Amphiporina

Infraorder AMPHIPORINA

Morphological circumscription

Eumonostilifera with a two-layered (inner longitudinal and outer circular) muscular rhynchocoel wall and a single vascular plug; exceptions include Carcinonemertes Coe, 1902, Ovicides Shields, 2001, Malacobdella Blainville, 1827, and certain species of Ototyphlonemertes Diesing, 1863, which lack a vascular plug.

Clade definition

Monostiliferous hoplonemerteans that are more closely related to Amphiporus lactifloreus (Johnston, 1828) than to Oerstedia dorsalis (Abildgaard, 1806).

Remarks

A clade of eumonostiliferan species more closely related to Amphiporus Ehrenberg, 1831 than to Oerstedia Quatrefages, 1846 was consistently recovered in previous molecular studies (Thollesson & Norenburg 2003; Strand & Sundberg 2005a; Sundberg & Strand 2007; Sundberg et al. 2007, 2009; Kajihara et al. 2011; Andrade et al. 2012; Kajihara & Kuris 2013; Taboada et al. 2013, 2018; Kvist et al. 2014, 2015; Chernyshev & Polyakova 2018a, b), but had lacked a formal scientific name before Chernyshev & Polyakova (2019) referred to it as Amphiporina, a rankfree clade name. The taxon is here assigned infraorder rank. As with the case in Oerstediina, I leave the ending as it was proposed by Chernyshev & Polyakova (2019), although the suffix -ina should be used for the name of a subtribe in accordance with Article 29.2 of the Code (ICZN 1999) when the taxon is placed in any of the family-group ranks. The name Amphiporina was first proposed by Ehrenberg (1828 –1831: 60) (see Nomenclatural notes below).

Table 5 lists species that have been molecularly ascertained to be affiliated with Amphiporina. Of the 54 species listed, 29 are known to have a single vascular plug (Table 6; Fig. 3 A–E). Among the remaining 25, the six species, Nemertovema hadalis Chernyshev & Polyakova, 2018a, Nemertovema norenburgi Chernyshev & Polyakova, 2019, Ototyphlonemertes correae Envall, 1996, Ovicides paralithodis Kajihara & Kuris, 2013, Proamphiporus crandalli Chernyshev & Polyakova, 2019, and Proamphiporus rectangulus (Strand et al., 2014) have been reported to lack a vascular plug; for the rest of 19 species, the anatomy of the blood vascular system is unknown. Within Eumonostilifera, species with a single vascular plug have so far been found only in Amphiporina; no species in Oerstediina have been ascertained to possess a single vascular plug (Tables 2, 3, 5). Therefore, it appears certain that the 147 species listed in Table 6 —species reported to possess a single vascular plug—likely belong in Amphiporina. As noted above, however, the absence of a single vascular plug does not necessarily denote an affiliation to Oerstediidae. Interestingly, Amphiporina harbours more species than Cratenemertea + Oerstediina combined.

One of the significant findings of Thollesson & Norenburg (2003) was that Malacobdella Blainville, 1827 nest-ed among monostiliferans. Species in Malacobdella live in the bivalve mantle cavity, attached to their host by their posterior ventral sucker (Vázquez et al. 2009). Unlike other nemerteans, they are suspension feeders (Gibson & Jennings 1969), and their proboscis completely lacks a stylet apparatus (e.g., Riepen 1933). Having a highly specialized morphology, Malacobdella had been placed outside Monostilifera in a different higher taxon variably named Bdellomorphae (Blanchard 1847), Bdellonemertini (Wijnhoff 1913), Bdellonemertea (Coe 1943; Iwata 1960), or Bdellomorpha (Chernyshev 2003a); Thollesson & Norenburg (2003) brought an end to these higher taxa. No mention has been made in previous literature to any contact between the rhynchocoel and the blood vascular system in Malacobdella (e.g., Kennel 1877 –1878; Oudemans 1885; Maclaren 1901; Riepen 1933; Gibson & Jennings 1967). My examination of Malacobdella japonica Takakura, 1897 revealed that the mid-dorsal blood vessel—while it does lack a single vascular plug—sends off at least three branches of vessels anterodorsally near its anterior end, each penetrating into the ventral portion of the rhynchocoel muscular wall without protruding into the rhynchocoel lumen, and terminating in a blind end that contains basophilic substances (Fig. 4 A–D). While these branches may compensate for the function of the vascular plug in terms of material transfer between the rhynchocoel and blood vessel, their fine structure and taxonomic distribution should be further studied to understand their physiological function and evolutionary significance.

In the tree in Thollesson & Norenburg (2003), Carcinonemertes cf. carcinophila imminuta (K̂lliker, 1845) was the sister group to all other Distromatonemertea. This topology was probably due to long-branch attraction (Felsenstein 1978; Philippe et al. 2005) caused by an increased substitution rate in these decapod-egg parasites. Subsequent molecular studies showed Carcinonemertes as either the sister group to all other Amphiporina (Andrade et al. 2012; Kajihara & Kuris 2013) or nested among the latter (Kvist et al. 2014). Species in Carcinonemertidae lack a middorsal vessel (Humes 1942) and thus do not have a vascular plug. In Ovicides paralithodis Kajihara & Kuris, 2013, the cephalic vessels do not protrude into the rhynchocoel as they pass through the cerebral ring, although they are tightly attached to the rhynchocoel wall (Kajihara & Kuris 2013: fig. 3).

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*Possesses a single vascular plug on the mid-dorsal vessel (see Table 6). †Reported to lack a vascular plug (see Table 4).

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