Pseudexogone imajimai, Salazar-Vallejo & Bailey-Brock & Dreyer, 2007

Salazar-Vallejo, Sergio I., Bailey-Brock, Julie H. & Dreyer, Jennifer C., 2007, Revision of Pseudexogone Augener, 1922 (Annelida, Polychaeta, Syllidae), and its transfer to Pilargidae, Zoosystema 29 (3), pp. 535-553 : 547-548

Pseudexogone imajimai n. sp.

( Figs 6 ; 7 )

Synelmis dineti – Imajima 1987: 158, fig. 5a-h (non Katzmann et al. 1974).

TYPE MATERIAL. — Northwestern Pacific Ocean. Japan, offTanega-shima, 30°37.8’N, 13°054.2’E, 45 m, VI.1975, M. Imajima coll., holotype ( NSMT- 85251).

Guam. Off Tanguisson, stn WWRC T-NE-R3, 13.5523°N, 144.8072°E, 43 m, 2001, 5 paratypes ( USNM). — Stn WRRC T-NW-R1, 13.5537°N, 144.8072°E, 50.3 m, 2001, 4 paratypes ( MNHN).

ADDITIONAL MATERIAL. — Guam. Off Tanguisson, stn WWRC T-NE-R3, 13.5523°N, 144.8072°E, 43 m (no date available), 1 specimen (gold-coated, ECOSUR). — Stn WRRC T-NW-R1, 13.5537°N, 144.8072°E, 50.3 m, 2001, 1 specimen (gold-coated, ECOSUR).

TYPE LOCALITY. — Off Tanega-shima, Kyūshū, Japan, in 45 m.

ETYMOLOGY. — The specific name is a modest homage to Dr. Minoru Imajima, in recognition of his very productive career in polychaete systematics, who has concentrated his efforts in Japanese fauna, and especially because of his publication on pilargids.

DESCRIPTION

Holotype complete, damaged, 5.5 mm long, 0.2 mm wide, 38 chaetigers. Because of the removal of chaetigers 1, 15, and 32, body breaking apart in those segments. Prostomium subtriangular ( Fig. 6A ), wider than long, with three similar medially constricted antennae; lateral antennae placed in the middle of prostomium, on the base of palps, not reaching palp tips; median antenna on posterior prostomial margin. Palps free from base, rounded, each with subdistal ventral papillae ( Figs 6B ; 7A, C ). Eyes not visible, but two large globular colourless structures present just behind lateral antennae. Nuchal organs placed in the outer external base of lateral antennae ( Fig. 7D ). Peristomium with two pairs of similar tentacular cirri, as long as median antenna, because they are lateroventral, dorsal cirri looks longer than ventral one. Ciliary bundles extended laterally from the median antenna insertion, and two others extend over the dorsal surface of peristomium ( Fig. 7 A-C).

Anterior parapodia uniramous. Median and posterior parapodia biramous. Dorsal cirri digitate throughout the body, ventral cirri basally swollen in anterior and median chaetigers. Anterior parapodia ( Fig. 6C ) with dorsal and ventral cirri with median constriction, of about same size; median ( Fig. 6D ) and posterior chaetigers, with dorsal cirri larger, digitate, ventral cirri basally swollen. Anterior parapodia with two denticulate capillaries, two pectinates, and a furcate chaeta ( Fig. 7E ). Following chaetigers with an emergent, brittle, sigmoid bidentate notospine ( Fig. 6F, G ), first present from chaetiger 6, continued to posterior end, more exposed in posterior chaetigers ( Fig. 7G ), each notospine with larger subdistal tooth; 1 or 2 finely denticulate capillaries and 1 or 2 pectinates per bundle ( Fig. 6E ). Furcates with a flaring rounded blade, slightly longer than the tapering smaller tine, with dorsal keel curved, with a subdistal notch ( Fig. 7E, F ).

Posterior end with two prepygidial achaetous segments; pygidium with two ventrolateral anal cirri as long as last achaetous segment and pygidium ( Figs 6H ; 7H ). Pharynx not everted.

VARIATION

Complete specimens were 3.0- 5.5 mm long, about 0.15 mm wide, with 30-45 chaetigers, often with an achaetous prepygidial segment. Oocytes visible in chaetigers 13-23, along a line, each of about 25-30 µm in length.

DISTRIBUTION

Japan to Guam, in shallow sublittoral depths. This is a new record for the polychaete fauna of Guam, Mariana Islands (Bailey-Brock 2003).

REMARKS

Pseudexogone imajimai n. sp. resembles P. backstromi by having furcates with a thin smaller tine. These two species differ in the relative shape of both prostomia and in the blade in furcates. Thus, in P. imajimai n. sp. the prostomium is wider than long, and the dorsal keel of furcates blade is curved with a subdistal notch. In P.backstromi , on the contrary, the prostomium is longer than wide, and the dorsal keel of furcates is straight with a subdistal hump. Since furcates are very short, it would make them barely exposed to fractures, and their shape is diagnostic. In the holotype of P. imajimai n. sp., there are bifid curved notospines from chaetiger 6 (on the right side) though over the left side, most are broken and there is one left on chaetiger 9. This could explain the finding by Imajima (1987: 158).

Figures

FIG. 6. — Pseudexogone imajimai n. sp.: A, anterior end in dorsal view; B, anterior end in lateral view; C, first parapodium, anterior view; D, 25th parapodium, anterior view; E, same, pectinate chaetae; F, G, notospines; H, posterior end in dorsal view (redrawn and rearranged from the original illustrations by Imajima 1987; without scale bars in the original). Abbreviations: see p. 536.

FIG. 7. — Pseudexogone imajimai n. sp.: A, anterior end in dorsal view,palps are free but collapsed after dehydration; B, same, lateral view; C, same in oblique view, showing ciliated bands; D, same, close up to show nuchal organ; E, anterior chaetiger with furcates; F, same, close up of furcate, pectinate, and base of denticulate capillary; G, posterior parapodium; H, posterior end, dorsal view. Abbreviations: see p. 536. Scale bars: A, B, H, 20 µm; C, 11.1 µm; D, 5.5 µm; E, 6.66 µm; F, 2.85 µm; G, 9.09 µm.