Euchone eniwetokensis Reish 1968

Euchone eniwetokensis Reish, 1968

Figure 17

Euchone eniwetokensis Reish, 1968, Fig. 4: 6–9.

Material examined. Type material. Eniwetok Atoll [USNM 38406, holotype], Marshall Islands, lagoon near Parry Island, 0.45 m, Coll. D. Reish.

Description. Colour, body shape, and size. Trunk cylindrical, posterior abdomen depressed. Holotype juvenile (Fig. 17 A). Body length 4 mm, width 0.3 mm.

Branchial lobes and branchial crown. Insertion of the branchial lobes exposed beyond collar (Fig. 17 B). Radioles: 5 pairs. Radiolar tips long (Figs 17 C–E). Ratio of the palmate membrane length versus branchial crown length: 0.25:1. Lateral flanges narrow. Dorsal lips and ventral lips not examined.

Peristomium. Anterior peristomial ring lobe not exposed beyond collar (Fig. 17 B). Posterior peristomial ring collar: dorsal and lateral margins of collar entire, ventral with a short incision, ventral margin higher than dorsal (Figs 17 A–B); entire length of mid-dorsal collar margins forms a narrow gap. Ratio of posterior peristomial ring collar length versus chaetiger 1 length, in ventral view: 1:1. Two peristomial eyes (Fig. 17 B).

Thorax. Elongate, narrowly hooded chaeta; two rows with three short, broadly hooded chaetae (Figs 17 H–I). Six acicular uncini per torus, with teeth of equal size above the main fang. Glandular ridges on chaetigers 2 and 5: glandular ridge of chaetiger 2 narrow, interrupted dorsal and ventrally; glandular ridge of chaetiger 5 narrow, irregular. Ventral shields weakly developed.

Abdomen. Abdominal segments: 14. Long, narrowly hooded chaetae; 5–8 modified abdominal uncini per torus on anterior and posterior abdomen (Fig. 17 F). Posterior end strongly depressed along the last six segments, with an anterior margin. Pygidium with rounded posterior margin (Fig. 17 G).

Methyl green staining. Ventral shields of thorax with large glands (Fig. 17 A). Dorsally, body is not coloured, laterally only narrow bands are coloured.

Remarks. Euchone eniwetokensis was described as having six chaetigers in the anal depression. Banse (1970) transferred the species to Chone because of the apparent lack of an anal depression; however, E. eniwetokensis has the posterior segments depressed (Fig. 17 A), short, broadly hooded thoracic notochaetae, which are characteristic for Euchone (Figs 17 H–I), and lacks paleate chaetae. Then, herein, the species is returned to Euchone. Euchone eniwetokensis and E. papillosa (Sars) have six segments in the anal depression, but they differ by the total number of segments, and by the number and length of radioles. Banse (1970) recognized that the full complement of chaetigers forming the anal depression is not present in juveniles. Bick and Randel (2005) confirmed this, such that no anal depression was found on the smallest specimens of E. analis, and smaller specimens bear only an enlarged faecal groove without lateral wings. Moreover, Bick and Randel (2005) stated that identifying small specimens of Euchone is difficult without information about the ontogenetic variation of characters (as in juveniles of E. analis). The same authors have also reported a great intraspecific variability in nearly all diagnostic characters of E. analis. The holotype of E. eniwetokensis is a juvenile; the glandular ridge on chaetiger 2 is inconspicuous (as in juveniles of E. analis).