Dendrocoelum dorsivittatum Kenk 1978

Dendrocoelum dorsivittatum

TYPE MATERIAL.—Holotype, set of serial sagittal sections on 10 slides, USNM 55239; paratype, sagittal sections of 1 specimen on 4 slides, USNM 55240.

Only two specimens of this species were collected in the profundal zone of Lake Ohrid, one sexually mature, the other not fully differentiated.

EXTERNAL FEATURES (Figure 18).—The larger specimen measured in life 13 mm long and 4 mm wide, the smaller one 10 and 2.5 mm, respectively. Unfortunately, the larger individual had an injured head. The anterior end of the smaller animal was broad and truncated, with an indistinct angular projection in the midline, to the sides of which the frontal margin sloped gently backward to the rounded lateral corners. There was no distinct narrowing behind the heatd. Posteriorly, the body ended in a blunt point.

The distance between the two eyes amounted to one-third, or a little more, of the width of the head. Their distance from the frontal margin was smaller than that from the lateral margins of the head.

The most characteristic feature of the species is its pigment pattern. The dorsal surface has a light yellowish-brown or grayish-brown ground color, which fades out toward the margins of the body. A dark brown band extends along the midline from a level behind the eyes to about the middle of the body (in the larger individual) or to close to the posterior end (in the smaller one). This band is less distinct than Figure 18 would suggest. To either side of the midline is a longitudinal row of from 6 to 10 unpigmented spots of different sizes and irregular shapes. The ventral side is lighter, whitish.

The locomotion of Dendrocoelum dorsivittatum is gliding, upon stimulation also crawling.

ANATOMY.—The anterior adhesive organ is a moderately developed field of adhesive epithelium, without special muscular differentiations. The pharynx is comparatively long and slender. In the two preserved specimens, its length amounts to 1.7 and 1.1 mm, corresponding to approximately one-seventh the length of the body.

The anatomy of the reproductive system places the species in close relationship to the remaining spotted species of Dendrocoelum. Only one fully mature specimen was available for the study of the genital organs. The testes are situated ventrally, dorsally, and at intermediate heights, extending from the level of the ovaries to close to the posterior end.

In the copulatory apparatus (Figure 42), the penis is located to the right, the bursal stalk and the adenodactyl to the left of the midline. The glandular area of thick epidermis around the genital aperture is typically developed. The penis has a rather large bulb enclosing a voluminous seminal vesicle (vs) into which the vasa deferentia (vd) empty separately from the anterolateral sides. The penis papilla consists of a thick-walled basal portion with a well-differentiated external circular muscle layer and a distal part with thin walls and a wide lumen. The two parts are separated externally by a circular groove.

The anterior enlargements or seminal receptacles of the oviducts are equipped with large tubal bursae. An outstanding feature is the great length of the common oviduct (odc) which is formed by the union of the two oviducts at a considerable distance behind the copulatory complex. The common oviduct runs anteriorly in an almost straight line and opens into the end part of the male atrium. The posterior two-thirds of the common oviduct and the terminal parts of the separate oviducts are connected with shell glands. The copulatory bursa (b) shows no peculiarities. Its outlet, the bursal duct (bd), starts as a narrow canal and gradually widens as it runs posteriorly, turning abruptly to the ventral side above the genital pore, and connecting with the common atrium. The adenodactyl (ad) is larger than the penis and has a very long lumen.

DISTRIBUTION AND ECOLOGY.—The two individuals of Dendrocoelum dorsivittatum were dredged in September 1935 in the profundal zone of Lake Ohrid, in Ohrid Bay, at depths between 47 and 55 m, on a bottom consisting of soft and hardened mud with few empty Dreissena shells.

TAXONOMIC POSITION.—I am establishing this new species with some hesitation, because we know so little about the variability of the color pattern of the undoubtedly closely allied forms of the Ohrid region. From related spotted species of Dendrocoelum, the new species differs mainly by the presence of the dark middorsal band. In the others (D. maculatum, D. sanctinaumi, D. ochridense, D. magnum, D. komareki, and D. decoratum), the dorsal midline has a color lighter than the ground shade of the dorsal surface. The dark midline shown in the figures for D. ochridense and young specimens of D. maculatum by Stankovi and Komárek (1927, text-fig. 5, 13) is erroneous. This was corrected in a later paper by Stankovi (1938, fig. 3, 4, 7). The great length of the common oviduct is perhaps the most remarkable anatomical characteristic of the species (which it shares with D. ochridense).

The name of the species, dorsivittatum (dorsum, Latin, back; vitta, Latin, band or stripe), alludes to the presence of the dark middorsal stripe.