Thelepus setosus (Quatrefages 1866)

How to Distinguish from Similar Species: Thelepus japonicus has notosetae absent from only about the last 10 segments. Thelepus crispus has uncini on the thoracic segments which curve to form an ellipse.

Lives in a weak tube of encrusted debris under rocks on rocky shores.

Geographical Range: Cosmopolitan: Pacific, Atlantic, Indian oceans, Mediterranean Sea, Red Sea. On our area found from British Columbia to southern CA; Japan

Depth Range: Low intertidal and nearby subtidal.

As with all terebellids, this species is benthic, few if any of the segments are longer than wide, the dorsal surface has no paleae, elytra, or felt, it has no cross-barred capillary setae, it has several long, threadlike, extensile, unbranched filamentlike tentacles closely associated with the mouth and are used in feeding but cannot be retracted into the mouth, the notosetae do not form transverse rows that extend nearly to the dorsal midline, the prostomium does not extend posteriorly as an elongated caruncle, and does not have a distinct caudal region which lacks setae, nor form an operculum. Terebellids have soft, light-colored pads on the ventral surface of all or most thoracic segments; these secrete the mucus which builds the tube. The anterior, "thoracic" portion of the body is often larger than the posterior portion. Thelepus setosus has no proboscislike extension on its peristomium, The uncini, if present in the thoracic segments, are in a single nearly straight row. The species has 3 pairs of slender, unbranched gills which are not obviously coiled, The capillarynotosetae begin on the 2nd or 3rd gill-bearing segment, Notosetae are absent from about the last 40 segments. Length up to 20 cm.

Biology/Natural History: Terebellids build thin, membranous, sand-encrusted tubes in soft mud and sand. They extend their feeding tentacles from the entrance of the tube like spaghetti over the surface of the nearby sediment. The tentacles are extended out from the burrow by ciliary creeping and they can be retraced by muscular action. Mucus and ciliary action on the feeding tentacles transports small organic particles from the sediment to the mouth, especially along a groove along one side. The tube of Thelepus crispus is usually made of fragments of shell, stone, and other detritus, is nearly 1 cm wide, and is cemented to the undersurface of or between rocks. It may leave its tube when disturbed and later build a new one. They circulate water through their burrows. The major phosphagen in these worms is phosphoarginine; phosphotaurocyamine is also present. Commensals include the polychaete scaleworms Halosydna brevisetosa, Hololepidella tuta, and Lepidasthenia sp. There are several genera of terebellids that are difficult to tell apart in the field. The feeding tentacles of Thelepus has threadlike, unbranched tentacles, and gills of similar length. Neoamphitrite and Terebella have dark, branching tentacles, which are longer than the gills which are white. Neoamphitrite has 17 thoracic segments while Terebella has 23 to 28. Members of Family Terebellidae have special pumping vessels at the base of the gills to actively pump blood through them.

“13. THELEPUS ANTARCTICUS.

(Pl. XLV., fig. 6.)

Thelepus antarcticus, Kinberg, J. G. H., 1866 ‘Annulata Nova,’ Oft,. Ak. Forh., p. 345.

It is noteworthy that not one of the three collections of Antarctic Polychaeta recently examined by Prof. Ehlers³ contained this highly characteristic species. Not only is it a true Thelepus, s. str., but it is probably co-specific with the northern type, Thelepus cincinnatus (Fabr.).

One fragment, comprising the head and thirty setigerous segments, upwards of 2 inches long, was dredged off Cape Adare in 8 fathoms, January 17, 1900.

The fore-part of the body or thoracic region comprises approxi­mately the first twenty-five setigerous segments, which differ from the abdominal segments in their greater girth, but not in respect of the parapodial armature.

A mid-thoracic segment measures 7 mm. crosswise by 2 mm. lengthwise ; an abdominal segment, 3 ½ mm. x 4 mm. ; so that an average abdominal segment is equal to half the width and twice the length of a typical thoracic segment.

The entire dorsal surface is characterised by a rich glandular pustulose structure which still preserves a slimy surface. The translucent hypodermal pustules are of varying sizes, irregular in their distribution, larger and more abundant than in the specimens of^. Thelepus cincinnatus which I have examined.

The tori uncinigeri commence on the third setigerous segment, and thereafter the tori and capillary fascicles co-exist to the end of the specimen (and of the body, Kinberg), there being no mutation of segments in regard to the distribution of setae, except that the tori become more pro-eminent in the abdominal segments, while the dorsal capillary fascicles suffer a slight reduction.

The simple filiform branchiae are more numerous on the second than on the third segment, the latter being the first setigerous segment.

The specimen offered certain chaetographical abnormalities. In the nineteenth setigerous segment there was no torus on the left side ; the twenty-fourth and twenty-fifth pharetrae (notopodium and neuro­podium) of the right side had no antimeres on the left side ; finally, the twenty-fifth right capillary fascicle was not subtended by the usual torus.

The uncini are uniserial throughout and exactly resemble those of Thelepus cincinnatus figured by von Marenzeller,¹ being charac­terised by the presence of two rows of accessory uncinules at the vertex and by a rounded process (mucro) at the distal end of the basal portion.

³ Ehlers, E., 1897. ‘Polychaeten.’ Ergebnisse Hamburg. Magelhaensischen Sammelreises, 11^th Lief.

Ehlers, E., 1900. ‘Magellanishe Anneliden der schwedischen Expedition.’ Nachr. Ges. Göttingen, Heft 2, p.206.

Ehlers, E. 1901. ‘Die Anneliden der Sammlung Plate.’ ‘Fauna Chilensis,’ Pd. II., Heft 2, p.251 (Suppt. V., Zool. Jahrb.).

¹Marenzeller, F. von, 1884. ‘Zur Kenntniss der adriatischen Anneliden,’ III. S.-B. Akad. Wien, Bd. 89, p.206.”

(Willey, 1902)