Research Article
Research Article
A new species of Polyonyx (Crustacea, Anomura, Porcellanidae) inhabiting polychaete-worm tubes (Annelida, Chaetopteridae) in the Indo-West Pacific
expand article infoBernd Werding, Alexandra Hiller§
‡ Justus-Liebig-Universität, Giessen, Germany
§ Smithsonian Tropical Research Institute, Panama, Panama
Open Access


Polyonyx socialis sp. n. from the South China Sea of Vietnam is described. The new species was collected in a previous study that compared the vertebrate and invertebrate symbiont communities living in the tubes of two syntopic species of the polychaete genus Chaetopterus. Polyonyx socialis sp. n. inhabits the tubes of the smaller polychaete species as a heterosexual pair, and frequently shares the cavity of the host’s tube with a larger porcellanid, P. heok, also present as a male-female pair, and with a species of trinchesiid nudibranch. Less frequently, the new species shares its host with a heterosexual pair of a larger species of pinnotherid crab. Polyonyx socialis sp. n. belongs to the P. sinensis group, a world-wide distributed morphological line within the heterogeneous genus Polyonyx. Most species in this group are obligate commensals of chaetopterid polychaetes. The crabs have a transversally cylindrical habitus, which enables them to move laterally along the worm tubes with ease. Polyonyx socialis sp. n. is a relatively small species that lives attached to the inner walls of the polychaete tube. The small size and flattened chelipeds and walking legs of the new species confers it an advantage to cohabiting the same worm tube with larger decapod species occupying most of the tube’s cavity.


adaptation, Chaetopterus , commensalism, polychaete-inhabiting


The porcellanid genus Polyonyx Stimpson is a diverse and heterogeneous taxon containing more than 30 species worldwide, most of which distributed in the Indo-West Pacific (IWP) (Johnson 1958; Haig 1960; Werding 2001; Osawa 2007; Osawa and McLaughlin 2010; Osawa and Ng 2016; Osawa et al. 2018; this study). Many species of this genus are known to live commensally with polychaete worms (Haig 1960, 1979; Ng and Sasekumar 1993; Werding 2001; Osawa et al. 2018; own observations). Johnson (1958) arranged the IWP species into three morphological groups, designated as Polyonyx biunguiculatus (Dana 1852), P. denticulatus Paul’son, 1875, and P. sinensis Stimpson, 1858. Nakasone and Miyake (1969) considered the P. denticulatus group as the new genus Aliaporcellana, which was later redefined by Haig (1978).

The largest of Johnson’s (1958) assemblages is the Polyonyx sinensis group, which is worldwide distributed, though most of the species in this group have an Indo-West Pacific (IWP) distribution. According to this author this group contains species with a “pronounced tendency towards commensalism”. Indeed, most species in this morphological line are commensal (Haig 1964), and have been reported to inhabit the tubes of tube-dwelling polychaetes, mainly of the family Chaetopteridae Audouin and Milne-Edwards, as heterosexual pairs (Pope 1946; Gray 1961; Haig 1965; Ng and Sasekumar 1993; Osawa 2001, 2007; Sanford 2006; Osawa and Poupin 2013; Britayev et al. 2017). In some cases commensalism seems to be facultative, as free-living individuals have been sporadically found in shallow waters under stones or in sand (Haig 1956, 1964; Werding 2001).

The Polyonyx biunguiculatus group contains six species, four distributed in the Central Pacific (Osawa 2015) and two in the Indian Ocean. The Central Pacific species, Polyonyx biunguiculatus, P. obesulus Miers 1884, P. similis Osawa 2015, and P. triunguiculatus Zehntner 1894, do not seem to have commensal relationships with other invertebrates, although they seem to prefer habitats characterized by corals and sponges (Haig 1964, 1979; Osawa 2007, 2015). The two Indian Ocean species, P. hendersoni Southwell 1909, and P. splendidus Sankolli 1963, have been rarely found, probably because they inhabit the ducts of sponges. Such life habit is reflected in the distinctive morphology of these two species (see Hiller et al. 2010), which made Werding (2001) consider them as conspecifics of a new genus. However, unpublished molecular data indicate that these species are aberrant forms of the P. biunguiculatus group.

