Pelagobia, GREEFF, 1879

Sardá, Rafael, Gil, João, Taboada, Sergi & Gili, Josep Maria, 2009, Polychaete species captured in sediment traps moored in northwestern Mediterranean submarine canyons, Zoological Journal of the Linnean Society 155 (1), pp. 1-21 : 11-15

GENUS PELAGOBIA GREEFF, 1879

PELAGOBIA LONGICIRRATA GREEFF, 1879

( FIGS 5A–F , 6A–F )

Pelagobia longicirrata Greeff, 1879: 247 , pl. XIV, figs 23–25; Pelagobia viguieri Gravier, 1911: 62–65 , pl. II, figs 22–25; Chamberlin, 1919: 122–124; Benham, 1921: 57, pl. 7, figs 59–60, 1927: 78; Treadwell, 1943: 33; Hartman, 1959: 175, 1964: 64, pl. 19, figs 7–8, 1968: 327, figs 1–3. Pelagobia erinensis Nolte, 1938: 278 , figs 224–225.

Material examined: 21 specimens in total – 16 from Planier and Lacaze-Duthiers submarine canyons, and five from Foix submarine canyon ( Table 1) – collected throughout the year, but specimens were more abundant in traps during spring. Two specimens were set aside for SEM analysis and were stored on stubs in the CMIMA-CSIC, and the remaining material was deposited with the author’s collection.

Description: Body short and flattened dorsoventrally in all specimens ( Fig. 5A ). Length up to 4 mm, and with between 15 and 18 chaetigers. Cephalic lobe composed of subtriangular prostomium, rounded anteriorly ( Fig. 6A–D ), and fused with peristomium. Two pairs of short antennae. Eyes not visible. Large occipital nuchal organ on each side of peristomium ( Fig. 6D ), shaped like lappets, and composed of three regions, each region with large reticulate space forming a field of pits, and each pit with partially extensible cilia-like structures ( Fig. 6D–F ). Several glands in reticulate spaces containing ciliary extensible formations of nuchal organs, and anterior prostomial region of cephalic lobe ( Fig. 6B–D ). Two pairs of long cylindrical tentacular cirri occur on first chaetiger, which has about six long compound chaetae and a single shorter simple chaeta ( Fig. 5B ). Second chaetiger without dorsal cirri. Remaining chaetigers with long cylindrical dorsal and ventral cirri of similar shape and size. Parapodia uniramous, with conical end processes, and about 20–30 long, thin, compound heterogomph chaetae, and one shorter simple chaeta, in the anterior part of every chaetal fascicle ( Fig. 5C ). Acicula extending slightly beyond parapodial tip. Long compound chaetae, with finely serrated blades and unidentate tips, 35–80 Mm long ( Fig. 5D ). Pharynx with two chitinized style-like jaws ( Fig. 5F ). Anal lobe with two rudimentary anal cirri that contain similar glands to those observed in the head ( Fig. 5E ).

Discussion: Although P. longicirrata seems to be a widely distributed bathypelagic species that has been sampled many times in plankton nets, its description has always been based on morphological characters seen using a light microscope. The present description, based on SEM microscopy, shows many particularly relevant features that were not well described before in this species, or in other members of the family. The most interesting feature relates to the fields of pits present in the nuchal organs. The presence of pits or ciliary pits in the nuchal organs of Lopadorhynchyidae has been used recently as an important morphological character ( Fauchald & Rouse, 1997). The morphological description of such structures is old; they are described as the ‘organe vibratile’ in Viguier (1886), eversible ciliary pouches in Claparède (1870) for Lopadorhynchus krohnii , or as nuchal organs in crests in Greeff (1879) for P. longicirrata , and as reticulate spaces in Rullier (1951) for L. krohnii . SEM examination enables a better description of these structures. They bear long, extensible, cilia-like structures that could be used both to improve floatability and as a mechanism for detecting prey. As Pelagobia is a carnivorous species, it is possible that it could be extending a fan of cilia around the head end to triangulate on prey (K. Fauchald, pers. comm.). A second important feature relates to the description of the pharynx of P. longicirrata : a preliminary drawing of the hooks was made by Orensanz & Ramirez (1973), and was somewhat different to that made by Friedrich (1949). The pharynx shown here ( Fig. 5F ) has a stylet with a basal plate that is not as large as that illustrated by Orensanz & Ramirez (1973). Finally, it is worth drawing attention to the presence of a short simple chaeta in the anterior part of every parapodium. The presence of this chaeta has not been included in previous descriptions of P. longicirrata , or in descriptions of any other species in the family.

FAMILY SYLLIDAE

Figures

Figure 5. Pelagobia longicirrata. A, general view. B, lateral view of anterior region. C, uniramous group of parapodia showing long dorsal and ventral cirri, and shorter simple chaeta in the anterior region. D, fascicle of heterogomph chaetae showing unidentate tip in the inset. E, pygidium. F, internal jaw. Scale bars: A, 500 Mm; B, 300 Mm; C, 200 Mm; E, 100 Mm; and D, F, 50 Mm (inset D, 3 Mm).

Figure 6. Pelagobia longicirrata. A, prostomium with two pairs of antennae. B, gland formations at the anterior of the prostomium. C, detail of B, showing glands and holes with cilia-like structures. D, lateral view of the nuchal organs, including the first chaetiger, where the simple chaeta position is observed (arrow). E, reticulate field of pits with cilia at the nuchal organ. F, view of reticulate area with the extensible cilia-like structure. Scale bars: A, D, 100 Mm; B, 20 Mm; C, F, 5 Mm; and E, 3 Mm.