Dendrocoelum magnum (Stankovic 1969)

Dendrocoelum magnum (Stankovi)

?Neodendrocoelum sp. 4 Stankovi 1955a, pl. 5: fig. 8.

Neodendrocoelum grande Paunovi and Rimsa, 1968:413 [nomen nudum].

Neodendrocoelum magnum Stankovi, 1969:414.

Dendrocoelum (Neodendrocoelum) magnum.—Gourbault, 1972:70.

Dendrocoelum magnum.—Kenk, 1974:18.

MATERIAL DEPOSITED.—Sagittal serial sections of 2 specimens on 11 slides, USNM 55278–55279.

This is a rather common species of the shell zone of Lake Ohrid.

EXTERNAL FEATURES (Figure 10).—The species is characterized by having a thick and bulky body, not as flattened as the similar D. sanctinaumi or D. maculatum. My specimens measured up to 22 mm in length and 6 mm in width. According to Stankovi, they may attain a length of 26 mm at full maturity. The anterior end is truncate with a very slightly bulging frontal margin and rounded lateral corners. Behind the head is only a slight indication of a neck constriction. The posterior end is rather bluntly pointed. The pigmentation of the dorsal side consists of a dark grayish-brown ground color, which fades out toward the body margins, and a pair of longitudinal rows of profusely branched whitish or light yellowish spots, 8–12 in each row. An indistinct light brown stripe runs along the dorsal midline, widening somewhat above the pharynx. On either side of the head is seen a short unpigmented streak, apparently corresponding to the auricular sense organs. The ventral surface is a dirty white or light yellow.

The two eyes are rather close together, at a distance of about one-fourth the width of the head. Their distance from the frontal margin is less than that from the lateral margins.

The locomotion of the animal is by a slow gliding, rarely by crawling movements.

ANATOMY.—The adhesive organ is a small (up to 170 μm in diameter) subterminal field of infranucleate epithelium in which the eosinophilic adhesive glands open and to which a few longitudinal fibers of the ventral subepidermal muscle layer are attached.

Unfortunately, I had no fully mature specimens of Dendrocoelum magnum at my disposal, only a few individuals in which the reproductive organs were in the anlage stage. For the analysis of the reproductive system we have, therefore, to rely on Stankovi's (1969:414–417) detailed description. The zone of rather small testes extends from the level of the ovaries to the posterior end. The testes are indicated as being predominantly dorsal (however, I observe also many testicular anlagen in the ventral parts of the mesenchyme in my specimens). The oviducts are equipped with well-developed yolk funnels and their anterior enlargements adjoining the ovaries show large typical tubal bursae. The adenodactyl and bursal duct are situated to the left, the penis to the right of the midline. The genital pore is surrounded by a field of epidermis pierced by numerous eosinophilic glands. The penis is shown in Stankovi's figure (see Figure 37) as a rounded bulb with a large spherical cavity, into which a rather folded penis papilla is completely invaginated. The long male atrium connects with the common atrium close to the gonopore, receiving the common oviduct in its terminal part.

DISTRIBUTION AND ECOLOGY.—Dendrocoelum magnum is a common species essentially confined to the sublittoral (shell zone) of Lake Ohrid. My specimens were dredged from depths between 19 and 30 m in Ohrid Bay, always associated with Dreissena, which appears to be their natural food. Stankovi reported its occurrence at depths of 20–50 m and stated that it deposits its cocoons of 3.5–4 mm diameter on dead Dreissena shells. The species may be kept in the laboratory at 10°C on a diet of beef liver and also tolerates temperatures above 20°C, according to Stankovi.

TAXONOMIC POSITION.—Dendrocoelum magnum agrees with the other pigmented species of Dendrocoelum of the Ohrid area in all its anatomical characters. From the similar D. maculatum and D. sanctinaumi, it differs by the bulkiness of its body and the pigmentation pattern of the dorsal side, as well as by the different ecological habitat. Paunovi and Rimsa (1968) analyzed the chromosomes of “Dendrocoelum grande” and found their number to be 2n = 32. One of the authors (D. Paunovi) informed me that the species was D. magnum, still undescribed at the time of their publication. A detailed analysis of the chromosome morphology was given by Paunovi (1977).

Dendrocoelum sanctinaumi (Stankovi and Komárek)

Neodendrocoelum St. Naumi Stankovi and Komárek, 1927:609.

Neodendrocoelum spec.—(immaturum II) Stankovi and Komárek, 1927:622.

Dendrocoelum (Dendrocoelum) st.–naumi.—Kenk 1930:300.

Dendrocoelum (Neodendrocoelum) Sancti-Naumi.—de Beauchamp, 1931:157.

Neodendrocoelum Sancti Naumi.—Komárek, 1953a:303.

Neodendrocoelum svetinaumi.—Geus, 1967:253.

MATERIAL. DEPOSITED.—Sagittal and transverse serial sections of 9 specimens on 29 slides, USNM 55285–55293.

Descriptions of Dendrocoelum sanctinaumi were presented by Stankovi and Komárek (1927:609–613) and by de Beauchamp (1932:216–219).

EXTERNAL FEATURES (Figure 3).—The species is comparatively short and broad, mature individuals attaining a maximum length of 15 mm and a width of 4 mm. The anterior end is truncate, with a bulging frontal outline that forms an indistinct, very blunt, median point in the quietly gliding animal. The rounded lateral lobes are not marked off from the central region of the frontal margin. Behind the head is an indistinct trace of a neck constriction. The lateral margins then diverge very gradually to reach the maximum width at about the posterior third of the body length. The tail end is bluntly pointed. At rest, the animal assumes a broad, ovoid shape.