In a recent study of symbionts of two syntopic species of chaetopterid polychaetes in the South China Sea of Vietnam, Britayev et al. (2017) found heterosexual pairs of a small porcellanid inhabiting one of this chaetopterid species. This porcellanid is an undescribed species of the Polyonyx sinensis group, which we here describe as Polyonyx socialis sp. n. The new species was reported by Britayev et al. (2017) to share its host either with P. heok Osawa & Ng, 2016, a rather large porcellanid, and a nudibranch species of the genus Phestilla Bergh, or with a male-female pair of a pinnotherid crab of the genus Tetrias Rathbun.

Materials and methods

Material of Polyonyx socialis sp. n. was provided by T Britayev (Severtzov Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow, Russian Federation) and D Martin (Department of Marine Ecology, Centre d’Estudis Avançats de Blanes, Blanes, Catalunya, Spain), and has been deposited in the Naturmuseum Senckenberg (SMF), Frankfurt a.M., Germany. Colour photographs were provided by T Britayev, and are included in the description. Measurements of carapace length and width (in mm) of type individuals follow collection information.


Family Porcellanidae

Polyonyx socialis sp. n.

Figs 1, 2a–g, 3


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.


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 2e–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.


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.


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.


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.


Currently known only from the Vietnamese coast of the South China Sea.


Polyonyx socialis sp. n. inhabits as a heterosexual pair the tubes of one of two syntopic species of Chaetopterus, which according to Britayev et al. (2017), may be a new undescribed species of polychaete from the Vietnamese South China Sea. These authors reported the new porcellanid frequently sharing the polychaete tube with a heterosexual pair of the significantly larger porcellanid P. heok and of the tergipedid nudibranch Phestilla sp. In one case Polyonyx socialis sp. n. shared its host with a male-female pair of the pinnotherid crab Tetrias sp. Interestingly, P. socialis sp. n. was not found in the tubes of the larger polychaete Chaetopterus cf. appendiculatus Grube, 1874, which is ecologically close to the porcellanid’s host. The inhabitants of the larger polychaete were either the porcellanid Eulenaios cometes (Walker, 1887) and the polynoid polychaete Ophthalmonoe pettiboneae Petersen & Britayev, 1997, or the carapid fish Onuxodon fowleri (Smith 1955). The presence of P. socialis sp. n. in the smaller and not the larger polychaete species may be explained by the crab’s host specificity, or by a lower tolerance of Chaetopterus cf. appendiculatus and its commensal inhabitants to sharing the space inside the tube.

The extremely broadened chelipeds and walking legs of P. socialis sp. n. are distinctive characters within Polyonyx, even when comparing the species with the morphologically closest P. boucheti and P. utinomii, and to all other tube-dwelling species of the genus. These characters are most likely adaptations to living tightly attached to the walls of the worm tube without being perceived as an obstacle for the larger crabs inhabiting the same tube. The new species is therefore morphologically adapted to cohabiting with a heterosexual pair of a congeneric larger crab, what is quite exceptional. The West Atlantic representative of the P. sinensis group, P. gibbesi Haig, 1956 inhabits the polychaete host Chaetopterus variopedatus (Renier 1804) as a male-female pair (Gray 1961), and very rarely shares its host with the pinnotherid crab Pinnixa chaetopterana Stimpson, 1860, which also inhabits the polychaete tube as a heterosexual pair (Grove and Woodin 1996). Studies in North Carolina and in the northern Gulf of Mexico (Pearse 1913, Gray 1961, Williams 1984, McDermott 2005) revealed that these two crab species do not coexist in one worm tube. In seldom cases, only juveniles of the two species were found in one tube. Apparently, the first to colonize the tube impedes the arrival of the other species. Once a heterosexual pair of either crab species has been established in the tube, it transitionally tolerates or completely rejects adults of the other species, which suggests that occupancy of the host by adult crabs involves intra- and interspecific competition (Sanford 2006). The cases where only one crab individual is present in the worm tube are probably transitional situations towards forming a male-female pair.

Polyonyx socialis sp. n. and P. heok comprise the first pair of porcellanid crabs observed to share the same host.


We thank T Britayev and D Martin for sending us the porcellanid material collected in Vietnam, and for providing us with collection data and the photograph of the described crab species. We thank M Robert for inking our drawings, and I Wehrtmann (Universidad de Costa Rica, San José) and E Macpherson (Centro de Estudios Avanzados de Blanes, Girona, Spain) for their valuable contributions to this paper.


