Ilyphagus bythincola Chamberlin, 1919

Salazar-Vallejo, Sergio I., 2012, Revision of Ilyphagus Chamberlin, 1919 (Polychaeta, Flabelligeridae), ZooKeys 190, pp. 1-19 : 5-8

Ilyphagus bythincola Chamberlin, 1919 Figures 12

Ilyphagus bythincola Chamberlin, 1919:402-403, pl. 69, Figs 4-9; Hartman 1960:131; Levenstein 1961b:136; Fauchald 1972:224-225.

Ilyphagus ascendens Chamberlin, 1919:403-404; Hartman 1960:131-132.

Type material.

Eastern Pacific Ocean. Holotype of Ilyphagus bythincola (USNM 19748), one paratype (USNM 19384), off Mexico, R/V Albatross, Stat. 3415 (14°46'N, 98°40'W), 1879 fathoms (3438.6 m), 10 Apr. 1891 (paratype two fragments; may belong to the same organism, not measured). Holotype of Ilyphagus ascendens (USNM 19735), off Hood Island, Galapagos Islands, 12 miles (19.3 km) SE Ripple Point, R/VAlbatross, Stat. 4649 (01°35'S, 89°30'W), 633 fathoms (1158.4 m), 10 Nov. 1904.

Additional material.

Eastern Pacific Ocean. One specimen (USNM 49080), off Mexico, R/V Albatross, Stat. 3415 (14°46'N, 98°40'W), 1879 fathoms (3438.6 m), 10 Apr. 1891 complete, K. Fauchald, id. (24 mm long, 9 mm wide, cephalic cage +17 mm long, 22 chaetigers). Two complete specimens (SIORAS-unumb.), R/V Akademik Kurchatov, Stat. 294 (08°23'S, 81°00'W), off Nazca Ridge, 6200-6240 m, Sigsbee trawl, 31 Oct./1 Nov. 1968 (43-75 mm long, 5-8 mm wide, cephalic cage 0-13 mm long (broken), 22-23 chaetigers). Many specimens (SIORAS unnumbered), off Northern Peru, R/V Akademik Kurchatov, Stat. 301 (05°51.7'S, 81°48.8'W), 5300 m, 4 Nov. 1968 (best specimen 48 mm long, 19 mm wide, cephalic cage 21 mm long, 22 chaetigers).

Description.

Holotype of Ilyphagus bythincola (USNM-19748) damaged, ovoid, body wall broken by compression, depressed (Fig. 1A); 48 mm long, 26 mm wide, cephalic cage chaetae broken, 21 chaetigers. Holotype of Ilyphagus ascendens (USNM 19735) with body digitate, ovoid, pointed anteriorly, rounded posteriorly (Fig. 1E); 55 mm long, 20 mm wide, cephalic cage 36 mm long (tips broken), 24 chaetigers. Body surface densely papillated, with fine sediment particles trapped between p apillae (Figs 1B, F, 2A); posterior region with longer papillae; each papillae filiform, most with tips pale, some with black tips.

Cephalic cage chaetae broken; scars present in chaetiger 1, short dorsal transverse row with 8-10 chaetal scars per side (Fig. 1C, D); holotype of Ilyphagus ascendens with chaetae at least twice as long as body width, perhaps as long as body length (Fig. 1E). Chaetiger 1 (and perhaps 2) involved in the cephalic cage, dorsal; most cephalic cage chaetae broken from the base, 8 notochaetae; neurochaetae lateral, 7 per side, arranged in a short row, transverse (oblique in Ilyphagus ascendens , Fig. 1G, H). Non-type specimens with chaetae as long as body length (Fig. 2A). Anterior dorsal margin of first chaetiger papillated. Anterior chaetigers without larger papillae. Anterior end observed in a non-type specimen (SIORAS).

Cephalic hood short, not exposed, margin smooth. Prostomium low, as dark as surrounding region; eyes not seen. Caruncle not seen. Palps large, thick, shorter than branchiae; palp lobes reduced. Lateral lips well developed; ventral and dorsal lips reduced.

