Two new species of Rhabdocoela (Polycystididae and Trigonostomidae) from China

Abstract

Two new species of Rhabdocoela, namely Alcha sinensis n. sp. (Polycystididae) and Trigonostomum sinensis n. sp. (Trigonostomidae), were discovered from the intertidal zone of eastern Shenzhen City, China. For A. sinensis n. sp., the stylet consists of two symmetrical triangular plates and one lamellar plate. All three plates are jagged at their posterior ends. The anterior end of the stylet connects to a thick muscular layer, which causes its movement. For T. sinensis n. sp., the copulatory organ consists of a long-tubular stylet and two "T"-shaped plates (plate I and plate II). The stylet bends 120° at 25% of its length from the base and extends straight distally. Two "T" plates are connected to each other and surround the stylet. Plate I is hook-shaped at its distal end, and plate II has a similar length but only half the width of plate I. The phylogenetic (18S rDNA and 28S rDNA) results also support the establishment of these two new species. On the basis of the molecular phylogeny and morphology of the copulatory organ and bursa appendage, we propose a new categorization of the species of Trigonostomum.

Taxonomy and phylogeny of Dalytyphloplanida Willems et al., 2006 (Platyhelminthes: Rhabdocoela), with the description of a new family, a new genus, and sixteen new species from Cuba and Panama

“…Weakly supported clades (UFboot < 95, SH-aLRT < 80, and pp < 0.95) were collapsed. Upon collecting and studying the morphological characteristics of Trigonostomum vanmecheleni, we observed a high degree of similarity to what has been described for T. sinensis Wang & Hu in Hu et al, 2019, prompting us to consider the possibility that these species might be synonyms (see results and discussion for further details). To investigate the genetic variations within and between these two species, we calculated the inter-and intraspecific genetic distances (ML distances) within Trigonostomum using a model-based approach in PAUP* v4.0 (Swofford, 2003).…”

“…More recently, Hu et al ( 2019) reviewed the grouping system for Trigonostomum and proposed its differentiation in four groups (type 1 to type 4). Our phylogenetic analysis largely corresponds with the analysis of Hu et al (2019); however, we were more conservative proposing species groups.…”

“…Although we named the species groups supported by our phylogenetic analysis, we are not proposing a new classification system for Trigonostomum due to the similarity of our results with those of Hu et al (2019). Particularly, we refrain to propose a species group to T. penicillatum because it is the only species of the group type 1 of Hu et al (2019) included in phylogenetic analyses.…”

“…Also the bursal appendage is very similar in both species, albeit slightly smaller in T. sinensis. The only discernible morphological difference that can be used to differentiate between both species is the angle of the proximal curvature of the stylet (120° in T. sinensis vs 90° in T. vanmecheleni) (for details see Hu et al, 2019). However, we consider the taxonomic value of this trait questionable and subject to the interpretation of authors when measuring the structures.…”