Flabesymbios, Salazar-Vallejo, 2012

Salazar-Vallejo, Sergio I., 2012, Revision of Flabelligera Sars, 1829 (Polychaeta: Flabelligeridae) 3203, Zootaxa 3203 (1), pp. 1-64 : 50-51

Flabesymbios n. gen.

Type species: Flabelligera commensalis Moore, 1909 .

Diagnosis. Body compressed, anteriorly blunt, slightly tapered posteriorly. Tunic very thin, not covering most body papillae. Cephalic cage chaetae very short. Notopodia lateral, poorly developed, with few short chaetae arranged in longitudinal row, covered by long, filiform clavate papillae; neuropodia well developed, approaching midventrally in median and posterior chaetigers. Notochaetae multiarticulated capillaries; neurohooks multiarticulated, crest entire. Symbiotic with sea urchins.

Etymology. Flabesymbios is a free combination between the first few letters of Flabelligera and the Greek word symbios, to indicate that members of the genus live associated with other invertebrates, especially sea urchins. Gender and declination are neuter.

Remarks. Flabesymbios n. gen. resembles Flabelliderma because in both genera the body papillae are not covered by the tunic. However, Flabesymbios n. gen. differs from Flabelliderma in that the parapodial papillae are delicate, easily eroded, and do not form notopodial lobes surrounding the notochaetae. Further, there are two additional features separating Flabesymbios n. gen. from Flabelliderma : Flabesymbios n. gen. has a very long first chaetiger and the neuropodia are displaced midventrally in median and posterior chaetigers. The illustrations by Hartman (1961, 1969 and repeated elsewhere) show lateral notopodial lobes and a swollen first chaetiger; these features do not match what can be observed in preserved organisms. Rather, her illustrations were based on a living specimen because the cephalic cage is held open, while the lateral notopodial arrangement is a distortion, probably due to their compression by a glass slide, or an attempt to make the body look more symmetrical.

Chloraema dujardini de Quatrefages, 1849 , which is the juvenile form of F. affinis , is associated with echinoderms. The sea urchins provide temporary shelter, probably against predation, but the flabelligerid does not show morphologic adaptations. de Saint-Joseph (1894:100) noticed that the smaller specimens had about half as many cephalic cage chaetae or branchiae and an almost papillae-free venter. This shelter/refuge idea was implicitly repeated by McIntosh (1915:107–108) and explicitly noticed by Fauvel (1927:113). Hartman (1961:117) had a contrary approach because she regarded F. haerens Chamberlin, 1919 , as the free-living stage of F. commensalis , which had been described as living on sea urchins. As shown below, these two species are synonyms and F. haerens is the juvenile and junior synonym.

Moore (1909a:288) noticed the modifications for living among sea urchin spines: “The color, lateral compression of the body, position of the neuropodia and the stout neuropodial hooks would be especially useful in this situ- ation.” Some of his specimens, which have been regarded as free-living, were collected from seagrass roots; it is unlikely that they are symbiotic with other polychaetes. A more careful procedure is required to evaluate potential tube-dweller hosts. The diagnostic features for the currently known species are included in the key below.

Distribution. Northeastern Pacific Ocean, living on sea urchins ( Centrostephanus and Strongylocentrotus ). There are seven species symbiotic with Strongylocentrotus and the relationship might be more widespread if there is a long history of co-evolution; however, some sea urchin specialists have not found the worms in congeneric echinoderms from other seas. There are two species groups in Strongylocentrotus ( Lee 2003) but closely allied species are apparently free from flabelligerid worms.The Western North American species are sympatric along most of their distribution ( Durham et al. 1980); this might imply either a recent independent colonization and speciation, or that the commensal flabelligerids all belong to the same species, being more abundant in subtidal, calm environments, as is the case with other symbionts ( Kretzler 1984).