Trigonostomum, Schmidt, 1852

Diez, Yander L., Monnens, Marlies, Wuyts, Arlien, Brendonck, Luc, Reygel, Patrick, Schmidt-Rhaesa, Andreas & Artois, Tom, 2023, 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, Organisms Diversity & Evolution (New York, N. Y.) 23 (4), pp. 631-681 : 676-677

Trigonostomum

Willems et al. (2004b) revised the genus Trigonostomum , which they subdivided into three groups (1A, 1B, and 2) based on the morphology of both the stylet and bursal appendage. Representatives of all three group were included in our phylogenetic analyses. For group 1A, this is T. venenosum , for 1B these are T. penicillatum , T. watsoni , T. vanmecheleni , T. tillicum , and T. sinensis , and for group 2, these are T. armatum , T. franki , T. setigerum , and T. tori . Also one species with an aberrant morphology was included: T. denhartogi . 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.

In the newly presented phylogeny, three clades of Trigonostomum are recovered ( Fig. 23 ), whereas T. penicillatum is the sister species to a monophyletic clade formed by these three clades. Only group 2 of Willems et al. (2004b) forms a monophyletic clade in our analysis (see clade ‘ armatum ’ in Fig. 23 ). All these species are characterised by a stylet showing a number of spirals (½ in T. franki to 5 ½ in T. tori ) and the mantle following the course of the stylet. In this group, the bursal appendage consists of two tubules attached by a ring to the bursa, with both tubes coiled over 360° (for more details see Willems et al., 2004b). 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.

Trigonostomum armatum is one of several known rhabdocoel species with an apparently worldwide distribution (see Willems et al., 2004b and references therein). However, our results suggest that T. armatum may in fact be a species complex. The two specimens included in the analysis do not constitute a monophyletic group, as the population from France is recovered as the sister species to the rest of clade 2, including a population of T. armatum from the Caribbean. Some marked morphological differences do exist between specimens from different populations of T. armatum (summarised in Willems et al., 2004b). In specimens from the northern Atlantic ( France), eastern Australia, and New Caledonia, pigmentation is lacking, whereas anterior dark spots are found in specimens from Cuba, Curaçao, and the southern Atlantic. The stylet of the Cuban specimens (333–369 µm) is more similar in length to that of the specimens from Curaçao (274–378 µm) and New Caledonia (386 µm). It is larger than specimens from France (158–196 µm) and Eastern Australia (183 µm long). The largest stylet is recorded for one specimen from the Southern Atlantic (434 µm). Similarly, the bursal appendage varies in length: 84–100 µm in the specimens from Cuba, 100–108 µm in the specimens from Curaçao, 127 µm in the specimen from New Caledonia, and 149 µm in the specimen from the southern Atlantic. However, it is necessary to include more populations in the phylogenetic analysis and perhaps additional, fast-evolving markers to elaborate the taxonomy of this species complex.

Clade 3 includes three recently described species of Trigonostomum which are very similar to each other. Trigonostomum tillicum is inferred as the sister to the clade containing T. vanmecheleni and T. sinensis . Until now, T. vanmecheleni was only known from channels of Venice ( Italy), but it is now recorded for the first time from the Caribbean coast of Cuba. The morphology of individuals from both populations is indistinguishable. Trigonostomum sinensis , described from China, is very similar to T. vanmecheleni . Measures of the male hard structures of T. sinensis are within the range of these structures in T. vanmecheleni . 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. In fact, both stylets appear almost identical when comparing their outline shape. Moreover, when considering the genetic distance, T. sinensis and T. vanmecheleni exhibit the smallest disparity among all species of Trigonostomum (ML distance 0.001). This is nearly ten times smaller than the closest lower distance observed between T. tillicum and T. sinensis / T. vanmecheleni (ML distance 0.010). Based on the morphological and genetic distance data, we propose the synonymisation of T. sinensis with T. vanmecheleni .

All the species in clade 3 are characterised by the sclerotised parts of the copulatory organ consisting of two mantle plates that run parallel to each other and differ in width—the thinner plate located in between the wider plate and the tubiform stylet. Moreover, all species have a bursal organ that consists of a heavily coiled tube that distally splits into a number of finer tubes. Both features could be morphological apomorphies for this clade.

Because of lack of material, T. yoandrisi sp. n. could not be included in the phylogenetic analyses. The morphology of the atrial organs of this species is similar to those of the species included in group 1B of Willems et al. (2004b), but these species are now spread out over our clades 1 and 3. Trigonostomum yoandrisi sp. n. shows the two typical features (apomorphies?) mentioned above for clade 3, and therefore, we provisionally consider it to belong to that clade. In what follows, we will compare the new species with the species of group 1B of Willems et al. (2004b).