Branchiae thick, digitate, in different sizes, sessile on branchial plate, in a horse-shoe pattern (Fig. 2B), one superior single row with four thick larger filaments, and six pairs of lateral filaments arranged in irregular double rows. Largest branchiae longer than palps. Nephridial lobes in branchial plate not seen. Chaetal transition from cephalic cage to body chaetae abrupt (most chaetae broken); first aristate neurospines in chaetiger 3. Gonopodial lobes not seen in holotype; non-type specimens with dark, digitate, small lobes in chaetiger 5 (Fig. 2C), or in chaetigers 5 and 6.

Parapodia slightly developed (Fig. 2D); notopodia without prominent lobe, chaetae emerge from the body wall. Median neuropodia ventrolateral, projected ridges. No additional longer papillae associated with chaetal lobes. Noto- and neuropodia lateroventral, very close to each other.

Median notochaetae arranged in oblique rows, as long as one-fourth or one-fifth of body width, 2-3 per bundle; all notochaetae thin, multiarticulated capillaries, with short articles basally, median-sized medially, longer distally (Fig. 2E). Neurochaetae anchylosed aristate spines, 6-8 per bundle; broken, with short anchylosed articles, arranged in an oblique line. Other chaetal features not examined in holotype. Paratype with noto- and neurochaetae broken; non-type specimens with slightly curved, hyaline, smooth tips (Fig. 2F).

Posterior end rounded; pygidium with anus ventral, without anal cirri.

Remarks.

The original body shape was digitate rather than sole-like; this distortion was the result of the sudden change of pressure, especially because of the sediment load in the dredge over its body. The damage resulted in the loss of all cephalic cage chaetae, but chaetal scars are visible in the corresponding position. This damage further compressed the body breaking its wall, and making it appear flat. Other specimens (SIORAS) are long, anteriorly swollen, posteriorly tapered; this is more pronounced among juveniles, and although the number of chaetigers is fixed early in development, counting depends on the presence of chaetae and they are often broken off.

Ilyphagus bythincola Chamberlin, 1919 and Ilyphagus ascendens Chamberlin, 1919 are herein regarded as synonyms. The latter has cephalic cage chaetae in an oblique row, rather than in a transverse one as in Ilyphagus bythincola . This displacement results in a larger area between chaetae and the anterior margin of chaetiger 1. However, because the anterior end of Ilyphagus bythincola is severely damaged, and because other body features are similar, there are insufficientdifferences to keep them separate as distinct species.

Ilyphagus bythincola resembles Ilyphagus hirsutus Monro, 1937, but they differ in the relative number of neurochaetae in chaetiger 1 and in the relative smoothness of neurochaetal tips. Thus, Ilyphagus bythincola has about 8 neurochaetae in the first chaetiger, whereas there are 3-4 in Ilyphagus hirsutus , and in the former, the neurochaetal tips are mostly smooth or barely hirsute, whereas in Ilyphagus hirsutus neurochaetae are markedly hirsute. At the same time, Ilyphagus bythincola resembles Ilyphagus wyvillei (McIntosh, 1885), but they differ in the relative number of neurochaetae in chaetiger 1 and in the number of branchial filaments. Thus, in Ilyphagus bythincola there are 8 neurochaetae per side and about 40 branchial filaments, whereas in Ilyphagus wyvillei there are 10-12 neurochaetae and about 16 branchial filaments.

Distribution.

Apparently restricted to deep water off southwestern Mexico, to Galapagos and Peru, in 1260-6000 m. There have been two other records for this abyssal species. Levenstein (1961b: 137, map), recorded it from the Java Trough, and Kirkegaard (1956:70, Fig. 9) recorded it from the Sunda Trench. The former (ZIRAS-9451) was based on a specimen broken in two, much damaged, collected in 6850 m depth (RV Vitjaz, Stat. 4535, 10 º 08'S, 107 º 55'E). It resembles Ilyphagus bythincola but better specimens are needed for a complete identification or description. On the other hand, the specimen from the Sunda Trench belongs to an undescribed species in Bradabyssa , and will be described elsewhere.