Hoplopera opaca Reisinger, 1924

Houben, Albrecht M., Monnens, Marlies, Proesmans, Willem & Artois, Tom J., 2022, Limnoterrestrial ‘ Typhloplanidae’ (Rhabdocoela, Platyhelminthes), with the description of four new species and a new genus, European Journal of Taxonomy 798, pp. 70-102 : 88-89

Hoplopera opaca Reisinger, 1924

Fig. 7D–F

Material examined

AUSTRIA • 1 spec., studied alive and sagittally sectioned; Hohe Rannach near Graz; 47°09′53″ N, 15°24′50″ E; 26 Aug. 2011; A.M. Houben and W. Proesmans leg.; leafy humus of a birch forest; XIV.2.42; HU .

Description

Our specimen is about 1 mm long and very dark. The anterior end is rounded, with the posterior end showing a distinct tail with pronounced eosinophilic tail glands ( Fig. 7D : tg). Small dermal rhabdites occur all over the body. Adenal rhabdites are rather inconspicuous and only visible on the sectioned specimen. Paired protonephridiopores ( Fig. 7D : pp) open posterior to the mouth. A rosulate pharynx ( Fig. 7D : ph) is positioned at ±75% of the body.

The gonopore ( Fig. 7E–F : gp) is situated at ±80% of the body and connected to a genital atrium ( Fig. 7D– F : ga), which is surrounded by muscles and lined with a high epithelium.

The vasa deferentia ( Fig. 7F : vde) enter the copulatory organ ( Fig. 7D–F : co) separately. This copulatory organ is a 20 µm long, oval structure surrounded by muscles of uncertain orientation. It contains sperm in its proximal part, which probably serves as a seminal vesicle. An ejaculatory duct was not observed. Intracapsular prostate glands ( Fig. 7E : gg) were observed on live specimens. The male duct is surrounded by circular muscles. The bipartite bursa ( Fig. 7D–F : bu) bears a thin-walled, proximal part containing sperm and a long distal part, the bursal stalk ( Fig. 7E–F : bs). This stalk is surrounded by circular muscles and contains a sclerotised structure ( Fig. 7E : ss). It seems to consist of several vertical bars, which are fused in the distal part.

The vitellaria were still developing in the studied specimen, but already reach up to ±33% of the body. The female duct ( Fig. 7F : fd) connects the oviduct ( Fig. 7F : od) and vitelloduct ( Fig. 7F : vd) to the genital atrium.

Discussion

See the general discussion on the genus Hoplopera .

Previously known distribution

Near Graz, Austria, in moist forest soils ( Reisinger 1924).

General discussion on Hoplopera

All species of Hoplopera are characterised by a combination of features: pharynx situated in the caudal body half, presence of dermal rhabdites and adenal rhabdites, the latter organised in rostral tracks (i.e., Stäbchenstassen), presence of tail glands, lack of a stylet, and the presence of a bursa copulatrix (see Van Steenkiste et al. 2010). This combination of features is also shown by Rhomboplanilla bryophila Schwank, 1980. However, species of Hoplopera all have a sclerotised structure in the bursa copulatrix, which is absent in R. bryophila.

Species of Hoplopera either occur in limnoterrestrial environments ( H. isis Houben, Proesmans & Artois sp. nov.; H. macropharynx Reisinger, 1924; H. maculata Reisinger, 1924; and H. opaca ), or in marine to brackish habitats (H. littoralis Karling, 1957 and H. pusilla Ehlers, 1974). However, the marine origin of H. littoralis and H. pusilla has been questioned. Based on its sampling locality, Karling 1957 and Ax 2008 suggested that H. littoralis might as well be a limnoterrestrial species. H. pusilla was found in the supratidal of salty meadows with salinity below 10‰ and was therefore deemed brackish instead of limnoterrestrial ( Armonies 1987).

All limnoterrestrial species possess a bipartite bursa, with the distal part (probably the bursal stalk) containing a sclerotised structure, while the bursa of the ‘marine’ animals consists of only one part. Furthermore, the vasa deferentia of the limnoterrestrial species enter the copulatory organ separately ( Reisinger 1924), while they fuse just before entering the copulatory organ in H. littoralis and H. pusilla ( Karling 1957; Ehlers 1974). Both the ecological and morphological differences suggest that Hoplopera may as well be split into two genera. Awaiting a thorough cladistic analysis, however, we prefer to maintain the taxon Hoplopera containing both limnoterrestrial and ‘marine’ animals.

The limnoterrestrial species of Hoplopera are most easily distinguished from each other by the shape of the sclerotised structure in the bursa. The fish pot-shaped sclerotic structure as seen in Hoplopera isis Houben, Proesmans & Artois sp. nov. is unique within the genus. In the other three limnoterrestrial species, this structure consists of 6–8 bars proximally connected to a ring (H. macropharynx), 4 (rarely 5) elongated, spoon-shaped structures accompanied with two longitudinal plates (H. maculata) or 6–10 longitudinal bars with broadened distal ends and small protrusions towards the genital atrium ( H. opaca ) ( Reisinger 1924). None of these species possesses the three horizontal bars present in H. isis Houben, Proesmans & Artois sp. nov.

Apart from the above-mentioned differences, H. isis Houben, Proesmans & Artois sp. nov. can also be distinguished from H. opaca and H. macropharynx by the presence of a seminal receptacle, a feature it shares with H. maculata. H. opaca has pronounced tail glands, while the tail glands of H. isis Houben, Proesmans & Artois sp. nov. are small and inconspicuous. H. macropharynx has a conspicuous, large, and oval pharynx, which is unique among the species discussed. H. maculata is the largest of all limnoterrestrial species of Hoplopera (1.5 mm vs. at most 1 mm for the other three species) and has typical parenchymal refracting spots (for details, see Reisinger 1924).

Figures

Fig. 7. Two species of Hoplopera Reisinger, 1924. A–C. Hoplopera isis Houben, Proesmans & Artois sp. nov. A. Internal organisation of a live specimen. B. Organisation of the genital system of a live specimen. C. Reconstruction of reproductive organs based on horizontal sections. D–F. Hoplopera opaca Reisinger, 1924. D. Internal organisation of a live specimen. E. Organisation of the genital system of a live specimen. F. Reconstruction of reproductive organs based on sagittal sections.