Idanthyrsus, KINBERG, 1867

Capa, María, Hutchings, Pat & Peart, Rachael, 2012, Systematic revision of Sabellariidae (Polychaeta) and their relationships with other polychaetes using morphological and DNA sequence data, Zoological Journal of the Linnean Society 164 (2), pp. 245-284 : 261

IDANTHYRSUS KINBERG, 1867

Idanthyrsus Kinberg, 1867: 350 ; Kirtley, 1994: 84.

Type species: Idanthyrsus macropaleus Schmarda, 1861 (after Kirtley, 1994). Type locality: Valparaiso Harbour, Chile.

Diagnosis: Idanthyrsus autapomorphies are outer paleae straight (no angle between shaft and blade), flat, and with pointed denticles along lateral margins and distal tips, and, if confirmed in all species of the genus, only fine lanceolate chaetae on neuropodia of parathoracic segments. Unlike other related genera they have three thoracic segments.

Description: Operculum longer than wide with lobes completely divided and distal end sloping posteriorly (oblique to longitudinal axis) and operculum papillae varying in number and size depending on species. Outer paleae arranged in semicircles with straight and flat blades and lateral and distal margins appearing sharply denticulated. Inner opercular chaetae arranged in one row along the inner margin of opercular lobes, with straight and cylindrical blades. One or two pairs of nuchal spines with bent tips (hooks) with or without limbations on the concave margin. Palps similar in length to the operculum. Median organ conspicuous with eyespots on its sides (on specimens examined). Tentacular filaments compound (branching); buccal flaps absent. Neuropodia of segment 1 achaetous, with conical cirri (one in all species examined). Segment 2 with two to four triangular lateral lobes. Three parathoracic segments with notochaetae consisting of lanceolate and capillary chaetae and only thin lanceolate neurochaetae (on specimens examined). Branchiae from segment 2 diminishing in size on posterior abdominal segments. Remarks: All species share the presence of straight, flat, denticulated outer paleae, unique among Sabellariidae . If the presence of only lanceolate chaetae in neuropodia of parathoracic segments is confirmed in all species of the genus this would be another autapomorphy for the group. In the revision of the family, Kirtley considered Lygdamis and Idanthyrsus as belonging to different groups (subfamilies) due to the difference in number of parathoracic segments. However, analyses show that these genera are closely related as they both share the overall shape of operculum with separated and long lobes with oblique distal end, inner paleae arranged in a single line along the inner margin of lobes, and presence of nuchal hooks. Besides the number of parathoracic segments in these two groups, they differ in the shape of outer paleae (smooth in Lygdamis ) and the presence on capillary chaetae alternating with the lanceolate neuropodia of parathoracic segments of Lygdamis and the latter only being present in Idanthyrsus . There are, at the time of writing, 19 species described in the genus ( Kirtley, 1994; Nishi & Kirtley, 1999). Some features, such as the presence of limbation on nuchal hooks, presence of neurochaetae on segment 1, number of lateral lobes on segment 2, or presence of eyespots near median organ, vary between species. They are generally found in shallow water in the tropics or temperate waters as isolated individuals but some species also inhabit boreal and deep water domains and some aggregations of individuals have also been found (e.g. Kirtley, 1994; Nishi & Kirtley, 1999).