Genus Arabella Grube, 1850

Type-species. Nereis iricolor Montagu, 1804

Diagnosis. Prostomium tapering anteriorly, without antennae; eyes, usually, present at posterior margin in number of two or four. Peristomium double ringed, rings clearly or inconspicuously separated. Maxillary carriers long and slender, two dorsal strongly sclerotized and one ventral moderately to poorly sclerotized. Five maxillary plates, shape of plates symmetric or asymmetric; MxI gracile or robust (sensu Colbath 1989b; Fig. 4B, C), distally dentate or falcate simple or bifid. Mandibles present, always fully developed. Branchiae absent. Notopodial cirri small papillae. Neuropodia prechaetal lobes usually shorter than postchaetal. Most chaetae limbate, geniculated at some extent, finely or coarsely serrated (Fig. 2 D); ventralmost chaeta in median and posterior parapodia taper abruptly (Fig. 3 L) or gradually (Fig. 2 E) to guards, which are always present; ventralmost chaeta acicular spine absent.

Remarks. Approximately 50 species have been described or placed within Arabella . Most descriptions are incomplete, leading to great amount of synonymisation and records of few cosmopolitan species, consequently making it difficult to identify specimens to species level. Large intraspecific variation has been recorded in maxillary features, such as shape of MxI and II (Colbath 1989b). However, the degree of intraspecific polymorphism is still unknown for most species, which poses an additional challenge to species identification.

Cenothrix Chamberlin, 1919 and Notopsilus Ehlers, 1868, recognized synonyms of Arabella, have been proposed as subgenera (Orensanz 1974). Shapes of MxI and ventralmost chaetae were used to differentiate among these subgenera and the nominal. Arabella and Cenothrix have MxI distally falcate. In Notopsilus, it is distally dentate. Ventralmost chaetae tapering to the guard is gradual in Arabella and abrupt in Cenothrix and Notopsilus . However, Notopsilus is not, currently, recognized because some species have both distally falcate and dentate MxI (Colbath 1989b).

Three species of Arabella had been previously reported from the Australian coast, Arabella (Arabella) iricolor (Montagu, 1804), A. (Cenothrix) mutans (Chamberlin, 1919) and A. (Arabella) longipedata Monro, 1931 (Hutchings & Yerman 2011) . Of these, we only recorded A. (C.) mutans and describe two new species to science.

Arabella (A.) iricolor and A. (C.) mutans are among the most recorded species for the genus. They are clearly differentiated by ventralmost chaetae tapering to the guard gradual and abrupt (sensu Colbath 1989b), which characterize them, respectively, as Arabella (Arabella) and Arabella (Cenothrix) . Arabella (A.) iricolor, described from England, is probably a species complex, which renders the inferred distribution range of this species questionable (Colbath 1989b). Arabella (A.) longipedata was described from a Great Barrier Reef specimen with posterior post-chaetal lobe longer than chaetae and around eight chaetae per parapodium, features absent in all specimens analysed in our study.

Traditionally, the main features used in species identification are: 1) ventralmost chaetae tapering abruptly or gradually to guards; 2) absence/presence of ridge on ventralmost chaetae; 3) shape of maxillary plates; 4) relative length of post-chaetal lobe; 5) shape of pygidium and pygidial cirri (Fauchald 1970; Orensanz 1974; Colbath 1989b). However, we did not include the ridge on ventralmost chaetae in our descriptions, because it is difficult to be certain about them using light microscopy. Shape of pygidium and pygidial cirri were useful features, as well as shape of mandible, which is usually poorly described and illustrated.