Myostenostomum styrensis, Kostenko & Gaponova, 2025

Myostenostomum styrensis sp. nov.

( Figures 1–2 View FIGURE 1 View FIGURE 2 )

Diagnosis

Myostenostomum with a solitary animal body length of 800–1000 μm, anterior brain lobes are dentate with elongated basal parts, posterior lobes saddle-like with folded lateral prolongations, mouth shaped like a transversely elongated, curved quadrilateral, the pharynx consists of two parts: the anterior oral part and the posterior esophageal part, pharyngeal glands with long stalks extend from the upper part of the intestine and open at the anterior margin of the pharynx, rounded posterior end of the body with well-distinguished cauda, rod-shaped rhabdoids, large well-defined oval-shaped ciliated pits, light-refracting bodies are absent.

Etymology

The name refers to the river Styr, to the basin of which the type location of the new species, Lake Bile, belongs.

Material examined

Holotype. UKRAINE: Rivne Region, Rudka village, Lake Bile , 51°29'10" N, 25°46'31" E, August 3, 2021, freshwater, sandy shoal without macrophytes, L. Gaponova leg. Whole specimen fixed in Bouin's solution and stored in 70% ethanol deposited in the collection of the IEE NAS Ukraine. Holotype voucher number CSM003. GoogleMaps

Other material examined. UKRAINE: same data as holotype. Three specimens were fixed in Bouin's solution and stored in microtubes with 70% ethanol. Paratypes voucher numbers CSM 004-006. Several specimens were studied alive, captured within the vegetation in stagnant waters. GoogleMaps

Description

Only solitary specimens were present. The body is 800 μm long but can be extended up to 1000 μm in length. The body is elongated and cylindrical in shape, with the anterior end being widely rounded. The length from the base of the posterior brain lobes to the anterior end of the body is 180 µm. The front end of the body can elongate, forming a characteristic duck-billed shape. The width of the body at the base of the posterior brain lobes is 130 µm. From this point, the body gradually expands, reaching its maximum width of 260 µm in the middle of the posterior third. The posterior end of the body is rounded and ends with a well-defined, intestine-free cauda measuring 60 µm in length and 25 µm in width. The epidermis is uniformly covered with cilia, with longer, stiffer sensory cilia (sc) up to 20 µm in length occasionally seen at the anterior end of the body ( Figure 1 View FIGURE 1 ).

Rod-shaped rhabdoids are found in the epidermis all over the entire surface. There are no light-refractive bodies. The ciliary pits (cp) are quite large, well-defined, measuring 60–65 x 35–40 μm.

The anterior brain lobes (ab) are deeply dentate on their internal surface and adjacent to ciliated pits. The basal parts of the anterior brain lobes are elongated and reach the level of the basal part of the posterior lobes. The posterior lobes (pb) and transverse commissure are fused, forming an arcuate ganglion with folded lateral prolongations.

The mouth (m) is shaped like a transversely elongated, curved quadrilateral. The pharynx consists of two parts. The anterior oral part, or the pharynx proper (php), is spherical in shape with a diameter of 65 μm. The muscle fibers create a uniquely deep relief on the surface of the pharynx that has not been previously reported for any known species of Myostenostomum ( Figure 2 View FIGURE 2 ). The posterior esophageal part (e) is cylindrical, approximately 50 μm long, and has a diameter of 25 μm. It connects the pharynx proper to the intestine. The surface of the esophageal region is characterized by shallow, weakly expressed transverse folds. The bodies of the pharyngeal glands are located, presumably, in the upper part of the intestine. The stalks of the pharyngeal glands (sg) run from the intestine along both the esophageal and oral pharyngeal parts, opening at the anterior edge of the pharynx proper. Before entering, the stalks expand (es) due to filling with secretions.

The muscular ‘gizzard’ (g), characteristic of the Myostenostomum species, divides the intestine into a short anterior part, adjacent to the esophageal part of the pharynx and a posterior part occupying the rest of the body leaving a short intestine-free portion right before the cauda. The length of the ‘gizzard’ is 110 μm, the outer diameter is 120 μm, and the walls of the inner channel through which the intestine passes almost touching each other in the middle of the ‘gizzard’, at its narrowest point. The function of the ‘gizzard’ is not clear. It may be used for mechanical processing of large food items.

Remarks

Myostenostomum styrensis sp. nov. is most similar to two species of the genus: M. bulbocaudatum and M. vanderlandi . M. styrensis and M. bulbocaudatum share the shape of the anterior brain lobes; however, the basal parts of the anterior brain lobes (as well as those of the posterior brain lobes) in M. styrensis are significantly elongated. Such elongations of the basal parts of the brain lobes can be seen in M. vanderlandi . Moreover, the latter species has a short but well-defined cauda comparable to that of M. styrensis . However, M. vanderlandi has metameric ganglia in lateral chains adjoining the anterior brain lobes anteriorly, while the new species has deeply dented (not metameric) anterior brain lobes.

A distinguishing feature of M. styrensis is the structure of its highly organized pharynx, which is divided into a voluminous spherical oral part and a tubular esophageal part. This type of pharyngeal structure has not yet been found in representatives of the genus Myostenostomum . In general, a discrete esophagus is extremely rare among representatives of the subphylum Catenulida. It is known in Myoretronectes paranaensis Noreña-Janssen & Faubel, 1996 from the family Retronectidae Sterrer & Rieger, 1974 .As shown in the sagittal reconstruction of the pharyngeal region of this species ( Noreña-Janssen & Faubel 1996, Fig. 2 View FIGURE 2 ), the esophagus is characterized by thickened epithelial walls and poorly developed musculature, unlike the pharynx, where the muscle fibers are significantly more developed and form a muscular pharyngeal complex. The esophagus and pharynx in M. paranaensis are well separated from each other. Another type of esophagus is represented in our new species and Stenostomum corderoi Marcus, 1945 (both from the family Stenostomidae ). In these species, the highly organized pharynx is divided into a large (2/3 of the length in S. corderoi ) voluminous oral part and a shorter tubular esophageal part (see our description and Noreña et al., 2005, Fig. 4B). Thus, it can be concluded that in the subphylum Catenulida, the esophagus develops either as a completely separate organ ( M. paranaensis ) or as part of a highly structured pharynx ( M. styrensis and S. corderoi ).

The arrangement and location of the long-stalked pharyngeal glands of M. styrensis are unique among the species of the genus.