Morphological circumscription
Oerstediina having a two-layered rhynchocoel wall; the cephalic vessels protrude into the rhynchocoel as they pass through cerebral ring; the mid-dorsal vessel does not penetrate the rhynchocoel to form a vascular plug.
Clade definition
Eumonostiliferans that are more closely related to Oerstedia dorsalis (Abildgaard, 1806) than to Plectonemertes sinensis Gibson, 1990a. See the clade definition for Plectonemertidae for the potential problem stemming from the absence of molecular sequence data for Plectonemertes sinensis.
Remarks
Thollesson & Norenburg (2003) were the first to recognize the clade Oerstediidae, as treated herein, which in their molecular phylogeny included Antarctonemertes varvarae Chernyshev, 1999; Nemertellina yamaokai Kajihara et al., 2000; Oerstedia polyorbis Iwata, 1954 [two specimens, erroneously identified as Oerstedia venusta Iwata, 1954 and Oerstedia zebra (Chernyshev, 1993), respectively; see Akhmatova et al. (2012) for the corrected identifications]; ‘ Tetrastemma ’ elegans (Girard, 1852); and ‘ Tetrastemma ’ wilsoni Coe, 1943. Thollesson & Norenburg (2003) referred to this clade as Tetrastemmatidae, probably because it included two species of ‘ Tetrastemma ’; furthermore, Gibson (1982a) had placed Nemertellina Friedrich, 1935b in Tetrastemmatidae, and Bürger (1895) had once placed Oerstedia Quatrefages, 1846 in Tetrastemmatidae. In a subsequent study by Strand & Sundberg (2005a), however, Tetrastemma flavidum Ehrenberg, 1828 (type species of Tetrastemmatidae) and Oerstedia dorsalis (Abildgaard, 1806) (type species of Oerstediidae) appeared in different clades, representing the Amphiporina and Oerstediina, respectively, as treated herein. The name Tetrastemmatidae should thus be used to denote a subclade in the Amphiporina, given the species identification of T. flavidum (see Nomenclatural notes below).
Listed in Table 3 are species that have been confirmed to constitute Oerstediidae by means of molecular phylogenetics. Among the 24 species listed, none is known to possess a mid-dorsal vessel that penetrates to form a single vascular plug, and 18 are known to lack a single vascular plug on the mid-dorsal vessel (see also Table 4; Fig. 2 A–I); the vascular-system anatomy is unknown for the remaining six species. Instead of a vascular plug from the mid-dorsal vessel, these 18 species tend to have cephalic vessels protruding into the rhynchocoel lumen as they pass through the cerebral ring (Fig. 2 F–G; Kajihara et al. 2000: fig, 19; Kajihara et al. 2011: fig. 24), although the protrusions can be subtle in smaller specimens (Fig. 2H, I). In larger species and specimens, the vascular epithelium in these protruding portions may be thickened (Fig. 2A) and specialized (Fig. 2D), so that these could be referred to as ‘vascular plugs’ (perhaps a different terminology for the ‘plugs’ in Oerstediina may prevent confusion: Gibson [pers comm.] suggests ‘monovascular plug’ and ‘bivascular plugs’ respectively, which I now propose). The ‘vascular plug’ (or, vascular protrusion into the rhynchocoel) in hoplonemerteans presumably functions to facilitate material transfer between the blood vascular system and the rhynchocoel (Crandall 1993a). Therefore, small-bodied species may actually lack a vascular plug because diffusion could compensate for its function.
*Lacks a single vascular plug from the mid-dorsal vessel (see Table 4; Fig. 2).
†Judging from the shape of the head, Antarctonemertes unilineata (Joubin, 1910) of Taboada et al. (2018) is different from Tetrastemma unilineatum Joubin, 1910 of Gibson & Tait (1984); furthermore, the latter has been reported i) to lack an accessory lateral nerve and ii) to possess a single vascular plug from the mid-dorsal vessel (see Table 6), each of which suggests a non- Antarctonemertes identity.
‡ “ Prosorhochmus nelsoni ” of Andrade et al. (2012) represents a different species, most likely as the result of mislabelling or a sample mix-up. Among specimens collected in Coquimbo (Chile) on 2 February 2009 by Per Sundberg were NemPhyl 27 (DNA105586 at Museum of Comparative Zoology, Harvard; COI accession number HQ848606) and the three specimens NemBar0842–0844 (COI accession numbers KU840164 – KU840166; identical sequences); NemPhyl 27 was used in Andrade et al. (2012). BLAST searches with KU840164 – KU840166 (NemBar0842–0844) indicate a 100% match with EF157586 [P. nelsoni identified by Maslakova & Norenburg (2008a)] and close similarity to other congeners [P. belizeanus (EF157591), P. claparedii (MH106532), and P. americanus (HQ848595)]. However, HQ848606 (NemPhyl 27) differs from KU840164 – KU840166 and EF157586 by 15.8% in uncorrected p -distance. Prosorhochmus nelsoni of Maslakova et al. (2005) (Table 6) has been reported to possess a vascular plug.
‘ Emplectonema ’ mitsuii Yamaoka, 1947, ‘ Tetrastemma ’ elegans (Girard, 1852), ‘ Tetrastemma ’ vittigerum (Bürger, 1904a), and ‘ Tetrastemma ’ wilsoni Coe, 1904 should each be transferred to a proper genus in the Oerstediidae, because the type species of Emplectonema and Tetrastemma belong in the Amphiporina. Within the Oerstediidae, at least Antarctonemertes Friedrich, 1955, Gononemertes Bergendal, 1900, and Oerstedia Quatrefages, 1846 (as currently recognized) seem to be non-monophyletic (Taboada et al. 2013, 2018; Kvist et al. 2015). Although some well-supported subclades are evident [e.g., Gononemertes Bergendal, 1900 + Vieitezia Junoy et al., 2011 (Ju-noy et al. 2011); Tetraneuronemertes Sundberg et al., 2007 + Nemertellina Friedrich, 1935b (Kajihara et al. 2011)], implementation of a subfamily classification seems to be premature, especially in the absence of sequence data for Obuergeria palma Corrêa, 1954, the type species for Obuergerinae.
Species that have been reported to lack a single vascular plug are listed in Table 4. Many of the species in Table 4 may belong to the Oerstediidae. However, lack of a vascular plug does not necessarily mean an oerstediid affiliation, because some Amphiporina species lack a single vascular plug from the mid-dorsal vessel (Table 5).
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