Trigonostomum yoandrisi sp. n. differs from all other species in group 1B in the shape of its stylet, which is proximally funnel-shaped and does not bend. Moreover, the stylet of T. yoandrisi sp. n. (~ 24 µm) is considerably shorter than that of all other species of the group, and the mantle of T. yoandrisi sp. n. forms two plates, an organisation only found in T. mirabile and T. nataschae ( Willems et al., 2004b) . However, in T. yoandrisi sp. n., one of the plates is X-shaped and forms two distal sharp projections, whereas, in the other two species both triangular plates form a single distal projection.

The bursal appendage of all species of group 1B of Willems et al. (2004b) shows an obvious striation, which is not the case in T. yoandrisi sp. n. The bursal appendage of T. yoandrisi sp. n. consists of a proximal part and two distallycoiled tubes (~ 360°) of ~ 48 µm long, which is comparable to the situation in T. watsoni . However, in T. watsoni , the bursal appendage is~ 70 µm long and the tubes are coiled more than 360°. The other species of this group also possess a bursal appendage larger than that of T. yoandrisi sp. n. and with a different number of tubes. The bursal appendage in T. breitfussi is 24 µm long and branches distally in 9 or 10 tubes; in T. coronatum , it is 78 µm long with a proximal, crown-like part and one terminally-bent, striated tube; in T. lilliei , it is ~ 97 µm long, heavily coiled, distally split into five or six tubes; in T. vanmecheleni , it is ~ 115 μm long, and distally split into six finer tubes; in T. mirabile , it is 66 µm long, proximally with a barrel-like casing and approximately ten distal tubules; in T. nataschae , it is ~ 65 µm long, with proximal barrel-like part and very narrow tubules, forming two bundles of curved tubules distally; and it is ~ 65 µm long, with a barrel-like casing and ~ 12 distal tubules in T. penicillatum (see Willems et al., 2004b). The unique morphology of the stylet (funnel-shaped, lacking the proximal bend) and mantle plates (one of which is X shaped with two distal tips) warrants the description as a new species for T. yoandrisi sp. n.

Adenorhynchidae Ax & Heller, 1970 status novus Updated diagnosis (after Ax & Heller, 1970). Representatives of Thalassotyphloplanida lacking eyes; female genital canal simple, an unpaired seminal receptaculum is present (except in Tvaerminnea direceptacula Ehlers et al., 1994 where there is a pair, and Listea simplex Ax & Heller, 1970 where there is none), a bursa may or may not be present, paired ovaries lying caudal; testes anterior to the pharynx (except in Litucivis ) and male copulatory organ with a funnel-shaped stylet. Type genus. Adenorhynchus Luther, 1948 , designated by Ax and Heller (1970).

Contained taxa. Cilionema , Coronhelmis , Tvaerminnea , Adenorhynchus Meixner, 1938 , Listea Ax & Heller, 1970 , Litucivis Ax & Heller, 1970 , and Scoliopharyngia Ehlers et al., 1994 .

Considering the fact that we suppressed Brinkmanniellinae and that other taxa previously included in this subfamily, except Gaziella , cluster in a fully supported clade in our phylogenetic analysis together with Litucivis serpens , we raise Adenorhynchinae to family rank ( Adenorhynchidae status novus). We also include in the new family all other members of Adenorhynchinae ( Adenorhynchus , Listea , and Scoliopharyngia ). The only representative of Adenorhynchinae included in the phylogenetic analyses was L. serpens . However, Adenorhynchinae is a morphologically homogeneous group ( Ax & Heller, 1970). These characteristics are also present in the brinkmanniellinids transferred to Adenorhynchidae status novus. Furthermore, all representatives of Adenorhynchidae status novus possess the testes anterior to the pharynx (except in Litucivis , where the testes are caudally located to the pharynx, a plesiomorphic condition).

Other representatives of the suppressed Brinkmanniellinae also share most features of Adenorhynchidae status novus. However, considering the diversity of the group, they are retained within Promesostomidae until new molecular information is available.

Figures

Fig. 23 Majority-rule consensus tree from the Bayesian analysis of the concatenated 18S rDNA+28S rDNA+cox1 of Trigonostomum. Branches with support values below the thresholds in the legend of the three analyses were collapsed. Symbols above branches represent posterior probabilities (: 0.90 ≤ pp <0.95). Symbols below branches indicate SH-aLRT (: 99 ≤ SH-aLRT <100;