Therochaeta pacifica Fauchald 1972

Therochaeta pacifica Fauchald, 1972

(Fig. 6)

Therochaeta pacifica Fauchald, 1972: 231-233, pl. 49, figs a-c.

Stylarioides collarifer Ehlers?. – Moore 1923: 221.

TYPE MATERIAL. — Eastern Pacific Ocean. Holotype (LACM-AHF 1032) and paratype (LACM-AHF 1033), R/V Velero III, Stat. 7047 (32°54’21”N, 117°29’33”W), 763 m, 7.V.1960, green sand, mud (8.5 mm long, 2.5 mm wide, cephalic cage 3 mm long, 20 chaetigers).

ADDITIONAL MATERIAL. — Eastern Pacific Ocean. One specimen (USNM 17386), R/V Albatross, Stat. 4351, off Point Loma Lighthouse (fide Moore 1923), or vicinity of San Diego, California (tag), 761-878 m, 14.III.1904, J.P. Moore. One specimen (LACM-AHF 4917), posterior fragment, R/V Ancona, Stat. 06a (44°40.7’N, 125°10.0’W), 1000 m, 5.VI.1965 (4 mm long, 1 mm wide, 11 chaetigers).

DISTRIBUTION. — Southern California to the Western Baja California, Mexico, in 600-1000 m.

DESCRIPTION

Holotype (LACM-AHF 1032) complete, previously dissected antero-ventrally, eversible anterior end exposed (Fig. 6A). Body whitish, depressed anteriorly, swollen posteriorly, distally tapered to form short cauda; 9 mm long, 2.5 mm wide, cephalic cage 6 mm long, 23 chaetigers.Tunic thick, adhering mostly white or transparent sediment grains and black sediment particles resulting in mottled appearance. Body papillae long, capitate, making large stiff tubercles, laterally continuous in chaetigers 1-2, discontinuous in following chaetigers, 3-4 in median chaetigers, larger dorsally (Fig. 6B); cauda without large tubercles.

Cephalic tube not exposed. Prostomium conical, low; eyes not seen. Caruncle short, not reaching posterior branchial plate margin. Palps fallen off, one left in vial is cylindrical, curled, whitish. Lateral and dorsal lips fused, well-developed; ventral lip reduced.

Branchiae mostly lost, short, digitate, sessile on branchial plate, arranged in three-four concentric rows with about 60 filaments (Fig. 6C), middorsal branchial scars larger. Size relationships with palps and among themselves, unknown. Nephridial lobes in branchial plate not seen.

Cephalic cage chaetae as long as 2/3 body length or more than twice as long as body width. Chaetigers 1-2 involved in cephalic cage; chaetae arranged in short lateral groups; chaetigers 1-2 with two noto- and one-two neurochaetae per bundle.

Anterior dorsal margin of first chaetiger triangular, papillate. Anterior chaetigers with long papillae in chaetal lobes. Chaetiger 2 longer than chaetiger 1, constricted posteriorly; chaetiger 3 shorter, as wide as chaetiger 2. Chaetal transition from cephalic cage chaetae to body chaetae gradual; pseudocompound neurochaetae in chaetigers 4-7, falcate neurohooks from chaetiger 8. Gonopodial lobes not seen.

Parapodia poorly-developed, lateral, chaetae emerging from the body wall; median neuropodia ventrolateral. Median noto- and neuropodia with large rounded interramal tubercles. Median notochaetae arranged in short oblique or transverse series; all notochaetae multiarticulated capillaries; articles medium-sized, becoming longer medial- and distally; notochaetae as long as ½–1/3 body width, one-two per bundle. Neurochaetae multiarticulated capillaries in chaetigers 1-3, two pseudocompound neurohooks per bundle in chaetigers 4-7, decreasing in size posteriorly, each with handle slightly expanded distally (Fig. 6D), with medium-sized feebly-defined articles and blade hyaline; falcate neurohooks from chaetiger 8, arranged in transverse series, two-three per bundle (Fig. 6E), decreasing to two in last two chaetigers, each falcate neurohooks with medium-sized anchylosed articles, slightly smaller basal- and subdistally, tips hyaline.

Posterior end tapered to blunt cone; pygidium with anus terminal, without anal cirri.

REMARKS

Therochaeta pacifica is very similar to T. collarifera but they differ in two main features: the relative shape of the region formed by chaetigers 1 and 2, and the relative size of the sediment tubercles in chaetigers 3 and 4. Thus, in T. pacifica the first two anterior chaetigers take a thicker and slightly depressed form, whereas in T. collarifera this region is thinner and swollen. Further, in T. pacifica the sediment tubercles in chaetigers 3 and 4 are not markedly projected as is the case in T. collarifera, a feature that as stated above is visible even in the smallest juveniles (3.5 mm long). The discrepancies between this redescription and the original one, in relation to the anterior chaetigers and their neurochaetae, may be explained by the fact that first two neuropodia are anteriorly displaced, such that to define their number the specimens must be seen laterally.