Cratenemertea

Suborder CRATENEMERTEA

Morphological circumscription

As given by Chernyshev (2003 a, 2011a), members are characterised by having a wickerwork rhynchocoel wall and a single vascular plug; the cerebral organs are located in the brain region, with the possible exception of Cratenemertes amboinensis (Bürger, 1890).

Clade definition Monostiliferans that are more closely related to Cratenemertes amboinensis (Bürger, 1890) than to Amphiporus lactifloreus (Johnston, 1828). For practical purposes, however, Cratenemertes amboinensis will have to be proxied by other cratenemertids for which sequences are available, such as Nipponnemertes pulchra (Johnston, 1837) (cf. Andrade et al. 2012), and until molecular data become available for the former. Confusion occurs, therefore, if Cratenemertes amboinensis does not nest within the clade represented by such proxies, once the former is placed in a molecular phylogenetic context.

Constituent subtaxa Currently, 28 species in seven genera are recognized in the Cratenemertea (Table 1). * Collarenemertes Chernyshev, 1993 shares the type species Amphiporus bimaculatus Coe, 1901.

Remarks

Thollesson & Norenburg (2003) performed an extensive molecular phylogenetic analysis covering all the major nemertean subtaxa—Palaeonemertea, Heteronemertea, Monostilifera, Polystilifera (Pelagica and Reptantia), and Bdellonemertea—based on 28S rRNA, histone H 3, 16S rRNA, and COI sequences. Within Monostilifera, Thollesson & Norenburg (2003) introduced two clade names, Cratenemertea and Distromatonemertea, stating that “We believe that effective communication is served by providing a name to the sister clade of cratenemertids, which we propose renaming as Cratenemertea...” (op. cit., p. 414). The problem is that the taxon concept of Cratenemerteacomprising “cratenemertids”—is ambiguous. Cratenemertea can be either the Cratenemertidae per se, or something like ‘hoplonemerteans with the rhynchocoel wall composed of a wickerwork of interwoven longitudinal and circular muscle fibres,’ as the name indicates (the Latin crâtis, meaning ‘wickerwork’).

One of the most remarkable recent findings in hoplonemertean systematics is that Uniporus Brinkmann, 1914 – 1915 has proven to belong actually in Monostilifera (Chernyshev & Polyakova 2018b), rather than in Reptantia (Polystilifera), as has long been thought (Brinkmann 1914 –1915, 1917; Stiasny-Wijnhoff 1934; Härlin & Sundberg 1995). The structure of the stylet apparatus is not known for any of four congeners [U. acutocaudatus Brinkmann, 1914 –1915; U. alisae Chernyshev & Polyakova, 2018b; U. borealis (Punnett, 1901a); U. hyalinus Brinkmann, 1914 –1915], as the proboscis tends to be lost while dredging. Uniporus was placed in Reptantia due to anatomical similarities to other reptantians, such as the interwoven rhynchocoel wall and the rhynchocoelic lateral diverticula; the latter is branched in Uniporus and unique among Reptantia. Chernyshev & Polyakova (2018b) carefully avoided the possibility of DNA contamination for their phylogenetic analysis, in which U. alisae was nested among Cratenemertea.

Another recent finding that might be of significance to cratenemertean systematics is that two of the planktonic forms, Achoronemertes scoresbyi (Wheeler, 1934) and Korotkevitschia pelagica (Korotkevitsch, 1961), may be juveniles of benthic adults (Chernyshev & Polyakova 2019). Among the yields of a vertical tow net from a depth of 250 m to the surface in the Northwest Pacific off Simusir Island in the Kuril Islands, Chernyshev & Polyakova (2019) found a planktonic nemertean. It was 6–7 mm in length, having transverse lateral cephalic furrows without secondary grooves, an orange-pink gut, no eyes, small cerebral organs with bifurcated canal, an interwoven rhynchocoel wall, and remarkably, neither the stylet apparatus nor bulb musculature (ibid, fig. 6A, F, G). In many respects, this planktonic specimen ‘ Cratenemertea 25DS’ resembles Achoronemertes scoresbyi and Korotkevitschia pelagica. Chernyshev & Polyakova (2019) suggested that Cratenemertea 25DS represented a swimming juvenile stage of a benthic species with a 0.4% difference in COI barcode sequence between a benthic adult specimen, ‘ Cratenemertidae sp. IZ 45644’ (Kvist et al. 2015). The latter was collected in the Sea of Okhotsk off Iturup Island in the Kuril Islands from depths of 183–213 m; it was 7.5 cm in body length, having well-developed lateral cephalic furrows with numerous secondary grooves (Chernyshev & Polyakova 2019, fig. 6B), no eyes (op. cit., fig. 6C), an interwoven rhynchocoel wall (op. cit., fig. 6K), and a stylet apparatus with doubled central stylets on the basis (op. cit., fig. 6D). The implications are that i) Korotkevitschiidae and Cratenemertidae may be junior synonyms of Uniporidae; and ii) Achoronemertes and Korotkevitschia may be junior synonyms of Nipponnemertes. Proper application of family names requires examining the placement of type species of each family in a molecular phylogeny, but no sequence data are yet available for Cratenemertes amboinensis (Bürger, 1890), Korotkevitschia pelagica (Korotkevitsch, 1961), or Uniporus hyalinus Brinkmann, 1914 –1915.

