Bryoplana belgica Houben, Proesmans & Artois, 2022

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 : 81-83

Bryoplana belgica Houben, Proesmans & Artois sp. nov.

urn:lsid:zoobank.org:act:1F08C372-CBAE-45E6-BE50-144752F3D44B

Fig. 5

Diagnosis

Species of Bryoplana with the ciliation reduced or completely lacking dorsally. Testes elongated, in the posterior body part, lateral to the copulatory organ. Copulatory organ with bent, sclerotised ejaculatory duct. Most proximal part of female duct forming a seminal receptacle.

Etymology

The epithet refers to the fact that the species was found in Belgium.

Material examined

Holotype BELGIUM • 1 spec., studied alive and serially sectioned; Koksijde , Oostduinkerke, nature reserve ‘De Zeebermduinen’ ; 51°08′22″ N, 02°41′26″ E; 30 Jul. 2011; A.M. Houben and W. Proesmans leg.; dry moss growing on dunes; KV.687 ; FMNH.

Description

The specimen is about 0.8 mm long. Both body ends are rounded ( Fig. 5A–B ). Free swimming specimens show a small tail. Adenal rhabdite glands ( Fig. 5A : ar) are situated in two groups at ±25% of the body. Dermal rhabdites and protonephridiopores were not observed. The epidermis on the ventral body side is 4.5 µm high and entirely covered with locomotory cilia, while on the dorsal body side it is 3.5 µm high and ciliation is strongly reduced, even almost completely lacking. A slightly forward-slanted rosulate pharynx ( Fig. 5A–B : ph) is located just rostral to the centre of the body.

The gonopore ( Fig. 5C–D : gp) is situated at ±80% of the body and connected to a genital atrium ( Fig. 5A, C–D : ga), which is lined with a high, nucleated epithelium and surrounded by muscles, the orientation of which could not be observed with certainty.

The elongated testes ( Fig. 5A, C–D : t) lie at ±70% of the body and ventrally to the paired vitellaria ( Fig. 5A–D : vi). They gradually taper into the broad vasa deferentia ( Fig. 5C : vde), which laterally enter the copulatory organ ( Fig. 5A : co). Circular muscles surround the 34 µm long copulatory organ, which includes an intracapsular seminal vesicle ( Fig. 5C–E : vs) and a 22 µm long, strongly sclerotised ejaculatory duct ( Fig. 5C–E : de). This ejaculatory duct is more or less straight at its proximal end and bends strongly (±100°) at the distal end. Coarse-grained eosinophilic glands are situated ventrally to the copulatory organ. Although the entrance into the copulatory organ could not be seen, these glands probably represent the prostate glands ( Fig. 5D : gg?).

The female duct ( Fig. 5C–D : fd) is relatively long and lined with a nuclear epithelium. Proximally, it widens into a seminal receptacle ( Fig. 5A, C–D : rs), which receives the short oviduct ( Fig. 5D : od). The vitelloduct and female glands ( Fig. 5D : fg) open into this female duct at the place where it connects to the seminal receptacle.

Discussion

The new species can readily be placed within ‘Typhloplanidae’ because it possesses all diagnostic features: a pharynx rosulatus, a single ovary, paired testes, and a single genital opening. Furthermore, the ventral position of the testes relative to the vitellaria indicates this species should be placed in ‘Protoplanellinae’, ‘Rhynchomesostominae’ Bresslau, 1933 or ‘Typhloplaninae’ Graff, 1905. However, the species’ general habitus and internal organisation differ markedly from the situation in the latter two subtaxa, and we therefore designate this species to ‘Protoplanellinae’.

Most representatives of ‘Protoplanellinae’ have the pharynx situated in the midbody or posterior body half. Only select species have a pharynx in the anterior half of the body, these belong to Achrochordonoposthia Reisinger, 1924; Bockia Reisinger, 1924; Bryoplana ; Microcalyptorhynchus Kepner & Ruebush, 1935, Prorhynchella Ruebush, 1939; and Protopharyngiellona Schwank, 1980. Some of these genera show very typical features: presence of a proboscis (Microcalyptorhynchus), presence of ciliated pits in the anterior body half (Prorhynchella), or the fact that the proboscis is of the doliiformis-type (Bockia). Lack of these features in our species indicates that it cannot be allocated to any of these genera. One of the most eye-catching features of B. belgica Houben, Proesmans & Artois sp. nov. is the lack of a bursa copulatrix, which in the remaining taxa is only the case for Bryoplana . We hence allocate our specimens to the latter genus, to which it indeed shows much resemblance.

Bryoplana belgica Houben, Proesmans & Artois sp. nov. differs from Bryoplana xerophila by the presence of a sclerotised, curved ejaculatory duct, the presence of a seminal receptacle in the female system, and the fact that the dorsal body ciliation is very much reduced, or even lacking. The latter feature was also mentioned by Kolasa (1977) for Ventrociliella romanae, which he suggested to be an adaptation to limnoterrestrial habitats. However, in all other typical limnoterrestrial taxa, even in B. xerophila , the dorsal ciliation is present. Both species of Bryoplana were recovered from dried out moss – B. xerophila in the USA and B. belgica Houben, Proesmans & Artois sp. nov. in Western Europe – but indeed differ markedly in the extent of the dorsal body ciliation, undermining Kolasa’s (1977) hypothesis. The presence/absence of a seminal receptacle in the female system as a distinguishing character should be used with care at this moment. Indeed, the seminal receptacle in B. belgica Houben, Proesmans & Artois sp. nov. is simply a swelling of the female duct, and not a separate organ. As such, the absence of this structure in B. xerophila may be due to the specimens of Van Steenkiste et al. (2010) not having mated yet. However, as Van Steenkiste et al. (2010) investigated several live specimens and twelve serial sections, this seems unlikely. Regardless, the differences in body ciliation and construction of the copulatory organ proper clearly distinguish both species.

Figures

Fig. 5. Bryoplana belgica Houben, Proesmans & Artois sp. nov. A. Internal organisation of a live specimen. B. Squash preparation of a live specimen. C. Organisation of the genital system of a live specimen. D. Reconstruction of reproductive organs based on sagittal sections of the holotype (KV.687). E. Detail of the genital system of the holotype (live photograph) (KV.687).