Euchone pallida Ehlers 1908

Euchone pallida Ehlers, 1908

Figure 1

Material examined. 6 specimens (2Ƥ, 23, 2 broken and undifferentiated) collected in St. DR1 (77 m depth) off Livingstone Island (6th January 2006); 14 specimens (7Ƥ and 73) collected at St. DR3 (100 m depth) off Deception Island. M.C. Gambi legit. 1 specimen is deposited at the PCZN (232), 13 specimens at the SZN - POLY (13).

Revised description. Medium sized sabellid with eight thoracic and 34 abdominal chaetigers, of which 14-15 form the anal depression (Fig. 1 D). Branchial lobes each with 20 radioles with small filiform tip (Fig. 1 C) and palmate membrane covering the ¾ of the length. Dorsal lips rounded, extending from inner, dorsal margin of branchial lobes, and terminating just dorsal to mouth, with poorly developed radiolar appendage. First pinnula of dorsalmost radioles separated from other pinnules and highly developed and ticker than the others, here defined as dorsal “pinnular appendages” (Fig. 1 E). Ventral lips pointed, up to twelve short ventral radiolar appendages (Fig. 1 E). Collar entire with edge terminating at same height all around and covering base of branchial crown, with thick pouches extending dorsally towards second chaetiger (Fig. 1 A). Methyl green staining pattern shows a homogeneous dark coloration of thorax and abdomen with distinct intra-segmental furrow, except for presence of thin glandular ridge on chaetiger two (Fig. 1 B). Inferior anterior thoracic chaetae narrowly hooded (Fig. 1 F). Thoracic uncini with long handle and series of small teeth of similar size above main fang (Fig. 1 G).

Abdominal uncini from pre-pygidial depression and depression chaetigers similar (Fig. 1 H, I), with main fang surmounted by few rows of small teeth, breast rectangular, not extending beyond distal end of proximal tooth, handles absent. Variation within a torus absent.

Remarks. Chaetiger and radiolar number, shape of the collar and posterior abdominal depression all correspond well to the original description by Krøyer (1856), as well as that of Banse (1972) for material from Alaska and that of Bick and Randel (2005) for material from Spitzbergen. On the basis of the available literature, the species present a peculiar bipolar geographical distribution, being present in Arctic and Antarctic regions.

The structures referred as “coils” or “glandular tubular organs” or, again “vascular loops”, present in the type species of the genus E. analis, were not detected. These structures situated dorsally in each side of the peristomium, are also present in Chone princei (= Jasmineira) (McIntosh, 1916), in the genus Fabrisabella (Fitzhugh, 1989) and in Claviramus grubei (Fitzhugh, 2002). However, in most Euchone species they are not easily discernible.

The examination of internal structures of the branchial crown confirms the presence of dorsal radiolar appendages without extension of radiolar skeleton (as occur in the type species of Euchone), and the presence of highly developed dorsal “pinnular appendages”. These structures appear similar to that observed in large specimens of Chone infundibuliformis identified by Malmgren’s (1866) (SMNH 6862) (Giangrande, 1992), and they appear not fused to the dorsal lips as observed in other Sabellinae genera.

Observation on egg size and mature sperm morphology of this species, based on material collected from the Weddell Sea (Antarctica), are described in Gambi et al. (2001) and in Patti et al. (2003). Here we provide for the first time TEM pictures of the mature spermatozoa (Fig. 2 A, B). Mature spermatozoa have an unusual cylindrical shape of the nucleus and a characteristic, conical, “nip-like” acrosome with a minute subacrosomal space; and up to 3–4 mitochondria present at the base of the axoneme.