PLECTONEMERTIDAE Gibson, 1900a

Kajihara, Hiroshi, 2021, Higher classification of the Monostilifera (Nemertea: Hoplonemertea), Zootaxa 4920 (2), pp. 151-199 : 158-159

Family PLECTONEMERTIDAE Gibson, 1900a

Morphological circumscription

Oerstediina having a wickerwork rhynchocoel wall; the cephalic vessels protrude into the rhynchocoel as they pass through the cerebral ring to form a pair of vascular plugs; the mid-dorsal vessel does not penetrate the rhynchocoel to form a vascular plug.

Clade definition

Eumonostiliferans that are more closely related to Plectonemertes sinensis Gibson, 1990a than to Oerstedia dorsalis ( Abildgaard, 1806) . Plectonemertes sinensis has hitherto been known only from the original morphological description ( Gibson 1990a). For practical purposes, therefore, it has to be proxied by other plectonemertids for which sequence data are available, such as Argonemertes australiensis ( Dendy, 1892) (cf. Kvist et al. 2014), until Plectonemertes sinensis is eventually sequenced. If Plectonemertes sinensis appears in a different clade, e.g., Cratenemertea , then this taxon will be referred to as either Acteonemertidae Chernyshev, 2005a or Potamonemertidae Chernyshev, 2005a .

Constituent subtaxa Currently, 14 species in eight genera are recognized in Plectonemertidae ( Table 2). Remarks

Crandall’s (2001) morphology-based cladistic analyses of monostiliferous hoplonemerteans with interwoven rhynchocoel musculature suggested that i) the marine genus Plectonemertes Gibson, 1990a , ii) the freshwater genera Campbellonemertes Moore & Gibson, 1972 and Potamonemertes Moore & Gibson, 1973 , and iii) the terrestrial genera Acteonemertes Pantin, 1961 , Antiponemertes Moore & Gibson, 1981 , Argonemertes Moore & Gibson, 1981 , Katechonemertes Moore & Gibson, 1981 , and Leptonemertes Girard, 1893 could each warrant a separate family. Chernyshev (2005a) implemented Crandall’s (2001) results and established Acteonemertidae (for Acteonemertes , Antiponemertes , Argonemertes , Katechonemertes , and Leptonemertes ) and Potamonemertidae (for Campbellonemertes and Potamonemertes ). However, recent molecular phylogenetic results are not necessarily concordant with Crandall’s (2001) morphology-based tree topology. For instance, in Kvist et al. (2014), Argonemertes australiensis ( Dendy, 1892) is more closely related to Potamonemertes percivali Moore & Gibson, 1973 than to Leptonemertes chalicophora ( Graff, 1879) , making Acteonemertidae (or, Acteonemertinae, if placed at the subfamily rank) nonmonophyletic. Implementation of a subfamily classification for Plectonemertidae (in the sense of this paper) thus seems premature.

The type species for the Plectonemertidae , Plectonemertes sinensis Gibson, 1990a (type locality: Hong Kong, subtidal, 21 m depth), has been described morphologically from only a single specimen. Sequence data for Plectonemertes sinensis are a prerequisite for plectonemertid systematics, but attempts to collect more individuals of this species have so far been unsuccessful ( Chernyshev 2011b, 2016).

Nomenclatural note

As discussed by Crandall (2001: 105), the authorship of the generic names Argonemertes and Antiponemertes is ascribed to Moore & Gibson (1985), who made the names nomenclaturally available, although these were first proposed without type designation by Moore & Gibson (1981).