Arabella (Cenothrix) mutans Chamberlin 1919
Creators: name: Zanol, Joana
name: Ruta, Christine
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Annelida
Polychaeta
Eunicida
Oenonidae
Arabella
Arabella mutans

description: Arabella (Cenothrix) mutans (Chamberlin 1919)
(Figs 3, 4A–E)
Cenothrix mutans Chamberlin, 1919: 330 –332.
Arabella mutans.— Crossland 1924: 71; Colbath 1989b: 294 –295.
Material examined. AM W.44798, MI QLD2429 (1), fixed in formalin, few median chaetigers fixed in ethanol; AM W.44855, MI QLD2439 (1), fixed in formalin; AM W.44892 (2), fixed in ethanol; AM W.44908 (3), fixed in formalin.
Measurements. Table 2.
Description. Live specimens whitish to yellowish with internal red hues due to blood (Fig. 3 A, B). Specimens fixed in formalin yellow to light brown, anterior and posterior ends lighter (Fig. 3 C–F); some chaetigers with dark brown discontinuous pigmentation on dorsal side.
Body long, slender, widest at anterior region, remaining part of the body about the same width along its whole length, abruptly tapering at posterior end (Fig. 3 B, G); dorsoventrally rounded, dorsal side more convex than ventral.
Prostomium around 2/3 as deep as, narrower to as wide as and longer than peristomium; tapering; posterior deeper than anterior end and constricted in the connection with peristomium; dorsally in a slope with median groove from posterior to anterior; ventrally flattened with median groove wider and bearing a projection at posterior end (Fig. 3 C–F). Four eyes evenly spaced (median a little closer) (Fig. 3 A). Median eyes a little more anterior or in line with lateral; smaller and more conspicuous than lateral; may be missing or doubled in some specimens. Peristomium double ringed, second ring a little longer, wider and deeper than first (Fig. 3 C–F).
Mandibles as far forward as maxillae in pharyngeal bulb while retracted; cutting plates longer than mandibular carrier, rounded, inner edge shorter than lateral (Fig. 4E). Maxillae dorsal carriers widest at anterior end; ventral carrier tapering, 2/3 as long as dorsal carriers (Fig. 4A). Maxillae with five pairs of plates (Fig. 4A, D); shape of plates symmetrical or asymmetrical. MxI and II polymorphic. 1) MxI gracile, basally dentate, distally falcate; MxII long on both sides (Fig. 4B). 2) One side MxI gracile, basally dentate, distally falcate, MxII long; opposite side MxI robust, completely dentate, distally bifid (most distal tooth inconspicuous), MxII short (Fig. 4A, C). Maxillary formula symmetric (n=2): (1,6(7)) + (1,6), 12 + 15(13), 7(4) + 6, 5(4) + 5(4), 1 + 1. Maxillary formula asymmetric (MxI left gracile + right robust dentate; n=4): (1,7) + 9, 12(11) + 7, 5(6) + 4, 4 + 4, 1 + 1. Maxillary formula asymmetric (MxI left robust dentate + right gracile; n=1): 9 + (1,5), 7 + 10, 5 + 5, 5 + 4, 1 + 1.
Pre-chaetal lobe rounded, as long as chaetal lobe. Post-chaetal tapering to round tips, twice as long as chaetal lobe. Notopodial cirri present as small papillae bearing one to two notoaciculae.
Neuroaciculae mucronate, 2–3 in most neuropodia, 1 in posterior ones. Chaetae length and limbate chaetae geniculation decrease from dorsal to ventral in all parapodia (Fig. 3 H–J). Limbate chaetae decrease in number from anterior to posterior end (4–2); most finely serrated, second dorsal coarsely serrated (9–4 denticles rows, decreasing towards posterior end; Fig. 3 H), except in anteriormost chaetigers. In anteriormost chaetigers, ventralmost chaetae limbate. Ventralmost chaeta tapering to distal guards abruptly present in all chaetigers except anteriormost ones, distally tapering to blunt end, guards mucronate, 1 in most parapodia, but some with 2 (Fig. 3 H, K–L).
Pygidium two lateral lobes bearing dorsal and ventral digitiform cirri placed laterally (Fig. 3 G); dorsal cirri about as long as to 1/3 (twice as long as) longer than ventral cirri.
Variation. Specimens may be more or less round in cross section, a characteristic that varies along the body. The widest region of the body is variable, but always anterior. The most conspicuous variations among examined specimens are the shape of MxI and II and number of median eyes. Maxillary plates I and II are, respectively, gracile and long on left side and robust dentate and short on the right side in most specimens (n=4). Two specimens have MxI gracile and MxII long on both sides. One (W.44908) has MxI robust dentate and MxII short on the left side and MxI gracile and MxII long on the right side, similar to that described for the type. Two median eyes are present in most specimens, but one or both may be missing and one may be duplicated. In sample W.44908, one specimen is missing the right median eye and the other has a third one posterior to it. Specimen W.44798 is missing both, presenting only two lateral eyes. This specimen also differs from the others in having: up to four geniculated limbate chaetae and always just one ventralmost chaeta tapering to distal guards abruptly in median chaetigers; and only four rows of denticles in coarsely serrated limbate.
Remarks. Relative length of peristomium rings varies in live specimen. The most different specimen (W.44798) was the only one collected in Mangrove Beach. All other ones came from Casuarina Beach.
The intraspecific variation of maxillae and poor species description led to the identification of this species as circumtropical. The presence of ventralmost chaetae tapering to distal guards abruptly is commonly used as the main diagnostic characteristic, although it is not restricted to this species and characterizes the subgenus Cenothrix (Fauchald 1970; Orensanz 1974; Colbath 1989b). Currently, the species is characterized by: 1) polymorphic MxI and II; 2) robust MxI dentate, when present; 3) ventralmost chaetae tapering to distal guards abruptly; 4) postchaetal lobe shorter than chaetae and 5) four pygidial cirri (Colbath 1989b). The type specimen is from Eastern Island. It has maxillae similar to those observed in W.44798, MxI robust dentate distally bifid and MxII short on the left side, MxI gracile and MxII long on the right side (Chamberlin 1919) and eyes under anterior peristomial fold, not illustrated in the original description (Perkins 1979). At least part of the unusual intraspecific variation described for this species appears to be real. Specimens from the same locality may differ only in the shape of the maxillary plates (Colbath 1989b), as observed here. In order to clearly understand the range of intraspecific variation, further studies using morphological and molecular tools are necessary.
Habitat. Sand, intertidal to 0.5 m deep.
Distribution. Tropical Pacific Ocean.
description: Published as part of Zanol, Joana & Ruta, Christine, 2015, New and previously known species of Oenonidae (Polychaeta: Annelida) from Lizard Island, Great Barrier Reef, Australia, pp. 745-772 in Zootaxa 4019 (1) on pages 752-755, DOI: 10.11646/zootaxa.4019.1.26, http://zenodo.org/record/235654