" Terebella " cf. Terebella gorgonae Monro, 1933

Figures 2 N–P, 6A–H

Terebella gorgonae: López et al. 1997: 66 .

Material examined. Coiba National Park, Panama: MNCN 16.01/10528 Coiba Island, 7º37'50''N 81º44'30''W, rocky intertidal, 30 Jun. 1996 (1 spec.) — AM W29707, same sample (1 spec. on SEM stub).

Description. Two specimens, one complete, 15 mm long, 3 mm wide at anterior chaetigers level, with 94 segments. Preserved material lacking pigmentation. Tentacles long and thin, whitish. Eyespots absent. Upper lip large and wide, lower lip thicker and shorter. Large tentacular membrane (Figure 6 A). Lateral lobes absent (Figure 6 B, C). Three pairs of pale branchiae inserted on segments 2, 3 and 5, increasing in size posteriorly, each with numerous dichotomous branches (Figure 6 B); first and third pair arranged almost in longitudinal line, second pair displaced laterally, almost in line with notopodia of anterior chaetigers (Figure 6 A, B). Nephridial papillae visible on segments 3–11. Fifteen ventral pads from segment 5, narrower posteriorly (Figure 6 C), mostly fused in pairs (Figure 6 C). Notopodia elongated, beginning on segment 4 and continuing for 83 segments, with two rows of different sized chaetae, each row exhibiting dorso-ventral gradation in length (Figure 6 D). Chaetae geniculate, with pointed serrations along one edge; decreasing in size distally (Figure 6 E). Neuropodial tori wider than ventral pads, contiguous. Uncini arranged in single rows on segments 5–10, with three rows of teeth over main fang, decreasing in size posteriorly, dental formula MF:3:2:4:6 (Figures 2 N, 6F). Neuropodial tori with double row of uncini arranged back to back, separated by wide gap, from segment 13 almost to pygidium; uncini similar to anterior ones, MF:3:2:4–9 (Figures 2 O, P, 6G, H). Posterior eight chaetigers without notopodia and with neuropodia with one single row. Pygidium consisting of broad rim surrounding anus, without papillae. Tube not seen.

Var ia t io n. There is no variation within these two specimens, and both are of similar size.

Remarks. These specimens agree with the Terebella gorgonae original description (Monro 1933) from Gorgona Island, Colombia and Infatigable Island, Galapagos, in most diagnostic characters such as the shape and arrangement of branchia and the presence of ventral pads to segment 20. However, the specimens from the Coiba National Park differ in the dental formula of the uncini having three or four rows of teeth while the original description only mentions two or three rows, a discrepancy that could be explained by the small size of the last one or two rows of secondary teeth, not easily recognisable under a dissecting microscope. In addition, Monro’s specimens have a larger proximal tooth on the edge of the geniculate notochaetae, while the specimens described herein lack this larger tooth, but have teeth decreasing in size progressively towards the tip of the chaetae.

A significant feature of the new material is the back to back arrangement of uncini in the double rows, a feature not described by Monro (1933). The relative position of uncini in double rows has been regarded as a stable generic character (McHugh 1995) of Terebellidae . It is characteristic of Lanice Malmgren, 1866, Loimia Malmgren, 1866 and Scionides Chamberlin, 1919 that uncini are arranged back to back, as it is in this material. Lanice and Loimia have lateral lobes on segment 3 and notochaetae smooth while our specimens from Coiba and species of Terebella lack lateral lobes and have serrated tip notochaetae. In Scionides, the lateral lobes are absent, as in Terebella, but notochaetae are smooth in Scionides and distally serrated in Terebella . Species of Terebella, when the character state has been recorded, have double rows of uncini arranged in alternate rows, face to face. Our specimens, placed for now as " Terebella " cf. T. gorgonae, could therefore represent a new genus.