Chaetacanthus Seidler, 1922

Salazar-Silva, Patricia, López-Sánchez, Daniel A. & Salazar-Vallejo, Sergio I., 2020, Revision of Chaetacanthus Seidler, 1922 (Annelida, Phyllodocida, Polynoidae), Zootaxa 4885 (3), pp. 395-422 : 397-398

Chaetacanthus Seidler, 1922

Chaetacanthus Seidler, 1922: 301 ; Seidler 1924: 97.

Type species. Iphione magnifica Grube, 1876 , by monotypy.

Diagnosis. Lepidonotinae with 26 segments. Prostomium Lepidonotinae-type, bilobed, without cephalic peaks, facial tubercle well-developed. Two pairs of circular eyes on posterior prostomial half, anterior eyes area often bulked. Three antennae, median antenna with ceratophore inserted frontally, style smooth, slender, subdistally swollen. Lateral antennae with ceratophores inserted terminally, style smooth, subdistally swollen. Palps thick with longitudinal rows of large papillae. Pharynx with 12 pairs of marginal, bottle-shaped papillae; jaws with cutting edge smooth. Tentacular segment not visible dorsally, tentaculophores with a few chaetae, tentacular cirri similar to antennae. Segment two projected over prostomium as a short nuchal lobe. Twelve pairs of elytra completely covering dorsum; elytra inserted on segments 2, 4, 5, 7, 9, 11, 13, 15, 17, 19, 21, and 23; first pair almost circular, following oval, outer posterior margins with long fimbriae, and a distinctive bundle on posterior border. Elytral surface with numerous spinous microtubercles and macrotubercles, sometimes with distinctive groups of ambercolour, honeycomb-like macrotubercles. Parapodia biramous. Neuropodia thick, longer than notopodia, truncate. Branchial filaments attached to lateral sides of notopodia. Dorsal cirri with thick cirrophores, cirrostyles resembling antennae. Cirrigerous segments without pseudelytra. Ventral cirri and venter smooth. Nephridial papillae of similar size throughout body. Notochaetae abundant; slender, long; neurochaetae unidentate, upper region short, with a few rows of subdistal spines.

Remarks. Seidler (1922) proposed Chaetacanthus and newly combined Iphione magnifica as Chaetacanthus magnificus ( Grube, 1876) , as its type species. In the same contribution, he redefined Euphione McIntosh, 1885 . He later ( Seidler 1924: 15) regarded Chaetacanthus Seidler, 1922 and Euphione McIntosh, 1885 as resembling each other by having similar numbers of segments and pairs of elytra, and branchial filaments on parapodia. These genera were separated because in Chaetacanthus neurochaetae are spinulose, with a few spread separate spines, and the dorsum does not have parelytrophores or pseudelytra, whereas in Euphione neurochaetae are fringed, with abundant thin spines, and the dorsum has parelytrophores or pseudelytra. The latter are well-defined, smooth, dorsal plate-like structures, visible in cirrigerous segments of Euphione species.

After Fauchald (1977), Chaetacanthus groups with eight other lepidonotin genera by having well-developed palps, 12 pairs or elytra, and 26 segments. However, only three genera have branchial filaments in interparapodial spaces: Euphione McIntosh, 1885 , Euphionella Monro, 1936 , and Chaetacanthus . These three genera can be separated by the presence of dorsal plates, or pseudelytra, and by the type of neurochaetae. Fauchald (1977) keyed these genera out by indicating that Euphionella has pseudoelytra, whereas the two other genera lack them, although Day (1967: 43; key to genera) indicated pseudelytra are present in Euphione , and they are visible in a photo of the type species available in the internet ( Natural History Museum 2014). The presence of pseudoelytra is uncertain; they were not mentioned by Imajima (1997) and in recent publications they have not been well illustrated. However, the relevant differences among these genera are in the neurochaetae; these three genera can be separated by their type of neurochaetae because they are completely smooth, without rows of spines in Euphionella ( Monro 1936) ; spinulose, with a few rows of spines in Chaetacanthus , and hirsute with long and fine spines in Euphione ( Imajima 1997) .

As herein redefined, Chaetacanthus Seidler, 1922 includes two groups of species regarding elytral ornamentation. The first group has large amber-colour macrotubercles, sclerotized, arranged in a honeycomb-like group, and includes the type species, herein included in the senior synonym, C. brasiliensis ( de Quatrefages, 1866) from the Grand Caribbean, and C. ornatus n. sp. from the Eastern Pacific. The other group does not have these honeycomblike groups of macrotubercles and includes C. harrisae n. sp., C. pilosus ( Treadwell, 1937) n. comb., reinstated, and C. pomareae ( Kinberg, 1856) from the South Central Pacific. It is interesting that Chaetacanthus has not been recorded from the Western African or Indian Ocean localities, but their low abundance, combined with the fact that they are apparently cryptic, living inside rock crevices, might explain the lack of records.

There are other types of branchiae among polynoids, especially in those living in hydrothermal vent environments. However, their branchiae are more complex and arise along elytrophoral margins; again, at least after their position, they are unlikely homologous with those present in lepidonotinae . Wehe (2006: 47) indicated that they can be globular, plicate in Branchiplicatinae Pettibone, 1985a , or arborescent in Branchinotogluminae Pettibone, 1985b and Branchipolynoinae Pettibone, 1984 . It is interesting that despite the differences in branchiae, and on the extent of elytral cover along dorsum, the two former subfamilies have been merged with Lepidonotopodinae Pettibone, 1983 after molecular studies ( Zhang et al. 2018; Bonifácio & Menot 2019; Hatch et al. 2020).