The two eyes lie rather close together at a distance of about one-third (or less) the width of the head. The mouth is situated a little behind the middle of the body, the genital pore halfway between the mouth and the posterior end.

The ground color of the dorsal surface is brown, varying in shade from a light grayish-brown to almost black. The light, yellowish color pattern resembles that of Dendrocoelum maculatum: it forms a rim along the margins of the body; a streak along the midline, beginning at the anterior point of the head, narrowing behind the eyes, widening again in the regions of the pharynx and the copulatory complex, and ending at the posterior tip; and a row of four to six lobed or irregularly branched spots in the lateral fields on each side of the midline. These spots are, in general, wider and less ramified than in D. maculatum, extending more in a transverse direction. They may, though rarely, show a tendency to fuse longitudinally. A pair of short oblique splashes above the lateral lobes of the head mark the position of the auricular sense organs. The ventral surface is uniformly gray.

Dendrocoelum sanctinaumi is slower and less active than D. maculatum. Its locomotion is either gliding or crawling. Already Stankovi (1960:260) observed that D. sanctinaumi usually contracts and remains still upon mechanical stimulation, while D. maculatum tries to escape by rapid crawling movements.

ANATOMY.—The anterior adhesive organ is very weak, forming a small subterminal spot, 70–100 μm in diameter, no longer than the width of the lateral adhesive zone from which it is separated by a small gap on either side. The pharynx is short, its length amounting to one-ninth or one-eighth the length of the body in preserved specimens. The ratio between the width and the length of the pharynx is 1:1.1 or 1:1.6.

The reproductive system of Dendrocoelum sanctinaumi has been analyzed in detail by Stankovi and Komárek and by de Beauchamp. The rather small testes are situated in the ventral, dorsal, and intermediate parts of the mesenchyme, extending from the level of the ovaries posteriorly to about midway between the gonopore and the tail end. The copulatory organs (Figure 38) are very similar to those of D. maculatum, with only some quantitative differences. The genital pore is surrounded by a circular area of modified epithelium with many gland ducts (gl), analogous to that described for D. maculatum. The general arrangement of the parts of the copulatory apparatus is the same as in the latter species, the adenodactyl and bursal stalk lying somewhat to the left and the penis and common oviduct to the right of the midline. The penis and the adenodactyl are of approximately equal size. In most specimens, the penis papilla (pp) is elongated, filling the entire male atrium (am), sometimes reaching down to the genital aperture. The penial lumen is a wide duct running from the bulb to the tip of the papilla. Its anterior portion may be somewhat expanded and is lined with a taller epithelium through which gland ducts open. This portion receives the separate openings of the two vasa deferentia (vd) and corresponds to a seminal vesicle (vs). The posterior part of the lumen, lined with a more flattened epithelium, may be considered to be a modified ejaculatory duct. In one of the eight animals examined, however, the distal part of the papilla was inverted into the lumen of the penis (Figure 55). It is possible that the inverted position is the normal condition in the living animal and that the eversion and elongation of the penis is brought about by a contraction of the muscles in the wall of the penis bulb when the animal is being killed. The arrangement of the muscles and glands of the penis is similar to that of D. maculatum.

From the junction of the two oviducts behind the copulatory complex the common oviduct (odc) passes anteriorly to the right of the bursal stalk and connects with the posterior part of the male atrium (am). The copulatory bursa (b) has a rounded or more or less lobate outline. The bursal duct or stalk (bd) widens somewhat above the common atrium (ac) but is not as excessively broadened and flattened as in D. maculatum. It curves ventrally to open into the common atrium.

ECOLOGY AND DISTRIBUTION.—Stankovi and Komárek collected mature specimens of Dendrocoelum sanctinaumi only in running water, in springs near the monastery of Sveti Naum. Immature animals were found also in the littoral zone of the lake but were rarer than D. maculatum. De Beauchamp's material also comprised mature individuals from Sveti Naum and a few immature specimens collected near the shore of the lake at the towns of Ohrid and Struga. In later papers, Stankovi (1955a:491; 1969:430) reported its occurrence also in the sublittoral zone. I found D. sanctinaumi to be fairly common in cold streams along the east side of the lake: Bej-Bunar, Studenište, and Sveti Naum. It occurs usually under stones, frequently in a strong current, often resting (feeding?) on small snails. As a rule, it is absent in the springs themselves and appears in the streams at some distance from their source. In the lake itself, the species occurs from the littoral zone down to the shell zone but is nowhere common. Some specimens collected in summer in the shell zone of Ohrid Bay (at depth from 16 to 26 m) were sexually mature. From these data we may conclude that D. sanctinaumi attains maturity in the cold habitats (streams, deeper zone of the lake) at any time of the year; in the littoral waters, which warm up considerably in the summer months, it apparently develops genital organs only during the colder season (see also Stankovi, 1969:430). It is, however, a eurythermal species and may be maintained in the laboratory for months at a temperature above 25°C (Stankovi, 1934:175).

I have kept D. sanctinaumi in laboratory culture for prolonged periods at 10°C. The animals refused to take beef liver as food and diminished in size. It is possible that aquatic snails are their normal food, as they are often associated with them in their natural habitat. No experiments were made to verify this assumption.

TAXONOMIC POSITION.—Dendrocoelum sanctinaumi fits well into the group of pigmented planarians of the Ohrid region. Its differences from D. maculatum, which resembles it superficially by its pigment pattern, are discussed under that species. According to Paunovi (1977), the chromosome numbers are the same in both species, 2n = 32.