Microdalyellia armigera (Schmidt, 1862) 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 armigera (Schmidt, 1862) Gieysztor, 1938

Fig. 8

Known distribution.

This species has been broadly recorded in the United States, Iceland, Faroe Islands, overall Europe ( United Kingdom, Ireland, Finland, Belgium, Germany, Austria, Switzerland, Czech Republic, Poland, Romania, Bulgaria, Latvia, Italy, France, and Spain), Russia, and Japan. See a summary of this distribution and main references in Van Steenkiste et al. (2011).

Material.

One specimen studied alive and whole mounted ( ZMH 13833), collected in Wandse river, submerged vegetation with organic matter, 0.1 m deep.

Remarks.

Live animal about 1 mm long, anterior margin rounded and posterior pointing, translucent, orange colouration due to parenchymal glands (Fig. 8 A ). A pair (Fig. 8 A : e) of eyes anterior to the pharynx. Barrel-shaped pharynx (Fig. 8 A : ph) 250 µm long. Testes located antero-lateral to the male copulatory bulb. This bulb encompasses a seminal vesicle (Fig. 8 B : sv), a prostate vesicle (Fig. 8 B : sv), and the stylet. The stylet (Fig. 8 C – E : st) is 123 µm long, H-shaped as typical for species of Microdalyellia . One arm carries five 17 - µm-long spines, and the other a single 58 - µm-long spine. The vitellaria (Fig. 8 A : vi) run beside the pharynx until the posterior third of the body, fusing before opening into the oviduct. The oval egg is 140 µm long (Fig. 8 A, C – F : eg).

Microdalyellia armigera shows a high morphological variability of the male sclerotised stylet, the main character to differentiate species in most microturbellarians. However, it would not be surprising if this variability represents cryptic speciation, as suggested by our phylogenetic analysis (see section Molecular Phylogenetic Analyses; Fig. 16 ). For example, the specimens of M. armigera collected by Müller and Faubel (1993) in the Elbe river present a stylet much larger than in our specimen (220 µm vs 123 µm respectively) and have 3–9 spines in one arm. The size of this structure in other North-European populations varies between 82 μm and 130 μm ( Luther 1955; Rixen 1961) and in specimens from Spain is 165 μm long ( Van Steenkiste et al. 2011). In general the number of spines in both arms can be variable (see Luther 1955; Bauchhenss 1971) but commonly there is one arm with one spine and the other with 4–5 ( Müller and Faubel 1993).

Figures

Figure 8. Microdalyellia armigera. A. Habitus of a swimming specimen; B. Seminal and prostate vesicles; C – F. Male stylet. e eye; eg egg; i intestine lumen; ph pharynx; pv prostate vesicle; st stylet; sv seminal vesicle; vi vitellaria. Scale bars: 100 μm (A); 50 μm (B – 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.