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).