Paramesacanthion Wieser, 1953

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 : 484-485

Genus Paramesacanthion Wieser, 1953

The genus is defined by the extreme anterior position of the cephalic setae and the double-jointed spicules and it was erected by Wieser (1953) including the original description of two species: Paramesacanthion inaequale Wieser, 1953 and Paramesacanthion estridium Wieser, 1953 . This author also transferred three other species to the new genus: Paramesacanthion oxycephalum ( Ditlevsen, 1926) Wieser, 1953 , Paramesacanthion tricuspis ( Schuurmans Stekhoven, 1950) Wieser, 1953 and Paramesacanthion klugei ( Filipjev, 1927) Wieser, 1953 . The first species was previously described as Enoplolaimus and the other latter two species as Mesacanthion . In the same manuscript, an identification key for the five species known at the time was provided. In addition, Enoplolaimus brevilabiatus Schuurmans Stekhoven, 1946 and Enoplolaimus microsetosus Allgén, 1932 were suggested as a synonym of P. tricuspis , but Warwick (1970b) was not convinced by the synonym and considered both species as inquirendae because they present only female and juvenile specimens. Later, other new species were described for the genus: Paramesacanthion allgeni Mawson,1958 ; Paramesacanthion paracanthospiculum Allgén,1959 ; Paramesacanthion paroxycephalus Allgén, 1959 and Paramesacanthion barbae Inglis, 1968 . In the seventies, Warwick (1970b) described two species ( Paramesacanthion hirsutus Warwick, 1970b and Paramesacanthion marei Warwick, 1970b) and provided a taxonomic key with nine species using the form of the male reproductive apparatus and the arrangement of the bristles at the posterior end of the body as differential features. In addition, Paramesacanthion catellus Boucher, 1970 and Paramesacanthion truncum Vitiello, 1971 were added to the genus meanwhile P. paroxycephalus and P. paracanthospiculum were invalidated by Boucher (1970). Posteriorly, Paramesacanthion abyssorum Bussau, 1993 and Paramesacanthion forceps Bussau, 1993 were described and the last species P. vietnamicum was described in the beginning of the current century. We modified and updated the taxonomic key of Warwick (1970b) using precloacal supplements, gubernaculum and spicules length as distinguishing characters.

Diagnosis from Smol et al. (2014): Outer labial and cephalic setae in front of the anterior end of cephalic capsule, about the level of anterior end of mandibles. Subcephalic setae at middle of cephalic capsule. Mandibles arch-shaped and with claws, consisting of two pieces united by an anterior bar only. Teeth shorter than mandibles. Spicules consisting of two portions, distal and proximal, articulating with each other. Males with or without precloacal supplement. Sexual dimorphism in the pilosity of the head. Marine.

Number of valid species: 14

1. Paramesacanthion abyssorum Bussau, 1993 ( Peru basin, lapsus abyssus)

2. Paramesacanthion allgeni Mawson, 1958 (Kerguelen Islands)

3. Paramesacanthion barbae Inglis, 1968 ( New Caledonia)

4. Paramesacanthion catellum Boucher, 1970 (Mediterranean, lapsus catellus )

5. Paramesacanthion estridium Wieser, 1953 ( Chile, lapsus estridia)

6. Paramesacanthion forceps Bussau, 1993 ( Peru basin)

7. Paramesacanthion hirsutum Warwick, 1970b (North Sea, lapsus hirsutus )

8. Paramesacanthion inaequale Wieser, 1953 ( Chile, lapsus inaequalis)

9. Paramesacanthion klugei ( Filipjev, 1927) Wieser, 1953 (Barents Sea)— type species

Syn. Enoplolaimus (Mesacanthion) klugei Filipjev, 1927

10. Paramesacanthion marei Warwick, 1970b ( North Sea )

11. Paramesacanthion oxycephalum ( Ditlevsen, 1926) Wieser, 1953 ( Falkland Islands)

Enoplolaimus oxycephalus Ditlevsen, 1926

Mesacanthion oxycephalum Schuurmans Stekhoven, 1946

12. Paramesacanthion tricuspis ( Schuurmans Stekhoven, 1950) Wieser, 1953 (Mediterranean)

Syn. Mesacanthion tricuspis Schuurmans Stekhoven, 1950

13. Paramesacanthion truncum Vitiello, 1971 (Mediterranean, lapsus truncus)

14. Paramesacanthion vietnamicum Nguyen Vu Than & Gagarin, 2013 ( Vietnam Coast)

SPECIES INQUIRENDAE

1. Paramesacanthion brevilabiatum ( Schuurmans Stekhoven, 1946) Wieser, 1953 (Skagerrak)

Syn. Enoplolaimus brevilabiatus Schuurmans Stekhoven, 1946

2. Paramesacanthion microsetosum ( Allgén, 1932) Wieser, 1953 ( Antarctica)

Syn. Enoplolaimus microsetosus Allgén, 1932

Syn. Mesacanthion microsetosum Allgén, 1959

3. Paramesacanthion paracanthospiculum Allgén, 1959 ( South Georgia)

4. Paramesacanthion paroxycephalum Allgén, 1959 ( Antarctica, lapsus paroxycephalus )