Magelona symmetrica Mortimer & Mackie 2006

Magelona cf. symmetrica Mortimer & Mackie, 2006

Figures 6 and 7 (A‒C)

Material examined. Mariana Islands, Guam, Northern District outfall, 13°29'14" N, 144°44'54" E, Jan/2007, 40‒ 52 m, Sta. N1R1, (1 complete specimen, BPBM-R 3874); Sta. N3R1 (3 af, BPBM-R 3875); Agana outfall, 13°33'08" N, 144°48'25" E, Jan/2007, 65‒ 100 m, Sta. A2R2 (2 af, BPBM-R 3876).

Diagnosis. Prostomium as long as wide, lacking prostomial horns. Notopodia and neuropodia of chaetigers 1‒8 with triangular postchaetal lamellae with smooth upper edges. Thoracic superior processes absent. Chaetiger nine with only simple bilimbate capillaries. Abdominal hooded hooks tridentate.

Description. One complete specimen observed, all others posteriorly incomplete; prostomium 0.28‒0.35 mm long, 0.25‒0.36 mm wide; thorax 1.40‒2.20 mm long (including prostomium), 0.27‒0.49 mm wide; abdomen 0.20‒0.48 mm wide; total length 5.57‒7.39 mm for 24‒42 chaetigers.

Prostomium as long as wide (L:W ratio 0.97‒1.12), flattened distally, anterior margin smooth or having discrete crenulations, lacking prostomial horns; (Figs 6A, C; 7A). Two pairs of longitudinal dorsal ridges; outer pair not distinct and adjacent to inner pair; the latter pair with a wide medial gap, diverging slightly distally (Fig. 6A, C). Burrowing organ not observed. Single palp observed, arising ventro-laterally from base of prostomium and extending to chaetiger 18 (Fig. 6A); measured 2.51 mm long; non‒papillated region very short, extending to chaetiger 1 (Fig. 6A). Papillae short proximally but long for majority of length, elongate; proximally and medially with six rows of papillae and distally reducing to four rows. Achaetous segment as long as chaetiger 1 (Fig. 6A).

Thoracic segments wider than long; chaetigers 1‒8 similar; parapodia biramous; notopodia and neuropodia similar, with low prechaetal lamellae and triangular postchaetal lamellae of similar size throughout thorax (Fig. 6B); notopodial lamellae distally smooth. Prechaetal superior processes (DML) and ventral digitiform lobes (VNL) absent in chaetigers 1‒8 (Fig. 6B). Chaetiger nine not distinctly reduced; notopodial prechaetal lamellae low, rounded, postchaetal lamellae triangular to elongate and longer than preceding thoracic chaetigers; superior processes (DML) absent (Fig. 6B). Neuropodia of chaetiger nine similar to notopodia. Additional ventral digitiform lobe (VNL) present on chaetiger nine, very discrete. Chaetae of thoracic chaetigers 1‒9 simple bilimbate capillaries; number of capillaries increasing towards posterior thorax, initially 8‒10 per rami increasing up to 20 per rami on chaetiger nine.

Abdominal segments longer than thoracic ones. Abdominal parapodia with elongate lateral lamellae (Fig. 6B); DML and VML observed as discrete bumps on a few segments, immediately above and below hooded hooks. Abdominal chaetae tridentate hooded hooks of similar size throughout; two same-sized teeth above main fang (Fig. 6D); internal arcuate chaetae not observed. Hooks in two groups, main fangs vis–à–vis. Abdominal segments with 4‒5 hooks per ramus throughout.

Lateral pouches (Σ and C configurations) not observed.

Pygidium round, anal cirri not observed.

All preserved specimens previously stained with Rose Bengal (see below).

Methyl Green Staining. Methyl green staining pattern not obvious due to previously staining with Rose Bengal. Rose Bengal stain obvious as dark glandular regions on chaetigers 1‒4 and chaetigers 8‒9, dorsally (Fig. 7B) and chaetigers 2‒4 ventrally (Fig. 7C). Abdominal region with two lateral bands per segment, not forming a complete ring, not staining dorsally or ventrally (Fig. 7B, C).

Remarks. The material collected from Guam agrees almost fully with the original description of Mortimer & Mackie (2006). The differences observed were in relation to a wider gap between the inner prostomial longitudinal ridges seen in the material from Guam and the presence of minute dorsal and ventral abdominal medial lobes, but these were not observed throughout the abdomen. In addition, the specimens examined herein presented discrete crenulations on the prostomial margin and crenulations have not been reported previously for this species (Mortimer & Mackie 2006; Mortimer et al. 2012). Other Magelona species lacking prostomial horns and specialized chaetae on chaetiger nine and having tridentate hooded hooks include M. alleni Wilson, 1958 and M. equilamellae Harmelin, 1964. These are different from M. symmetrica on the presence of distinct thoracic and abdominal pigmentation and unequal abdominal parapodial lamellae in M. alleni (Wilson 1958; Harmelin 1964). Mortimer et al. (2012) described specimens of M. symmetrica from the Arabian Gulf having pale reddish pigment in the posterior thoracic region not observed in the type material or in the specimens herein examined.

Distribution. Magelona symmetrica was originally described from the Seychelles (Mortimer & Mackie 2006). Specimens identified as M. cf. symmetrica were collected in the Arabian Gulf (Mortimer et al. 2012). This is a new record of Magelona species from Guam in the Mariana Islands.