Dendrocoelum albidum Kenk 1978

Dendrocoelum albidum

TYPE MATERIAL.—Holotype, set of serial sagittal sections on 2 slides, USNM 55233; paratypes, sagittal sections of 2 specimens on 6 slides, USNM 55234–55235.

This is a small, whitish Dendrocoelum inhabiting the shell zone of Lake Ohrid.

EXTERNAL FEATURES (Figures 14, 19).—Mature specimens are 7–9 mm long and 1–1.5 mm wide. The truncated head has a straight or slightly bulging frontal margin and rounded lateral edges not projecting laterally. During locomotion, the frontal contour may vary and a transitory short projection may appear in the midline, corresponding to the site of the adhesive organ, which may be recognized in life as a small opaque spot. There is no narrowing or neck behind the head. The lateral margins diverge for a short distance behind the anterior end, converge again in the posterior region, and meet at the rather pointed posterior end.

Eyes are normally two, separated from each other by about one-third, or slightly more, the transverse diameter of the head; their distance from the frontal margin is about equal to, or a little greater than, their distance from each other. A division of the principal eyes into two or three cups is frequently observed. The median trunk of the intestine reaches anteriorly almost to the level of the eyes.

The general color is usually whitish, with the darker intestine shining through when filled. Many specimens, however, show a light reddish-brown pigmentation on the dorsal side. This pigmentation may be easily discernible or it may appear as a very light tint visible only in the starving animal when it is not obscured by the color of the intestinal contents. There are all intergrades between purely white and distinctly pigmented individuals. In a group of white specimens collected at one time and kept under observation for about three months, several animals developed different degrees of pigment while others remained white. When it occurs, the pigment is arranged in two broad longitudinal stripes, one on each side of the midline (Figure 19). Each stripe has a sharp medial border, while laterally it fades out toward the margin. No pigment is present on the head, along the midline, and in a rather broad rim along the margins of the body. The ventral surface is always unpigmented.

The pharynx is situated approximately in the middle of the body. The copulatory complex occupies up to one-half of the postpharyngeal region.

Dendrocoelum albidum is in life easily recognized by its peculiar pigment pattern. Unpigmented specimens, however, cannot be readily distinguished from other small white species of Lake Ohrid, such as D. minimum, D. sinisai, and even Phagocata ochridana.

ANATOMY.—The adhesive organ (Figure 29) consists of a deep tubular pit (ao) opening on the underside of the head close to the frontal margin. It is lined with an infranucleate epithelium pierced by gland ducts containing an eosinophilic secretion. The same secretion also fills the cavity of the pit. The musculature of the organ consists mainly of fibers belonging to the longitudinal layer of the ventral subepidermal muscles. In a median section, these fibers are seen adhering to the wall of the pit and running backward, in more or less curved lines, passing below the cerebral commissure (cc) to enter the ventral muscle layer. The ventral layers of circular and diagonal fibers (mvc) are not modified in the region of the adhesive organ, while the inner layer of transversal muscles (mt) is absent in that region. The cell bodies of the adhesive glands are situated at a considerable distance behind the anterior end. Their outlets (gl) pass above the cerebral commissure.

In the reproductive system, the numerous testes occupy both the dorsal and ventral parts of the mesenchyme extending from the level of the ovaries to near the posterior end. The ovaries and yolk glands show no peculiarities.

The anatomy of the copulatory complex (Figure 46) conforms, in general, with that of related species of the Dendrocoelum adenodactylosum group. The bursal duct (bd) is located to the left of the midline, the adenodactyl (ad) beneath, or somewhat to the left of, the penis. The genital pore (gp) is surrounded by an epithelium somewhat thicker than the general surface epithelium of the ventral side. The cells of the thickened area contain no typical rhabdites and are apparently of a glandular nature.

The penis is of moderate size and has an inversible papilla (ppi). When the papilla is inverted, the penis bulb appears large and contains an expanded lumen (vs) into which the two vasa deferentia (vd) empty. On the other hand, when the papilla is completely everted (Figure 53A), the bulb is contracted and the lumen is reduced in size. The papilla consists of a basal portion with a thick wall and a strong musculature (particularly a layer of circular muscles underlying the outer epithelium) and a thin-walled distal part of variable shape. The male atrium (am) is lined by a thick, glandular epithelium and is surrounded by a strong coat of circular and longitudinal muscle fibers.

The common oviduct opens into the end part of the male atrium. Both the terminal portions of the paired oviducts and the greater part of the common oviduct connect with numerous eosinophilic shell glands. The copulatory bursa (b) is a rounded or more irregularly shaped sac of moderate to rather small size. The bursal duct (bd) is at first a narrow canal running posteriorly to the left of the penis. In its posterior part it widens considerably and then opens into the common atrium from the dorsal side. Before its opening it forms a well-developed, rather characteristic, posterior caecum (cbd). The adenodactyl is of moderate size, about as large as the penis or somewhat larger. It has a conical papilla and a long lumen.

ECOLOGY AND DISTRIBUTION.—Dendrocoelum albidum was dredged in the shell zone of Lake Ohrid, in Ohrid Bay, at depths ranging from 16 to 30 m. Both mature and young individuals were taken. The species seems to be rather common in the shell zone but is easily overlooked on account of its small size. Several young animals were reared to maturity in aquaria on a diet of fresh beef liver.

TAXONOMIC POSITION.—Dendrocoelum albidum is undoubtedly a member of the D. adenodactylosum group, considering the anatomy of its reproductive system. Among its outstanding characteristics is the frequent development of a specific pigment pattern and the presence of a rather highly differentiated adhesive organ. It shares the latter characteristic with a closely allied species of the profundal zone, D. sinisai, from which it differs externally by having the eyes situated more closely together and by the fact that its intestinal area reaches anteriorly to the level of the eyes.

The name of the species, albidum (albidus, Latin, whitish), refers to the white or near-white color of the animal.