Arabella Grube, 1850

Steiner, Tatiana M. & Amaral, Antonia Cecília Z., 2009, Arabella aracaensis, a new species with growth rings on its mandibles, and some remarks on the endoparasitic Labrorostratus prolificus (Polychaeta: Oenonidae) from southeast Brazil, Journal of Natural History 43 (41 - 42), pp. 2537-2551 : 2544-2545

Arabella Grube, 1850

Type species: Nereis iricolor Montagu, 1804, p. 82 .

Diagnosis

Conical prostomium, two to four eyes, two peristomial rings. One pair of mandibles, one pair of partially fused maxillary support plus one unpaired median (which is absent in the parasitic species). Symmetrical or asymmetrical maxillae: MI falcate distally, with or without basal dentition on the internal margin, or dentate along the entire margin; MII to MIV dentate; MV, in general, with one tooth. Sub-biramous parapodia, with small notopodial papillae. Simple limbate geniculate setae, with or without serrated margin. Simple modified ventral seta (termed hooded acicular setae or hooded simple setae) sometimes present; acicular spine absent.

Traditionally, all species with the acicular spine absent and the MI with a falcate or dentate internal margin are included in Arabella . Some authors have adopted Cenothrix and Notopsilus as subgenera of Arabella , in the following arrangement: (a) Arabella (Arabella) – MI falcate, and modified ventral seta absent; (b) Arabella (Cenothrix) – MI falcate, and modified ventral seta present; (c) Arabella (Notopsilus) – MI dentate, and modified ventral seta present (according to Orensanz 1974) or absent (according to Perkins [1979] and Hilbig [1995]).

The genus Cenothrix was created by Chamberlin (1919) to include species with a modified ventral seta. Chamberlin (1919) considered that such a seta was absent in Arabella . According to Crossland (1924) and Colbath (1989), the modified ventral seta is similar to other setae of the parapodium, and differs from the others in the degree of modification but not in its structure. It is possible that all the species of Arabella have this kind of seta, but the diameter and degree of variation may differ among them. This seta may have not been observed in some of these species. In the new species of Arabella described later, this seta is present, but nevertheless we opted not to place it in the genus Cenothrix.

According to Colbath (1989), and as observed in the present study, the modified ventral seta has no hood, as mentioned by some authors. The term acicular, adopted by some authors, does not seem appropriate for this seta, and therefore it will be referred to herein as the modified ventral seta.

The polymorphism of the maxillae (symmetry and shape, mainly MI and MII) in some species of the genus has been little studied until now. For A. iricolor, Kielan-Jaworowska (1966) examined 13 specimens deposited at the British Museum (Natural History) and observed five different types of arrangement of maxillae, one symmetrical and four asymmetrical (mainly MI and MII). Colbath (1989) examined many specimens of A. iricolor deposited in the National Museum of Natural History (Smithsonian Institution), and observed that all of them had asymmetrical maxillae, except one population in which the left MI was bifid.

There are no studies on how the modified ventral setae and the symmetry of the maxillae vary in different populations of one species or among species, or how the ontogenetic changes appear during the life of an individual.

Because of the scarcity of external characters to determine the species of the genus, the morphology of the jaws has been considered taxonomically important. All maxillae can be examined from the dorsal position, so that many other characters can be analyzed.