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Ilyphagus bythincola Chamberlin, 1919, by original designation.
Body digitiform, rounded at both ends, densely covered by thin, abundant papillae. Body wall thin. Cephalic cage well developed; notochaetae dorsal, arranged in transverse rows. Parapodia biramous, inconspicuous. Notochaetae multiarticulated capillaries; neurochaetae thicker, anchylosed aristate spines.
The type species, Ilyphagus bythincola Chamberlin, 1919, originally described as lacking cephalic cage, in reality has one but the chaetae are broken and only their embedded bases or some chaetal scars are left. The other species described as lacking a cephalic cage, Ilyphagus pluto Chamberlin, 1919, is not a polychaete but an abyssal holothurian which belongs to the synallactid genus Meseres (identified by the late Dr. Cynthia Ahearn, USNM). The inclusion of Ilyphagus pluto allowed a rather broad concept for body shape and chaetal patterns, because this species is rather cylindrical and completely lacks a cephalic cage. Further, Chamberlin (1919) emphasized the lack of large, wide papillae that resemble tubercles, as those found in some species of Brada. Thus, he recognized the difference and restricted the inclusion to those species lacking thick papillae (or tubercles).
Ilyphagus and Bradabyssa Hartman, 1967 are closely allied (Salazar-Vallejo et al. 2008) because they have anchylosed neurospines which are basally annulated (anchylosed short articles) and distally hyaline, tapering into an arista (hence aristate spines). However, there are four main differences between these genera. First, the relative position of the cephalic cage chaetae: they are arranged as transverse dorsal rows in Ilyphagus, whereas in Bradabyssa they are lateral, fewer and smaller. Second, the development of the body wall: most Bradabyssa species have a thick muscular body wall, whereas in Ilyphagus species it is reduced with poorly-developed muscle layers. Third, the branchial features: in Ilyphagus there are a few thick branchial filaments arranged in a horse-shoe pattern, whereas in Bradabyssa they are abundant and medially separated by the caruncle into two half-moon shaped groups. Fourth, and derived from the latter: the species of Bradabyssa have a well-developed caruncle whereas in Ilyphagus it is reduced or absent.
Besides the type species, Ilyphagus bythincola Chamberlin, 1919 from the Eastern Pacific (including Ilyphagus ascendens Chamberlin, 1919), the genus contains Ilyphagus coronatus Monro, 1939 from the Antarctic Ocean, Ilyphagus hirsutus Monro, 1937 from the Central Indian Ocean, and Ilyphagus wyvillei (McIntosh, 1885) from the Antarctic Ocean.
There are several species that have been previously placed in Ilyphagus but belong elsewhere. Thus, Ilyphagus antarcticus Hartman, 1978, Ilyphagus ilyvestis Hartman, 1960 and Ilyphagus minutus Amoureux, 1986 belong in Bradabyssa, Ilyphagus caudatus Rioja, 1963 belongs in Therochaeta Chamberlin, 1919, and Ilyphagus octobranchus Hartman, 1965 belongs in Diplocirrus Haase, 1915 as emphasized elsewhere (Day 1973, Salazar-Vallejo et al. 2008, Salazar-Vallejo and Buzhinskaja 2011). Lastly, as indicated above, Ilyphagus pluto Chamberlin, 1919 is not a polychaete but an holothurian.
The species of this genus have representatives living in deep to very deep sea sediments (1260–7000 m), from the Pacific, Indian and Antarctic Oceans.
Key to species of Ilyphagus Chamberlin, 1919 restricted
- Salazar-Vallejo, S; 2012: Revision of Ilyphagus Chamberlin, 1919 (Polychaeta, Flabelligeridae) ZooKeys, 190: 1-19. doi
- Hartman O (1965) Deep-water benthic polychaetous annelids off New England to Bermuda and other North Atlantic areas. Allan Hancock Foundation, Occasional Papers 28: 1-378.
- Fauchald K (1977) The polychaete worms: Definitions and keys to the orders, families and genera. Natural History Museum of Los Angeles County, Science Series 28: 1-190.
- Chamberlin R (1919) The Annelida Polychaeta of the Albatross Tropical Pacific Expedition, 1891–1905. Memoirs of the Museum of Comparative Zoology, Harvard College 48: 1-514.
- Salazar-Vallejo S, Carrera-Parra L, Fauchald K (2008) Phylogeny of Flabelligeridae (Annelida: Polychaeta). Journal of Systematic Zoology and Evolutionary Research 46: 203-215. doi: 10.1111/j.1439-0469.2008.00464.x
- Day J (1973) New Polychaeta from Beaufort, with a key to all species recorded from North Carolina. National Oceanographic and Atmospheric Administration, Technical Report of the National Marine Fisheries Service, Circular 375: 1-153.
- Salazar-Vallejo S, Buzhinskaja G (2011) Revision of Diplocirrus Haase, 1915, including Bradiella Rullier, 1965 and Diversibranchius Buzhinskaja, 1993 (Polychaeta, Flabelligeridae). ZooKeys 106: 1-45. doi: 10.3897/zookeys.106.795