Nomenclatural notes

Chernyshev (2003a) argued that the Cratenemertea of Thollesson & Norenburg (2003) was nomenclaturally unavailable because it was “accompanied with neither a diagnosis, nor even a single synapomorphy; therefore, it should be referred to as nom. nudum” (op. cit., p. S63). In fact, however, Article 13 of the Code (ICZN 1999) is not relevant to names at the ranks of phylum, class, order, etc., as Article 1.2.2 provides that “Articles 1–4, 7–10, 11.1–11.3, 14, 27, 28 and 32.5.2.5 also regulate names of taxa at ranks above the family group”. That is, higher taxon names above the family group are supposed to be available even if they were established without being “accompanied by a description or definition that states in words characters that are purported to differentiate the taxon” as stipulated in Article 13.1.1 of the Code (ICZN 1999), as long as these names satisfy other articles given in Article 1.2.2., i.e., Articles 21–22 (Date of Publication) and 50–51 (Authorship) of the Code (ICZN 1999). There is thus no rule to be followed to determine the authorship of the name Cratenemertea under the current Code (ICZN 1999). For this reason, in this paper, author names and date of publication have intentionally been omitted for names at ranks above the family group.

The name Nipponnemertes was originally proposed by Friedrich (1968: 34) for the following seven nominal species without type designation: Amphiporus bergendali Gering, 1912; Amphiporus drepanophoroides Griffin, 1898; Amphiporus pacificus Coe, 1905; Amphiporus occidentalis Coe, 1905; Amphiporus punctatulus Coe, 1905; Cratenemertes danae Friedrich, 1957; and Cratenemertes madagascarensis Kirsteuer, 1965. In accordance with Article 13.3 of the Code (ICZN 1999), Nipponnemertes had been nomenclaturally unavailable before Gibson & Crandall (1989: 463) made it available by giving a diagnosis and designating Amphiporus drepanophoroides Griffin, 1898 as the type species. Crandall (2001: 106) invoked Articles 69.1 and 69.1.1 of the Code (ICZN 1999), argu-ing Amphiporus punctatulus Coe, 1905 be the type species for Nipponnemertes. Crandall’s (2001: 106) “subsequent designation” is invalid as a nomenclatural act, because the type fixation had been already done in Gibson & Crandall (1989). The authorship of Nipponnemertes is thus ascribed to Gibson & Crandall (1989).

Cosmocephala Stimpson, 1857 was established for Cosmocephala beringiana Stimpson, 1857 and Cosmocephala japonica Stimpson, 1857, without type designation (Stimpson 1857). These two species are currently placed in Amphiporus (Appendix Table); therefore, Cosmocephala is currently invalid (Gibson 1995). One of these, Amphiporus japonica resembles Nipponnemertes punctatula (Coe, 1905) (Iwata 1952; Kajihara 2017). If validated, the name Cosmocephala Stimpson, 1857 is to compete for nomenclatural precedence with Nipponnemertes Gibson & Crandall, 1989.

A proposal to conserve Valdivianemertes Stiasny-Wijnhoff, 1923 against the senior synonym Akrostomum Grube, 1840 (Crandall 1993 b, 1994) has been rejected in Opinion 2016 (ICZN 2003). The name Valdivianemertes is thus invalid.