Ceratopera complicata, Van Steenkiste & Leander, 2018

Van Steenkiste, Niels W. L. & Leander, Brian S., 2018, Molecular phylogeny of trigonostomine turbellarians (Platyhelminthes: Rhabdocoela: Trigonostomidae), including four new species from the Northeast Pacific Ocean, Zoological Journal of the Linnean Society 182, pp. 237-257 : 249-250

CERATOPERA COMPLICATA SP. NOV.

( FIG. 5B, E–H)

Etymology: The species epithet refers to the complex stylet.

Type locality: Clover Point , Victoria, British Columbia, Canada (48°24 ′ 12 ″ N, 123°21 ′ 03 ″ W), algae in rocky lower intertidal (14/11/2016) .

Type material: One whole mount which is designated as the holotype ( SMNH Type-8920) .

Other material: Observations on two live animals. 18S rRNA (GenBank accession # MF321747 ), 28S rRNA (GenBank accession # MF321757 ) .

Diagnosis: Species of Ceratopera with a very complex stylet. Stylet proper 66 μ m long with a long base and rounded tip. Mantle with numerous plates and a tubular spine. Base of stylet with window-like opening and several ridges. Bursal appendage 48 μ m long, slightly protruding in the bursa, with proximal funnel, slightly sclerotized ring, and two coiled insemination ducts.

Description: Animal about 1 mm long, robust with large rostral rhabdite tracks, lenticular eyes and some parenchymatous brownish coloration ( Fig. 5B). Pharynx slightly anterior to the midpoint of the body. Epidermis packed with oblong rhabdites.

Internal organization typical for Ceratopera and other Trigonostominae with paired testes posterior to the pharynx, paired seminal vesicles entering the prostate vesicle, an elongated bursa with a sclerotized bursal appendage, paired ovaries and vitellaria.

The stylet is 66 μ m long (non-axial: 65 μ m). It is very complex and consists of (1) a curved 66- μ m-long stylet proper with a rounded tip (x1) and a very long base (x2); and (2) a mantle enveloping and connecting to the stylet ( Fig. 5E, F). This mantle is modified into a 52- μ m-long slender tubular spine with a pointed tip (x3), a 33- μ m-long sickle-shaped plate (x4) running more or less parallel to the stylet proper, and a broad plate with a rounded, slightly combed edge (x5). Where this combed edge connects to the stylet proper, some smaller overlapping plates (x6) are present. All these structures come together in the proximal part of the stylet, which has a window-like opening (arrow in Fig. 5E) and on which several ridges originate.

The bursa is long and slender in one animal, likely because of the absence of sperm. In the other animal, the distal part of the bursa is swollen. The latter part is provided with the 48- μ m-long bursal appendage consisting of a proximal 12- μ m-long funnel that splits into two spirally curled, 36- μ m-long insemination tubes ( Fig. 5G). The thick wall of the proximal funnel is confluent with the bursa wall through several sclerotized ridges, but also partly extends into the bursa (arrow in Fig. 5H). A ring marks the transition between the funnel and the insemination tubes (arrow in Fig. 5G). This ring seems to be an outward extension of the bursa wall.

Discussion: The morphological characters in C. complicata sp. nov. are reminiscent of several taxa within Trigonostominae including Ceratopera sensu Den Hartog , Messoplana , Trigonostomum and Ptychopera . The stylet is very complex as in some species of Trigonostomum and Ptychopera . This is unusual for species of Ceratopera sensu Den Hartog and Messoplana of which most species only have a curved, tubular stylet proper of varying length connected to a tubular spine or plate. These are also the two most prominent features in the stylet of C. complicata sp. nov., but additional plates and ridges result in a very ornate stylet. The bitubular appendage is typical for Messoplana , but the thick-walled, funnel-shaped proximal part is rather wide at its base and connects to the bursa through sclerotized ridges, not unlike species of Ceratopera sensu Den Hartog (e.g. C. pilifera ).

Den Hartog (1966a) and Ax (1971) mention a close relationship between Messoplana and Ceratopera sensu Den Hartog (see also general discussion). Both authors clearly define Messoplana by the position of the pharynx in the mid or hind part of the body, the construction of the stylet and its accessory mantle piece, and the morphology of the bursal appendage. However, the position of the pharynx is a dubious feature as species in both genera have representatives with a pharynx in or close to the middle of the body.

The morphology of the stylet can be very similar between representatives of Ceratopera sensu Den Hartog and Messoplana . For instance, some species of Ceratopera sensu Den Hartog also have the accessory mantle piece modified into either a separate duct or a tubular spine as in Messoplana ; however, according to Den Hartog (1964) the accessory mantle piece in Ceratopera sensu Den Hartog distally connects to and sheaths the straight part of the stylet. This distal connection is not always present in several species of Ceratopera sensu Den Hartog , including C. axi , C. ehlersi , C. pilifera and C. steinboecki (Riedl, 1959) Den Hartog, 1964 . Moreover, in several species of Messoplana , such as M. elegans (Luther, 1948) Den Hartog, 1966a , M. canariensis Ehlers & Ehlers, 1980 and ‘ M. pacifica ’ (now C. pacifica comb. nov.), the stylet proper is also sheathed by the mantle.

The bursal appendage is another important diagnostic character used to separate Ceratopera sensu Den Hartog and Messoplana . It consists of a single, funnel-shaped, curved or coiled and strongly sclerotized tube in Ceratopera sensu Den Hartog , while Messoplana typically has two slender ducts surrounded by a sclerotized ring confluent with the bursa wall. The latter type of bursal appendage is shared with Proxenetes ; however, Proxenetes differs from Messoplana by its typical stylet surrounded by a closed, funnel-shaped mantle. When looking at the bursal appendage in several species of Ceratopera sensu Den Hartog and Messoplana more closely, the difference in its morphology becomes less apparent. In some species of Ceratopera sensu Den Hartog , including C. cascadiensis sp. nov., C. axi and C. pilifera , the bursal appendage distally bifurcates into two smaller tubes, sometimes with a faintly sclerotized ring around the transition zone (e.g. C. axi ). In most species of Messoplana , the proximal part is undivided and partly extends into the bursa. This undivided proximal part can be longer, shorter or the same length as the two tubes. In ‘ M. globulifera Artois et al., 2000 ’, a distal bifurcation is lacking altogether.

The examples and discussion above show that the characters separating Messoplana from Ceratopera sensu Den Hartog are equivocal. Not surprisingly, the phylogenetic position of C. pacifica comb. nov. and C. complicata sp. nov. ( Fig. 7A), two species that could either be placed within Ceratopera sensu Den Hartog or Messoplana , reflects the ambiguous taxonomic status of these two genera. It is clear that Ceratopera sensu Den Hartog is not monophyletic when assigning C. complicata sp. nov. to Messoplana and keeping C. pacifica comb. nov. in its original genus. Therefore, the genus Messoplana is suppressed and its representatives are transferred to Ceratopera .