The habitus of Scutopus robustus Salvini-Plawen, 1970 (Caudofoveata, Limifossoridae), a rare mollusc from the NW Iberian Peninsula

Abstract

Caudofoveata are vermiform, aplacophoran molluscs with reduced foot and a mantle covered with calcareous sclerites. Most Caudofoveata live at great depths mainly burrowing in muddy bottoms. They are usually damaged during their collection, thus the iconography available is incomplete, except for the patterns of the sclerites. A sample of Scutopus robustus was collected during the oceanographic expedition MASPROGAL 2013, carried out by the EBMG-USC, on the bottoms of polymetallic nodules and corals of the NW Iberian Peninsula at a depth of 600–700 m (43°48,000’N, 8°51,760’W). S. robustus is a rare species recorded for the first time in this geographic area, which confirms the suspicions of Salvini-Plawen and Garcia-Alvarez (2014) about its presence off the Iberian Peninsula; so far, it has only been found off Norway, Iceland, northern England, southwest Ireland and the western Mediterranean Sea (Salvini-Plawen 1970; Ivanov and Scheltema 2001, Salvini-Plawen and Garcia-Alvarez 2014). Specimen: 5 mm long × 0.25–0.5 mm wide. White in 70% ethanol. Postoral buccal shield slightly flanking the mouth. Three body regions (Fig. 1a): anterium, trunk and tassel, covered in sclerites lying longitudinally on the mantle. Anterium (Fig. 1b), with flat, smooth, lanceolate sclerites (40–70 μm long × 15–30 μm wide) and truncated base (Fig. 1e, f). Trunk narrowing in its medial region (Fig. 1c), with two types of sclerites: first type lanceolate (80–130 × 25–45 μm) with smooth waist, truncated base with slight notch and reinforced apical end of the blade (Fig. 1g–j) on the first part of the trunk; second type (100–150 × 20–50 μm) like the first but with a round base (Fig. 1k, l) at the end of the trunk. Tassel bell-shaped (Fig. 1d) with two types of sclerites: first type lanceolate (150–200 × 30–40 μm) with slight waist and round base longer than the blade (Fig. 1m); second type acicular (180–220 × 10–15 μm) and smooth (Fig. 1n). The iconography of S. robustus is scarce, and only two low-quality photographs of the habitus and some drawings of the sclerites are known (Salvini-Plawen 1970, 1972, 1975). The detailed photographs of the habitus and sclerites under and optical microscope and SEM presented in this paper are the first for this species. The studied specimen differs from those already described (Salvini-Plawen 1970, 1972, 1975), as the small, triangular sclerites described from the anterior part were not observed, probably due to the strong contraction of the anterior part, and the acicular sclerites of the tassel are longer than those of previous descriptions. The fact that the trunk Communicated by P. Martinez Arbizu