Micronephthys platycephala

Description. Holotype incomplete, 20 mm long, 1 mm wide, with 51 chaetigers, robust. No pigment present on body or prostomium. Prostomium broad, flattened dorsoventrally, partially semi-transparent; anterior margin straight; antennae at corners of anterior margin; palps at base of lateral extensions of prostomium, widely separated from antennae; anterolateral margin (between palps and antennae) convex; posterior margin of prostomium V-shaped (Figs 6 A – C, 7 A – C). Antennae and palps long, tapering with swollen tips, antennae about 3 / 4 length of palps (Figs 6 A – B, 7 A – B). Nuchal pits round (Fig. 7 A). Pair of subdermal eyes, not visible, at posterior margin of chaetiger 2. Pharynx not everted, description from paratype specimen (AM W. 47398): pharynx with 19 bifid terminal papillae, simple middorsal terminal papilla absent, 22 subdistal rows of elongate papillae (distalmost papillae of mid row as long as or longer than palps), with 8 – 15 papillae per row, proximally decreasing in size to small micropapillae which extend to base of pharynx; elongate middorsal and midventral subdistal papillae absent (Fig. 6 D – E). Chaetiger 1 anteriorly directed, of similar size to subsequent chaetigers, with ventral cirri elongate, similar in shape and length to palps (Figs 6 A, 7 A, 7 C), dorsal cirri present as very small papilla (Fig. 7 C – D). Following chaetigers with small dorsal and ventral cirri, ovoid to spherical in shape (Figs 6 F, 7 F – H). Interramal branchiae absent. Interramal patches of cilia present (Fig. 7 F). Parapodial acicular lobes conical to pointed (Fig. 7 I – J), aciculae golden-coloured, distally curved. Anterior parapodial postchaetal lobes poorly developed, low, rounded, subequal in length to acicular lobes, and longer than rudimentary prechaetal lobes; posteriorly postchaetal lobes slightly longer than acicular lobes (Fig. 7 H – I). Chaetae of 4 types: chaetiger 1 with capillary chaetae in noto- and neuropodia, barred chaetae present in preacicular position in notopodia only (Fig. 7 D – E); following chaetigers with barred preacicular chaetae (Fig. 7 J – K) and spinose postacicular chaetae in both noto- and neuropodia (Fig. 7 G – H, J – K); lyrate chaetae with unequal rami present in postacicular position (Fig. 7 G, 7 J), starting from chaetiger 5. Variation. Paratypes are all incomplete. Three complete specimens from AM W. 47229, each possess a single long tapering terminal cirrus, up to eight posterior segments in length.

Remarks. Micronephthys platycephala n. sp. is distinguished from other species by a combination of characters: body large and robust, pharynx with 19 bifid terminal papillae and 22 rows of subdistal papillae, absence of single middorsal terminal papillae and elongated middorsal and midventral subdistal papillae, presence of micropapillae proximally on the pharynx, prostomium anteriorly flattened and semitransparent, eyes not visible externally but deeply subdermal in segment 2, presence of capillary, barred, spinose and lyrate chaetae (dentate chaetae absent from the first notopodia), spherical dorsal cirri, absence of interramal branchiae, and poorly developed parapodial lobes and lamellae. There appears to be no other species described that possesses this combination of features, and none described as having this unusual form of the prostomium. This species shares similarities with Micronephthys sphaerocirrata (Wesenberg-Lund, 1949) — the form and arrangement of pharyngeal subdistal papillae and proximal micropapillae — but differs from it by possessing a larger, robust body, anteriorly flattened and semitransparent prostomium, the absence of visible eyes, and the absence of a single middorsal papilla on the terminal ring of papillae of the pharynx. While the type material was collected in a plankton tow, all the additional material was collected as benthos and we suggest that the type material was accidently dislodged from surface sediments by the plankton tow.

Distribution. Australia (Queensland: Lizard Island, Great Barrier Reef).

Etymology. The species name is derived from the greek words " platys " (flat) and " kephale " (head) and refers to the flattened prostomium. Habitat. Sand and muddy sand, from shallow subtidal depths to 30 m.

Type locality. Carter Reef, Great Barrier Reef, Queensland.

Type material. Holotype: AM W. 47397, Queensland, Great Barrier Reef, Carter Reef, 14 ° 34 ' S, 145 ° 34 ' E, 30 – 36 m, plankton tow, 31 Jan 1982. Paratypes: AM W. 46844 (1 on SEM, 1 photographed), AM W. 47398 (2, 1 photographed), from same sample as holotype. Other material examined. Queensland, Lizard Island: AM W. 47229 (3), 100 m off Chinamans Ridge, 14 ° 40 ' S, 145 ° 27 ' E, sand, 9 m, 13 Oct 1978; AM W. 47396 (3), between bommies inside lagoon entrance, 14 ° 40 ' S, 145 ° 28 ' E, medium to fine sediment, 18 m, 9 Oct 1978.