Arabella (Cenothrix) robusta Zanol & Ruta, 2015, n. sp.
Creators: name: Zanol, Joana
name: Ruta, Christine
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Annelida
Polychaeta
Eunicida
Oenonidae
Arabella
Arabella robusta

description: Arabella (Cenothrix) robusta n. sp.
(Figs 4 F–K, 5, 6)
Material examined. Holotype: AM W.44355, MI QLD2371 (1), fixed in formalin, few posterior chaetigers fixed in ethanol. Paratype: AM W.43916, MI QLD2331 (1), fixed in ethanol.
Measurements. Table 2.
Description. Live specimens whitish with orange (dark) spots regularly distributed on dorsal side; on prostomium, between median and lateral eyes; on peristomium, a pair on anterior and posterior margins and a median on the posterior margin of both rings; on chaetigers, a pair on anterior and posterior margins and a median on posterior margin (Fig. 5 B, C). Specimen fixed in ethanol beige with same pigmentation pattern of live specimens (Fig. 5 D). Specimen fixed in formalin beige with dark brown spots scattered around (Fig. 5 E).
Body long, slender, widest at anterior region (chaetiger 13–19), evenly tapering towards posterior end of the body (Fig. 5 A); dorsoventrally rounded, more convex dorsally than ventrally at anterior end (Fig. 5 H). Prostomium as wide as, as deep as and longer than peristomium; tapering with rounded lateral margins (triangular); dorsally flat; ventrally round (Fig. 5 D–J). Four round eyes in line, median closer to lateral than to each other; median smaller than lateral (Fig. 5 D, E). Peristomium double ringed, division between segments inconspicuous dorsally, first ring a little longer than second (Fig. 5 D, E).
Mandibles as far forward as maxillae in pharyngeal bulb while retracted; cutting plates about as long as mandibular carriers, diverging at anterior margin, inner edge shorter than lateral (Fig. 4 F). Maxillae dorsal carriers widest at anterior end, connected at posterior end, ventral carrier not observed (Fig. 4 G). Maxillae with five pairs of plates, asymmetric (Fig. 4 G–I). Left side MxI robust distally falcate and Mx II short, right side MxI gracile distally falcate and MxII long (Fig. 4 H). Maxillary formula: (1,10 (7)) + (1,7), 6 + 12 (10), 5 + 5 (4), 5 + 5, 2 + 2.
Pre-chaetal lobe rounded, as long as chaetal lobe. Post-chaetal tapering to round tips, more than twice as long as chaetal lobe anteriorly, digitiform and twice as long as chaetal lobe posteriorly. Notopodial cirri present as small papillae, with at least 1 notoacicula.
Neuroaciculae mucronate, 1–2 per neuropodia. Chaetae length and limbate chaetae geniculation decrease from dorsal to ventral in all parapodia. All chaetae robust in median and posterior chaetigers, thick in relation to parapodia (Fig. 4 J–K, 6B). Limbate chaetae decrease in number from anterior to posterior end (3–2); most finely serrated, second dorsal coarsely serrated (7–5 denticles rows, decreasing towards posterior end; Fig. 4 J). 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 per parapodium (Fig. 4 K, 6A, B).
Pygidium two swollen pads lacking cirri (Fig. 5 K).
Variation. Live specimens differ in the colour of pigmentation, which may be orange or black. The pigmentation is retained in specimens fixed in ethanol. In specimens fixed in formalin, the wide orange spots of pigmentation were replaced by scattered small brown spots. Shape and depth of prostomium varied from round to tapering; proximal end about as deep as or deeper than distal end. Pharynx is slightly everted in the holotype, making the peristomium seem a little longer ventrally than dorsally.
Remarks. The flat body of the paratype may be an artefact of the fixation in ethanol. MxI and II showed no variation among examined specimens.
The robust chaetae and MxV with two teeth are unique features in this species. Arabella (Cenothrix) atlantica Crossland, 1924 and Arabella (Cenothrix) monroi Colbath, 1989b are the most similar to A. (C.) robusta n. sp. They share features of the prostomium and peristomium; chaetae, except for the number, which is smaller in A. (C.) robusta n. sp.; and similar maxillary plates and formula, except for MxV, which has just one tooth in A. (C.) atlantica and A. (C.) monroi. Arabella (C.) monroi and A. (C.) robusta n. sp. also share the swollen pygidial pads and the lack of pygidial cirri. Features of the pygidium are not known for A. (C.) atlantica (Colbath 1989b).
Etymology. Name refers to robust chaetae, limbate and ventralmost chaetae tapering to distal guards abruptly thicker in relation to parapodia than in other species.
Habitat. Coral rubble, 5–9 m deep.
Type locality. Lizard Island, Big Vicki's reef.
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 755-758, DOI: 10.11646/zootaxa.4019.1.26, http://zenodo.org/record/235654