Dugesia crassimentula Sluys & Stocchino, 2024

Dugesia crassimentula Sluys & Stocchino , sp. nov.

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Material examined

Holotype: ZMA V.Pl. 786.1 , Route Nationale d’Ivohibe (700), south-east Madagascar (22°28 ʹ 51.78 ʹʹ S 46°53 ʹ 9.10 ʹʹ E), 1950, coll. J. Millot, sagittal sections on four slides. GoogleMaps

Etymology

The specific epithet is based on the Latin adjective crassus, thick, stout, and the noun mentula, penis; it alludes to the plump penis papilla of the species.

Diagnosis

A Dugesia species characterized by the following features: high triangular head with a distinctly pointed auricle on either side; mouth opening at the hind end of the pharyngeal pocket; distinct layer of transverse muscles directly dorsally to the ventral nerve cords; plump, barrel-shaped penis papilla, provided with a large dorsal penial fold and a small ventral fold; bundles of longitudinal muscles traversing the penis papilla and attaching to the penial folds; large, conical diaphragm; distinct seminal vesicle absent; ectal reinforcement on bursal canal absent; highly asymmetrical openings of oviducts into bursal canal.

Geographical distribution

Known only from the type locality.

Description

As this specimen was originally assigned to the species Dugesia milloti (cf. de Vries 1988: 363), it is assumed that its external appearance is similar to that of the latter. Length of preserved specimens ~ 9 mm (as measured on histological sections).

Apart from the usual subepidermal body musculature there is a distinct layer of transverse muscle located directly dorsally to the ventral nerve cords and extending throughout the body length ( Fig. 7 View Figure 7 ).

Pharynx located in the middle of the body, measuring about a quarter of the body length (as measured on histological sections). No extra layer of longitudinal muscle in the outer pharynx musculature. Mouth opening at the hind end of the pharyngeal pouch.

Many small, dorsal testes are distributed throughout the body length, extending from directly behind the ovaries far into the posterior end of the body.

At about the level of the root of the pharynx, the vasa deferentia expand to form spermiducal vesicles, which are filled with sperm. Posteriorly to the pharyngeal pocket the sperm ducts turn dorso-medially, after which they first decrease in diameter and then separately penetrate the antero-lateral wall of the penis bulb. Hereafter, the sperm ducts traverse the penis bulb to separately open into the funnel-shaped, proximal, posterior portion of the lumen of the diaphragm ( Fig. 8A View Figure 8 ). This lumen is lined with a cuboidal, nucleated epithelium that is penetrated by a fine-grained, cyanophil secretion.

The large, conical diaphragm is covered by a cuboidal, nucleated epithelium, which continues on the proximal, anterior half of the ejaculatory duct and is penetrated by the orange secretion of penis glands. The distal part of the wide ejaculatory duct shows a few folds before it opens at the blunt tip of the penis papilla. The latter is a plump, barrel-shaped structure that is provided with penial annexes (cf. Stocchino et al. 2017) in the form of two penial folds. One, small fold is located at the ventral surface of the papilla, rather close to its root, while the other fold occurs dorsally and is much larger ( Figs 7 View Figure 7 and 8A View Figure 8 ). In addition to these penial folds, the ventral lip of the penis papilla exhibits at its tip an invagination that may represent a non-permanent structure, presumably being based on a contraction artefact.

The musculature of the penis bulb extends posteriorly beyond the root of the penis papilla and attaches to two small, dorsally and ventrally located atrial folds, situated near the root of the papilla ( Figs 7 View Figure 7 and 8A View Figure 8 ). Bundles of well-developed longitudinal muscle fibres run from the penis bulb through the papilla and attach to the penial folds. The epithelium of the penis papilla is underlain by a thin layer of circular muscle.

The ovaries are situated at some distance (~500 μm) behind the brain, i.e. at about a quarter of the distance between the brain and the root of the pharynx. The infranucleated oviducts run posteriorly to about the level of the gonopore, from where they turn medio-dorsad. The left oviduct runs far towards the dorsal body surface and then bends antero-ventrally to open through the dorsal wall of the bursal canal. The right oviduct opens into the broad, basal portion of the bursal canal, not far from where it communicates with the atrium. Shell glands discharge an orange secretion into the bursal canal immediately ventrally to the oviducal openings.

