Oxyonchus ( Filipjev, 1927 ) De Coninck & Schuurmans Stekhoven, 1933

De Souza, João V. & Maria, Tatiana F., 2023, Taxonomic review of Thoracostomopsidae (Nematoda, Enoplida): state of the art, list of valid species and dichotomous keys, Zootaxa 5361 (4), pp. 463-496 : 482

Genus Oxyonchus ( Filipjev, 1927) De Coninck & Schuurmans Stekhoven, 1933

First, Oxyonchus was created by Filipjev (1927) as a new subgenus of Enoplolaimus including Enoplolaimus (Oxyonchus) hamatus Steiner, 1916 , Enoplolaimus (Oxyonchus) acantholaimus Ssaweljev, 1912 ), Enoplolaimus (Oxyonchus) australis de Man, 1904 and Enoplolaimus (Oxyonchus) dentatus Ditlevsen, 1918 . Filipjev (1927) also described Enoplolaimus (Oxyonchus) polaris Filipjev, 1927 and synonymized this species with Enoplolaimus hamatus sensu Ditlevsen, 1926 . Oxyonchus differs from Enoplolaimus by the buccal cavity bearing two well-developed teeth and onchia and De Coninck & Schuurmans Stekhoven (1933) raised it to genus level bringing to it most of the Filipjev`s species, except Enoplolaimus (Oxyonchus) polaris Filipjev, 1927 which was considered as a synonym of O. dentatus , but this synonym was invalidated later by Wieser (1953). Oxyonchus stateni Allgén, 1930 and Enoplolaimus campbelli Allgén, 1932 were also considered a synonym of O. australis by De Coninck & Schuurmans Stekhoven (1933). These authors transferred Enoplolaimus dubius Filipjev, 1918 , Enoplolaimus elegans Schulz, 1932 and Enoplolaimus crassidens Ditlevsen, 1930 to Oxyonchus and provided a key to the seven known species of Oxyonchus . Wieser (1953) revised the genus confirming the synonym of O. stateni with O. australis previously indicated by De Coninck & Schuurmans Stekhoven (1933) and pointed out some problems with the validity of Filipjev (1927) species, for example O. polaris was considered as valid species and Oxyonchus elegans (Schulz, 1926) De Coninck & Schuurmans, 1933 and Oxyonchus crassidens ( Ditlevsen, 1930) De Coninck & Schuurmans, 1933 were suggested to be synonyms of O. dentatus . In this case, seven species were included in Wieser’ keys based on the length of the cephalic setae related to head diameter. Several species were added to this genus in the middle of last century and Gerlach & Riemann (1974) provided a species list of 16 species, in some cases the species epithet was readequated to comply with the rules of Latin grammar. Posteriorly, Keppner (1988) described Oxyonchus striatus Keppner, 1988 and considered Oxyonchus brachysetosus Allgén, 1959 , Oxyonchus crassicolis Allgén, 1959 , Oxyonchus macrodon Allgén, 1959 , Oxyonchus notodentatus Allgén, 1959 , and Oxyonchus parastateni Allgén, 1959 as doubtful species because their description and illustrations were insufficiently adequate to distinguish these species from species previously described. Then, a key for 12 valid species was proposed based on cuticle ornamentation, spicules length and presence of apophysis in the gubernaculum ( Keppner, 1988). Later, Nicholas (2004) updated the key given by Keppner (1988) including his new species ( Oxyonchus evelynae Nicholas, 2004 and Oxyonchus longisetosus Nicholas, 2004 ) and agreed with the doubtful species presented in Keppner (1988). Recently, three new species were described by Fadeeva et al. (2012): O. nicholasi , Oxyonchus orientalis Fadeeva, Mordukhovic & Zograf, 2012 and Oxyonchus sakhalinensis Fadeeva, Mordukhovic & Zograf, 2012 . Here, we consider O. dubius as species inquirenda because its original description is based on a single female and we do not find any male redescription although it was included in the keys provided by Keppner (1988) and Nicholas (2004).

Diagnosis from Smol et al. (2014): Lips high or low. Cuticle smooth or striated. Cephalic organ present or absent. Mandibles well-developed, arch-shaped, rods connected by broad transverse bar with claws, denticles can be present at the inner surface of the mandibular plate. Teeth unequal; two large ventrosublateral teeth that extend to the anterior end of the mandibles. Dorsal tooth small. Spicules short (one to two cloacal body diameters). Precloacal supplement present or absent. Gubernaculum with or without apophysis. Marine.