  • Britayev TA, Mekhova E, Deart Y, Martin D (2017) Do syntopic host species harbour similar symbiotic communities? The case of Chaetopterus spp. (Annelida: Chaetopteridae). PeerJ 5: e2930.
  • Dana JD (1852) Crustacea. Part I. United States Exploring Expedition. During the years 1838, 1839, 1840, 1841, 1842. Under the command of Charles Wilkes, U.S.N. 13. C Sherman, Philadelphia, 685 pp.
  • Grove MW, Woodin SA (1996) Conspecific recognition and host choice in a pea crab, Pinnixa chaetopterana (Brachyura: Pinnotheridae). Biological Bulletin 190: 359–366.
  • Haig J (1956) The Galatheidea (CrustaceaAnomura) of the Allan Hancock Atlantic Expedition with a review of the Porcellanidae of the western North Atlantic. Allan Hancock Atlantic Expedition Report 8: 1–44.
  • Haig J (1960) The Porcellanidae (CrustaceaAnomura) of the eastern Pacific. Allan Hancock Pacific Expeditions 24: 1–440.
  • Haig J (1964) Papers from Dr. Th. Mortensen’s Pacific Expedition 1914–1916. 81. Porcellanid crabs from the Indo-west Pacific, Part I. Videnskabelige Meddelelser fra Dansk Naturhistorisk Forening i Kjobenhavn 126: 355–386.
  • Haig J (1965) The Porcellanidae (Crustacea, Anomura) of Western Australia, with descriptions of four new Australian species. Journal of The Royal Society of Western Australia 48: 97–118.
  • Haig J (1978) Contribution toward a revision of the porcellanid genus Porcellana (Crustacea: Decapoda: Anomura). Proceedings of the Biological Society of Washington 91: 706–714.
  • Haig J (1979) Expédition Rumphius II (1975) Crustacés parasites, commensaux, etc. Th. Monod et R. Sèrene, éd. V. Porcellanidae (Crustacea, Decapoda, Anomura). Bulletin du Muséum national d’Histoire naturelle, Section A, Zoologie, Biologie et Ecologie Animales, Paris, 4e série 1: 119–136.
  • Hiller A, Harkantra S, Werding B (2010) Porcellanid crabs from Goa, Eastern Arabian Sea (Crustacea: Decapoda: Porcellanidae). Journal of the Bombay Natural History Society 107: 201–212.
  • Johnson DS (1958) The Indo-West Pacific species of the genus Polyonyx (Crustacea, Decapoda, Porcellanidae). Annals of Zoology (Agra) 2: 95–118.
  • McDermott JJ (2005) Biology of the brachyuran crab Pinnixa chaetopterana Stimpson (Decapoda: Pinnotheridae) symbiotic with tubicolous polychaetes along the Atlantic coast of the United States, with additional notes on other polychaete associations. Proceedings of the Biological Society of Washington 118: 742–764.[742:BOTBCP]2.0.CO;2
  • Miers EJ (1884) Crustacea. Report of the Zoological Collections made in the Indo-Pacific Ocean during the voyage of HMS ‘Alert’, 1881–1882: 178–331, Part 2: 513–575.
  • Miyake S (1943) Studies on the crab-shaped Anomura of Nippon and adjacent waters. Journal of the Department of Agriculture, Kyūsyū Imperial University 7: 49–158.
  • Nakasone Y (1969) A new porcellanid crab (Anomura: Porcellanidae) from Japan (Aliaporcellana kikuchii gen. et sp. nov.), with description of two species of the new genus. Publications from the Amakusa Marine Biological Laboratory, Kyushu University: 17–32.
  • Ng PKL, Sasekumar A (1993) A new species of Polyonyx Stimpson, 1858, of the P. sinensis group (Crustacea: Decapoda: Anomura: Porcellanidae) commensal with a chaetopterid worm from Peninsular Malaysia. Zoologische Mededelingen, Leiden 67: 467–472.
  • Osawa M (2001) Heteropolyonyx biforma new genus and species, from Japan, and redescription of Polyonyx utinomii (Decapoda: Porcellanidae). Journal of Crustacean Biology 21: 506–520.
  • Osawa M (2007) Porcellanidae (Crustacea: Decapoda: Anomura) from New Caledonia and the Loyalty Islands. Zootaxa 1548: 1–49.
  • Osawa M (2015) A new species of Polyonyx Stimpson, 1858 (Crustacea: Decapoda: Anomura: Porcellanidae) from the PANGLAO 2004 Marine Biodiversity Project in the Philippines. The Raffles Bulletin of Zoology 63: 536–545.