The bursal canal arises from the postero-lateral wall of the atrium and at first has an oblique orientation into postero-dorsal direction before it curves sharply anteriad.The obliquely oriented portion of the bursal canal is already rather wide and after the bend in anterior direction it gradually enlarges even more until it is as wide as the copulatory bursa, thus passing smoothly into the latter ( Fig. 8B View Figure 8 ). The bursal canal is lined with an infranucleated epithelium that is underlain by a single, subepithelial layer of longitudinal muscle, followed by an only slightly thicker layer of circular muscle; ectal reinforcement is absent. Bursal canal and copulatory bursa contain a large spermatophore of which the sac-shaped capsule is packed with sperm.

Comparative discussion

This specimen initially was identified by de Vries (1988) as D. milloti , as she was of the opinion that contraction of the longitudinal muscles running from the penis bulb and attaching to the posterior atrial fold would considerably shorten the atrium, so that the anterior fold would sit at the base of the penis papilla. Further, she argued that contraction of the penis bulb musculature would narrow the seminal vesicle and alter the shape of the diaphragm. However, we believe that the morphology of this specimen cannot be explained as merely due to contraction artefacts but that it exhibits a suite of essentially different character states concerning the following structures: penis papilla, penial and atrial folds, diaphragm, seminal vesicle, ectal reinforcement, oviducts, mouth opening, and transverse musculature. These differences between D. milloti and D. crassimentula will be discussed below.

An immediately obvious difference between D. milloti and D. crassimentula concerns the size and shape of the penis papilla, which in the former is always a slim cone (cf. de Vries 1988: figs 9, 10; Fig. 3A View Figure 3 ), whereas in the latter it is a plump, barrel-shaped structure that completely fills the male atrium (cf. de Vries 1988: fig. 14C; Fig. 7 View Figure 7 ). Further, in D. crassimentula the penis papilla is provided with two folds, a large dorsal one and a small ventral fold, which are absent in D. milloti .

The atrium of D. milloti is provided with two annular folds or valves, with the posteriorly extending musculature of the penis bulb attaching to the posterior valve (cf. de Vries 1988: figs 9, 10; Fig. 8A, B View Figure 8 ). In contrast, D. crassimentula only has two small atrial folds, not valves, one dorsal and one ventral, receiving the longitudinal muscles coming from the penis bulb.

The diaphragm of D. milloti is of the stubby, valve-shaped type (cf. de Vries 1988: figs 9, 10; Figs 3A, B View Figure 3 , 5 View Figure 5 ), whereas that of D. crassimentula is of the pointed, conical type ( Fig. 8A View Figure 8 ). Another aspect of the penis that differs between D. milloti and D. crassimentula is the presence of a well-developed seminal vesicle in the former (cf. de Vries 1988: figs 9, 10; Fig. 3A, B View Figure 3 ) and the virtual absence of such a vesicle in the latter ( Fig. 8A View Figure 8 ).

In D. milloti the connection between copulatory bursa and bursal canal is established in the usual manner, while the diameter of the latter also shows the usual dimension (cf. de Vries 1988: figs 9, 10; Fig. 5 View Figure 5 ). In contrast, in D. crassimentula the diameter of the bursal canal expands until it has reached the same size as the copulatory bursa, after which it almost imperceptibly grades into the bursa ( Fig. 8B View Figure 8 ). With respect to the female copulatory apparatus, another difference between D. milloti and D. crassimentula resides in the development of the ectal reinforcement on the bursal canal. In D. milloti this extra layer of longitudinal muscles extends from the vaginal area to the copulatory bursa or at least to about halfway along the bursal canal, whereas in D. crassimentula ectal reinforcement is completely absent. Another aspect of the female reproductive system concerns the oviduct, which consists of nucleated cells in D. milloti and infranucleated cells in D. crassimentula , while in the former the ducts open symmetrically into the bursal canal, whereas in D. crassimentula the oviducal openings are highly asymmetrical.

In D. crassimentula the mouth opening is located at the posterior end of the pharyngeal pouch, whereas in D. milloti it is shifted anteriad, being located at about one-sixth of the distance between the hind wall of the pharyngeal pouch and the root of the pharynx (see D. milloti description).

De Vries (1988) already noted that the development of the transverse musculature, located directly dorsally to the ventral nerve cords, varies between species of Dugesia . In D. milloti it is poorly developed, whereas in D. crassimentula the transverse muscles are quite distinct ( Fig. 7 View Figure 7 ).

Thus, there are ample anatomical differences between D. milloti and D. crassimentula , supporting the separate specific status of the latter, as it neither conforms to any other known species of Dugesia .