Microdalyellia schmidtii ( Graff, 1882 ) Gieysztor, 1938

Diez, Yander L. & Schmidt-Rhaesa, Andreas, 2024, Little neighbours in Hamburg: free-living aquatic flatworms (Platyhelminthes), Evolutionary Systematics 8 (2), pp. 279-310 : 279-310

Microdalyellia schmidtii ( Graff, 1882) Gieysztor, 1938

Figs 9 , 10

Known distribution.

Species with a known distribution in West Europe ( United Kingdom, Finland, Germany, and Switzerland) ( Graff 1882, 1913; Fuhrmann 1894; Luther 1955; Rixen 1961; Young 1970), and Russia ( Nasonov 1919, 1926).

Material.

Observations on eight live animals, five whole mounted afterwards ( ZMH 13834–13838) and the other three preserved in absolute ethanol (two already sequenced for molecular analyses); collected in Wittenberg, Rissen, on tree holes filled with water and litter, 20–50 cm over the ground level.

Remarks.

Animals 704–1010 µm long (x ̄ = 840 µm; n = 3), with a pair of eyes (Fig. 9 A : e) anterior to the pharynx. The yellowish to orange colouration is due to parenchymal glands. The barrel-shaped pharynx (Fig. 9 A : ph) is 231–289 µm long (x ̄ = 262 µm; n = 3), when the animal is not contracted.

The atrial organs are located in the posterior body third. The pair of testes (Fig. 9 B, C : t) is located beside the male copulatory organ. The copulatory organ encompasses a seminal vesicle (Fig. 9 B, C : sv), a prostatic vesicle (Fig. 9 B, C : pv), and the stylet. The stylet (Figs 9 B, C : st, E, F, 10) is 147–151 µm long (x ̄ = 149 µm; n = 4) and each arm is armed with a conical spine. The larger spine is 64–75 µm long (x ̄ = 69 µm; n = 4) and the smaller is 48–52 µm long (x ̄ = 50 µm; n = 4), both with the distal tip bent outside and hook shaped. Each vitellarium (Fig. 9 A : vi) runs beside the pharynx until the posterior third of the body, and fuses before entering the oviduct. The ovary (Fig. 9 D : ov) is 157–212 µm long (x ̄ = 185 µm; n = 2). The distal most three oocytes are of similar size and organised in a row, the rest of the ovary is full of smaller oocytes with different degrees of development. The eggs (Fig. 9 A : ov) are oval, 148–206 µm long (x ̄ = 177 µm; n = 4).

Around 44 species of Microdalyellia have been recorded globally ( Tyler et al. 2006–2024). Among them, M. schmidtii stands out as the only one possessing a single, funnel-shaped spine on each arm of the stylet, leading to its stylet being described as ‘ plow-shaped’ ( Graff 1882; Luther 1955). However, doubts have been raised regarding the validity of this species due to its similarity to M. armigera and M. kupelwieseri ( Meixner 1915; Ruebush and Hayes 1939). It has been suggested that M. schmidtii and M. kupelwieseri might be forms of M. armigera with a greatly reduced number of spines ( Luther 1955; Young 1970).

Nasonov (1919, 1926) provides insight into the close relationship among the previously mentioned species, particularly in Russian populations. Some specimens of M. schmidtii from Russia ( Nasonov 1919: fig. 8) resemble M. kupelwieseri due to the presence of a few smaller spines alongside the main spine of one arm (see Meixner 1915; Luther 1955; Young 1970). However, Nasonov (1926) also recorded specimens of M. schmidtii with a single spine per arm, some displaying transitional forms between extreme morphologies of M. schmidtii and M. armigera . Nevertheless, populations of M. kupelwieseri from Germany ( Rixen 1961), the United Kingdom ( Young 1970), and Spain ( Farías et al. 1995) show morphological convergence, with stylets measuring 100–110 µm and carrying one spine in one arm and three in the other.

The Hamburg population of M. schmidtii is notable for its significantly larger stylet (140 µm) compared to other recorded populations: 92 µm in the United Kingdom ( Young 1970), 66 µm in Finland ( Luther 1955), and 100 µm in Germany ( Rixen 1961). Stylet sizes in M. kupelwieseri populations are also smaller (100–110 µm). The stylet in the new studied material l ( ZMH V 13848 – V 13856 ) of M. kupelwieseri populations displays a triangular spine in the bridge linking the two branches of the stylet (Fig. 11 B, D : sp), a structure lacking in both M. schmidtii and M. armigera . In addition, one of the spiny arms of M. kupelwieseri carries three spines, whereas the number is always one in M. schmidtii from Hamburg. We suspect, therefore, that the high similarity between the two species and the sympatric distribution of both in some localities, contributed with their misidentification. Indeed, the re-examination of two specimens identified as M. kupelwieseri collected in Belgium and stored in the reference collection of Hasselt University (VII. 4.20 and VII. 4.24) allowed their reclassification as M. schmidtii . This action is made considering that their stylets lack a triangular spine in the bridge. A third specimen collected in Belgium, together the two previously mentioned, and sharing the same morphology, was included in the phylogenetic analysis by Van Steenkiste et al. (2013) and it clustered in our phylogeny with specimens of M. schmidtii collected in Hamburg (see section Molecular Phylogenetic Analyses; Fig. 16 ). These findings support that the presence or not, of a triangular spine in the stylet’s bridge is a diagnostic character for M. kupelwieseri and M. schmidtii , respectively.

Until now, the relationships among M. armigera , M. kupelwieseri , and M. schmidtii , and the validity of the latter two species, remained unclear. However, it is suggested that the group M. kupelwieseri – schmidtii can be distinguished from M. armigera by the reduced spine number in one arm, as well as the larger size of the single spines in M. schmidtii and the main spines of M. kupelwieseri compared to those in M. armigera . In this sense, our phylogenetic analysis contributed to clarify that the three species represent distinct lineages and support their validity. However, M. armigera could represent a complex of cryptic species (see section Molecular Phylogenetic Analyses; Fig. 16 ).

Figures

Figure 9. Microdalyellia schmidtii. A. Habitus of a swimming specimen; B, C. Male atrial organs; D. Ovary; E, F. Male stylet. Scale bars: 100 μm (A); 50 μm (B – F).

Figure 10. Microdalyellia schmidtii. A – F. Male stylet. Scale bars: 50 μm (A – F).

Figure 11. Microdalyellia kupelwieseri. A – F. Male stylet; D. Detail of the bridge’s spine; E – F. Detail of the spiny branches. Scale bars: 50 μm (A – F).

Figure 16. Majority-rule consensus tree from the Bayesian analysis of the concatenated 18 S + 28 S rDNA dataset of Rhabdocoela. Branches with support values below the thresholds in the legend of the three analyses were collapsed. Support values are represented in the order of posterior probabilities / SH-aLRT / ultrafast bootstrap. Branches without symbols have pp = 1, SH-aLRT = 100, and UFboot = 100. Taxa from which new sequences were obtained for this study are highlighted in bold.