2016; 4.0 mm x 4.3 mm (Fig. 1). Paratypes: male-female pair cohabiting with a pair of Polyonyx heok , SMF -52401, South China Sea, south coast of Vietnam, Nhatrang Bay, Mun Island, 16-20 m, silty sand, hand collection from tube of Chaetopterus sp. No 4, coll. Britayev and Martin, 04. 2016; male 4.6 mm x 5.7 mm (Fig. 2), female (ov) 5.1 mm x 5.6 mm (Fig. 3), both with bopyrid infestation and therefore, largely deformed. Figure 1. Polyonyx socialis sp. n., female holotype, dorsal view, SMF 52400, South China Sea, south coast of Vietnam, Nhatrang Bay, Tre Island, Dam Bay, dorsal view. Scale bar: 2.0 mm. Description. Carapace (Fig. 1) round to subovate, 1.1 to 1.2 times wider than long, broadest at epibranchial level, moderately convex, surface smooth, shining, covered with distant, shallow transversal striae. Regions scarcely demarcated. Hepatic margin roundly produced, crested. Branchial margins evenly rounded, crested. Rostrum broad, transverse (holotype) or weakly trilobate (paratypes), median lobe forwardly directed, lateral lobes rounded. Orbits shallow, outer orbital angles rounded. Side walls entire, not visible from above. Third thoracic sternite (Fig. 2a) broad, anterior margin rounded, lateral lobes broad, forwardly directed. Telson composed of seven plates (Fig. 2b). Basal article of antennular peduncle unarmed. First antennal article broadly in contact with lower orbital margin, movable articles smooth, second elongate; flagellum long, reaching to tip of chelae. Third maxilliped (Fig. 2c) with broad ischium and rounded inner projection; merus with subrectangular rounded inner lobe. Exopod long, slender, overreaching middle of merus. Figure 2. Polyonyx socialis sp. n., male paratype, SMF 52401, South China Sea, south coast of Vietnam, Nhatrang Bay, Mun Island. a Third thoracic sternite b Telson c Right third maxilliped, dorsal view (setae omitted) d Left (larger) cheliped, dorsal and ventral views e First and second right walking legs, dorsal view f Detail of dorsal view of propodus and dactylus of right third walking leg. Scale bars: 0.5 mm ( a-c , f), 2.0 mm (d), 1 mm (e). Chelipeds (Figs 1, 2d) similar in both sexes, heterochaely not very pronounced. Merus with some transverse rugae on upper surface, with large, laminate, forwardly projected lobe; carpus swollen, with similar lobe that makes carpus appear nearly as broad as long; upper surface with some transversal rugae, proximal border concave, outer border with faint, scale-like rugae, and scattered, short setae; distal portion with tuft of simple setae. Manus compact, swollen above, outer border evenly curved outside. Fingers short, approximately 1/3 of total length of manus. Outer border with narrow fringe of densely set, very fine, simple setae. Fingers closing on entire length, movable finger with upper border of cutting edge with fringe of upwardly standing simple setae. Walking legs (Figs 2 e-f ) relatively short, merus flattened, ovate from above, 1.4 to 1.5 times longer than wide, surface with scattered, transversal ridges and scattered setae. Carpus and propodus elongate, of similar length, with scattered simple setae, propodus spineless except for terminal triplet. Dactylus elongate, terminating in curved, bifurcate claws, the upper one being smaller, inner margin with two or sometimes three smaller spines. Males with pair of pleopods on second abdominal segment. The overall coloration of Polyonyx socialis sp. n. (Fig. 3) is light brown with a symmetric pattern of white marks on the carapace. The chelipeds have whitish marks upon the articulation between merus and carpus, and at the level of the articulation with the dactylus. The walking legs have white areas on the proximal part of the merus, and around the articulations. Figure 3. Polyonyx socialis sp. n., female paratype, SMF 52401, South China Sea, south coast of Vietnam, Nhatrang Bay, Mun Island. Right side of carapace deformed by parasitic isopods ( Bopyridae ). Scale bar: 3.0 mm. Ecology. Polyonyx socialis sp. n. inhabits the tubes of a Chaetopterus sp. worm as heterosexual pairs, and shares its host with other symbionts, either a male-female pair of the porcellanid P. heok and the aeolid nudibranch Phestilla sp., or a male-female pair of the pinnotherid crab Tetrias sp. The hosts were collected between 6 and 20 m depth. Etymology. The specific name socialis, from the Latin, meaning social, refers to the sociable behaviour of the new species, as it tolerates and is tolerated by other symbionts inhabiting the same polychaete host. Remarks. The new species is morphologically similar and probably systematically close to P. utinomii Miyake, 1943 and P. boucheti Osawa, 2007, both of which also live in Chaetopterus tubes ( Osawa 2001 ). The new species is distinguished from P. utinomii and P. boucheti by 1) the narrower carapace, with the rostrum being less transverse, 2) the extremely extended and forwardly directed lobes on merus and carpus of the chelipeds, 3) the wide and flattened merus of the walking legs, and 4) the extremely fine and transparent fringes of setae on the chelipeds. Distribution. Currently known only from the Vietnamese coast of the South China Sea.