A new species of Polyonyx (Crustacea, Anomura, Porcellanidae) inhabiting polychaete-worm tubes (Annelida, Chaetopteridae) in the Indo-West Pacific

ZooKeys 818: 25–34 (2019) A new species of Polyonyx inhabiting polychaete-worm tubes... doi: 10.3897/zookeys.818.30587 A peer-reviewed open-access journal RESEARCH ARTICLE http://zookeys.pensoft.net 25 Launched to accelerate biodiversity research A new species of Polyonyx (Crustacea, Anomura, Porcellanidae) inhabiting polychaete-worm tubes (Annelida, Chaetopteridae) in the Indo-West Pacific Bernd Werding1, Alexandra Hiller2 1 Institut für Tierökologie und Spezielle Zoologie der Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 29 (Tierhaus), D-35392 Giessen, Germany 2 Smithsonian Tropical Research Institute, Apartado 0843-03092, Panamá, República de Panamá Corresponding author: Alexandra Hiller () Academic editor: I. Wehrtmann | Received 20 October 2018 | Accepted 20 December 2018 | Published 17 January 2019 http://zoobank.org/95F1F6EC-005E-4D64-8FD6-8730220ECE47 Citation: Werding B, Hiller A (2019) A new species of Polyonyx (Crustacea, Anomura, Porcellanidae) inhabiting polychaete-worm tubes (Annelida, Chaetopteridae) in the Indo-West Pacific. ZooKeys 818: 25–34. https://doi. org/10.3897/zookeys.818.30587 Abstract 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. Keywords adaptation, Chaetopterus, commensalism, polychaete-inhabiting Copyright Bernd Werding, Alexandra Hiller. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. 26 Bernd Werding & Alexandra Hiller / ZooKeys 818: 25–34 (2019) Introduction 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 MilneEdwards, 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. A new species of Polyonyx inhabiting polychaete-worm tubes... 27 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. Systematics Family Porcellanidae Polyonyx socialis sp. n. http://zoobank.org/C8712225-9D4B-40AA-87CA-0B6FCDB3174A Figs 1, 2a–g, 3 Material. Holotype: female, SMF 52400, South China Sea, south coast of Vietnam, Nhatrang Bay, Tre Island, Dam (...truncated)