Pseudexogone backstromi Augener, 1922

(Figs 1; 2)

Pseudexogone backstromi Augener, 1922:192-194, textfig.7ac, only pl. 4-4b. — Rozbaczylo 1985: 75 (ref., distr.).

TYPE MATERIAL. — The type specimen of the type species was not deposited in Sweden, although it was collected during a Swedish expediton, nor in Hamburg, where Augener lived. Museum curators were unable to find it, therefore the type and only specimen is being regarded as lost.

TYPE LOCALITY. — Másatierra (Robinson Crusoe) Island, Juan Fernández Archipelago, Chile. It was collected from a dead hydroid theca in a sample of calcareous algae, dredged in 30-45 m depth.

MATERIAL EXAMINED. — Hawaii. Oahu, Off Barber’s Point, stn ZR1, 21°16’53.5”N, 158°01’30.3”W, I.2001, 63.7 m, 1 specimen (WRRC unnumb.). — Off Mamala, stn 26, 21°17.35’N, 157°56’49.0”W, 32.9 m, VIII.2001, 1 specimen (WRRC unnumb.). — Stn 27, 21°17’38.2”N, 157°55’31.8”W, 20.1 m, VIII.2001, 2 specimens (WRRC unnumb.). — Off Mokapu, stn B2R2 (no coordinates), 32.3 m, III.1998, 2 specimens (WRRC unnumb.). — Stn B2R3 (no coordinates), 32.3 m, III.1998, 2 specimens (WRRC unnumb.). — Off Waianae, stn ZWR6 (no coordinates), 36.6 m, X.1996, 1 specimen (WRRC unnumb.). — Stn ZWR2 (no coordinates), 33 m, V.1998, 2 specimens (one WRRC unnumb., 1 gold-coated in ECOSUR). — Stn ZWR1 (no coordinates), 36.3 m, V.2001, 1 specimen (WRRC unnumb.).

DISTRIBUTION. — Previously only known from the type locality in sublittoral rocky bottoms; the additional specimens were collected in Hawaii. They are regarded as belonging to P. backstromi, which implies a very wide distribution from the southeastern Pacific to the central north Pacific.The tiny size of P. backstromi and the presence of this species with sessile invertebrates, which may be widely distributed as foulers or as members of drifting communities, may explain this wide distribution.

DESCRIPTION OF THE TYPE SPECIMEN

(based on the original description and drawings) Body smooth, cylindrical, tapering towards anterior and posterior ends (Fig. 1A); 9 mm long, 0.05 mm wide, c. 64 chaetigers. Prostomium subtriangular; palps simple, almost completely free from each other; one pair of large dorsal eyes; three antennae, the right lateral one placed ahead of the right eye. Two pairs of tentacular cirri, apparently the dorsal one longer than the ventral one (Fig. 1B). Parapodia with dorsal and ventral cirri similar to each other. Median and posterior chaetigers with a single bidentate sigmoid notospine; each spine with large proximal tooth and smaller distal tooth (Fig. 1C). Neurochaetae include finely denticulate capillaries (Fig. 1D). Posterior end with one achaetous segment; pygidium with two ventrolateral anal cirri, each as long as pygidium plus achaetous segment (Fig. 1E). Pharynx everted with margin smooth.

DESCRIPTION OF NON- TYPE SPECIMENS

Complete non-type specimens transparent, long, cylindrical, rarely yellowish, slightly wider anteriorly, tapering posteriorly (Fig. 2A), 5.0- 7.5 mm long, 0.12-0.30 mm wide, with 41-61 chaetigers.

Prostomium subtriangular, about as long as wide, slightly narrower than peristomium. Three cirriform antennae of about the same size; lateral antennae located in the center of prostomium, median antenna placed on the posterior prostomial margin. Eyes dark brown, visible in all specimens. Palps anteriorly rounded, tapering, free from each other distally (Fig. 2B); ventrolateral papillae cirriform, as long as antennae, placed half way along the palp length. Two pairs of cirriform tentacular cirri, equal-sized, most specimens with dorsal cirri longer, few with dorsal cirri slightly shorter, with a small median knob; ventral pair placed ventrally, appear smaller than dorsal one (Fig. 2C). Ciliary bundles eroded.

Parapodia uniramous in chaetigers 1-6, thereafter biramous. Parapodial cirri cirriform throughout body. Anterior parapodia with slightly emergent bidentate notospines, denticulate capillaries and furcates (Fig. 2D). Dorsal cirri digitate, ventral cirri cirriform; dorsal ones slightly longer. Notopodia with large sigmoid bidentate spines starting on chaetiger 7 (in smaller specimens on chaetiger 5, rarely on chaetiger 8), continued to the last chaetiger. Neuropodia include furcates in anterior chaetigers, pectinates and denticu- late capillaries, mostly broken. In median and posterior chaetigers, about two pectinates and two denticulate capillaries per bundle. Furcates (Fig. 2E) with unequal tines, longer tine with a short flange, smaller tine cylindrical. Median chaetigers (Fig. 2F) without furcates; notospines emerge slightly. Notospines with larger subdistal tooth (Fig. 2G).

Posterior end tapering (Fig. 2H), one achaetous segment visible in about half the specimens. Pygidium conical, rounded, with two lateral anal cirri (right cirrus lost). Anus terminal. Pharynx everted in few specimens, as long as the prostomium and palps, tubular, with smooth margin; ventral pharyngeal organ enlarged (Fig. 2C). Brain lobes extended posteriorly to chaetiger 4. Oocytes present in larger specimens in chaetigers 15-40, measure 40-45 µm in diameter.

VARIATION

Notospines start on chaetigers 6-8 (5 in small specimens). Undamaged parapodia had one furcate, and two of each: pectinates and denticulate capillaries.

REMARKS

Pseudexogone backstromi resembles P.imajimai n. sp. and P. williamsae n. sp. by having well-defined eyes; they differ because of the type of furcates. In P. williamsae n. sp., they have a thicker, smaller tine while in the other two, they are thinner. The main differences between P. backstromi and P. imajimai n. sp. are on the relative shape of both prostomia and the larger tine of furcates. Thus, in P. backstromi the prostomium is about as long as wide, and the larger tine in furcates has a dorsal keel straight with a subdistal hump. On the contrary, in P. imajimai n. sp. the prostomium is markedly wider than long, and the larger tine in furcates has a dorsal keel curved with a subdistal notch. Furcates are very short and thus would be barely exposed to sediment abrasion or fractures, rendering their shape useful as a diagnostic feature.

Because the specimens were collected in Hawaii, far away from the type locality, they should not be employed for establishing a neotype. Further, the specimens from Hawaii have faintly pigmented eyes, whereas in the type specimen they were very large. This difference may be due to sexual maturity since the type specimen was about 9 mm long, while these specimens were up to 7.5 mm long, or alternatively due to preservation and storing in ethanol. The start of bidentate notospines was not specified in the original description, but they start in chaetigers 6-8 in all Pseudexogone species, so that this feature is of very little use. A comparison between the original illustrations, the specimens and the SEM photos indicates a very close resemblance. Although the distribution is very wide, these pilargids may occupy a large geographic area with the same ecological conditions.