  • Osawa M, McLaughlin PA (2010) Annotated checklist of anomuran decapod crustaceans of the world (exclusive of the Kiwaoidea and families Chirostylidae and Galatheidae of the Galatheoidea) Part II – Porcellanidae. The Raffles Bulletin of Zoology 23 (Supplement): 109–129.
  • Osawa M, Poupin J (2013) First record of the porcellanid crab, Polyonyx boucheti (Crustacea: Decapoda: Anomura) from the Indian Ocean. Marine Biodiversity Records 6: 1–5.
  • Osawa M, Ng PKL (2016) Revision of Polyonyx pedalis Nobili, 1906 (Crustacea: Decapoda: Anomura: Porcellanidae), with descriptions of three new species. Raffles Bulletin of Zoology 34 (Supplement): 499–518.
  • Osawa M, Naruse T, Ng PKL (2018) New records of species of the Polyonyx sinensis group (Crustacea: Decapoda: Anomura: Porcellanidae) from Japan, the Philippines, Singapore, and Malaysia, with descriptions of two new species. Zootaxa 4429: 303–323.
  • Paul’son O (1875) Studies on Crustacea of the Red Sea with notes regarding other seas. Part I. Podophthalmata and Edriophthalmata (Cumacea). SV Kul’zhenko, Kiev 14: 92–96. [In Russian, English translation by the Israel Program for Scientific Translations, Jerusalem, 1961]
  • Pearse AS (1913) On the habits of the crustaceans found in Chaetopterus tubes at Woods Hole, Massachusetts. The Biological Bulletin 24: 102–114.
  • Petersen ME, Britayev TA (1997) A new genus and species of polynoid scaleworm commensal with Chaetopterus appendiculatus Grube from the Banda Sea (Annelida: Polychaeta), with a review of commensals of Chaetopteridae. Bulletin of Marine Science 60: 261–276.
  • Pope EC (1946) Chaetopterus, a strange and beautiful worm. Australian Museum Magazine 9: 87–92.
  • Renier SA (1804) Prospetto della Classe dei Vermi, nominati el ordinati secondo il Sistemo de Bosc. Padua 38: 15–27.
  • Sanford F (2006) Polyonyx gibbesi (Decapoda: Anomura: Porcellanidae) and Pinnixa chaetopterana (Decapoda: Brachyura: Pinnotheridae), two crab commensals in tubes of the polychaete worm Chaetopterus sp., in the northern Gulf of Mexico. Crustacean Research 6: 23–31.
  • Sankolli KN (1963) On a new species of porcellanid crab (Decapoda, Anomura) from India. Journal of the Marine Biological Association of India 5: 280–283.
  • Southwell T (1909) Report on the Anomura collected by Mr. James Hornell at Okhamandal in Kattiawar in 1905-6. Okhamandal Marine Zoology Report, 105–123.
  • Stimpson W (1858) Prodromus descriptionis animalium evertebratorum, quae in Expeditione ad Oceanum Pacificum Septentrionalem, a Republica Federata missa, Cadwaladaro Ringgold et Johanne Rodgers Ducibus, observavit et descripsit. Pars VII, CrustaceaAnomura. Proceedings of the Academy of Natural Sciences of Philadelphia 10: 225–252.
  • Stimpson W (1860) Prodromus descriptionis animalium evertebratorum, quae in Expeditione ad Oceanum Pacificum Septentrionalem, a Republica Federata missa, Cadwaladaro Ringgold et Johanne Rodgers Ducibus, observavit et descripsit. Pars VIII, Crustacea Macrura. Proceedings of the Academy of Natural Sciences of Philadelphia 12: 22–47.
  • Werding B (2001) Description of two new species of Polyonyx Stimpson, 1858 from the Indo-West Pacific, with a key to the species of the Polyonyx siensis group (Crustacea: Decapoda: Porcellanidae). Proceedings of the Biological Society of Washington 114: 109–119.
  • Williams AB (1984) Shrimps, Lobsters, and Crabs of the Atlantic Coast of the Eastern United States, Maine to Florida. Smithsonian Institution Press. Washington, DC, 550 pp.
  • Zehntner L (1894) Voyage de MM. M. Bedot et C. Pictet dans l’Archipel Malais. Crustacés de l’Archipel Malais. Revue Suisse de Zoologie et Annales du Musée d’Histoire Naturelle de Genève 2: 135–214.
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