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

Polyonyxsocialissp. 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 genusChaetopterus.Polyonyxsocialissp. 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.Polyonyxsocialissp. n.belongs to theP.sinensisgroup, a world-wide distributed morphological line within the heterogeneous genusPolyonyx. 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.Polyonyxsocialissp. 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.

Abundance, size and habitat relation of reef fish on biogenic structures (structure-forming invertebrates) at Anacapa Island, southern California

A biogenic structure ecosystem of extensive worm tubes of Chaetopterus sp. harbouring different fish species was investigated at Anacapa Island, southern California to determine the numerical abundance, species composition, body size, and seasonality of the ichthyofauna associated, and their relation with the worm patch size. Bimonthly, a 30-m swimming transect and a quadrat-based survey estimated the length of each fish, the type of habitat it occupied, and the worm tube patch size-class. Larger worm mats harboured higher densities of fish, mainly the dominants Rhinogobiops nicholsii and Chaenopsis alepidota, which live inside the worm tube patches. Fish population density varied between the ‘inside’ and ‘edge’ of Chaetopterus beds as smaller and younger individuals were hindered from reaching the middle of the patch by larger, older and territorial individuals. The prevalence of positive and significant correlations between the abundance of smaller individuals of R. nicholsii and specific habitat features (e.g. edge) suggests that the abundance and distribution of juveniles might be habitat-dependent besides intraspecific competition between older and younger individuals. There was a decreasing density of R. nicholsii at greater than 20 cm away from worm tubes and there were no individuals beyond 6 m away from structure. Chaenopsis alepidota was not recorded when the patch size was less than 50% tube coverage. Our results revealed that complex habitat structure had a positive influence on the abundance of juvenile and adult of C. alepidota and R. nicholsii, and suggest that these structured areas are preferentially utilized, which contributes to its patchy distribution pattern. The presence of biological structures in low-relief sedimentary habitats can have critical functional significance even for reef fish. These important habitat features may need to be identified and protected.

Devonian Dentalium martini Whitfield, 1882, is not a mollusk but a worm

Dentalium martini Whitfield, 1882, is a calcareous worm tube, not a mollusk, and is placed in synonymy of Coleolus crenatocinctum Hall, 1879. The growth of D. martini mimics logarithmic curvature in the holotype, but in other specimens shape varies from irregularly curved to straight. A prime reason for reassignment is variation of the wall thickness around the circumference in D. martini, which in late Paleozoic and Recent scaphopods of the same size is of uniform thickness.