Dendrocoelum lacteum (Müller 1774)

Dendrocoelum lacteum (Müller)

MATERIAL DEPOSITED.—Sagittal serial sections of 3 specimens on 23 slides, USNM 55266–55268.

Stankovi and Komárek (1927:661) and later Stankovi (in several papers) reported the occurrenc of this common European species in Lake Ohrid. Arndt (1938:52) also found an individual of this species on a specimen of a sponge, Ochridaspongia rotunda, dredged in Ohrid Bay. The lake form differs from the typical species, however, in some constant characters.

EXTERNAL FEATURES.—The animals in the lake attain a length of 25 mm and a width of 4–5 mm at maturity. The general shape of the body and the configuration of the head are the same as in the typical species. The head is truncate and is flanked by a pair of rounded auricular lobes that project laterally. The greater part of the frontal margin is differentiated into a large, somewhat lobed and folded adhesive cushion. The eyes are far apart from each other at a distance of about three-fifths the transverse diameter of the head. In mature animals, the pharynx lies approximately in the middle of the body, and the copulatory apparatus occupies the anterior third of the postpharyngeal region.

The lake form is characterized by a light, but distinct, reddish-brown coloration, distributed almost evenly over both the dorsal and ventral surfaces of the body. Only a very thin, white middorsal line remains free of pigment. This line is not always well distinguishable when the gut is filled, as the color of the intestinal contents may shine through on the dorsal side. In the fasting animal, however, it is clearly visible.

ANATOMY.—The lake form agrees with the typical species in all essential anatomical characters. The testes are situated in both the dorsal and ventral parts of the mesenchyme, in the prepharyngeal as well as the postpharyngeal regions. In the copulatory complex (Figure 50), the adenodactyl (ad) lies slightly to the left of the midline and is smaller than the penis. The bursal duct (bd) opens into the common atrium rather far dorsally, above the base of the papilla of the adenodactyl. The shape of the penis shows a wide range of variation in the four specimens examined. The penis bulb is generally of ellipsoidal shape and contains a seminal vesicle (vs), the walls of which have villuslike projections protruding into the cavity. The size of the vesicle varies within wide limits. In the specimen depicted in Figure 50, the vesicle was very voluminous and did not extend posteriorly beyond the penis bulb. In another case, the bulb was more contracted, the cavity narrower and situated for the greater part within the papilla of the penis. The two vasa deferentia (vd) open separately, generally far apart from each other, into the anterolateral sides of the seminal vesicle. The outlet of the vesicle, or ejaculatory duct, is rather short. At the transition between this outlet and the vesicle is seen the flagellum (fl). In two of the specimens, the flagellum was inverted into the vesicle, in the other two it projected into the male atrium. The flagellum is perhaps a trifle smaller than that of the typical species. A similar wide variation in the relative size of the penis bulb, the papilla, and the seminal vesicle, as we encounter in the lake form, is observed also in the typical Dendrocoelum lacteum (see particularly Gelei, 1928:10–11). The greater part of these apparent modifications is doubtlessly due to different states of contraction of the muscular systems of the organs, particularly that of the penis bulb. When the muscles of the bulb are relaxed, the bulb appears large, its cavity voluminous, and the penis papilla short. A contraction of the muscles, on the other hand, will reduce the size of the bulb, constrict the seminal vesicle, and push a considerable part of the penis tissues posteriorly; the papilla will then appear comparatively larger and will contain part or all of the seminal vesicle.

One of the four specimens examined showed a definite glandular area surrounding the genital pore. The epithelium was thicker than that of the general ventral surface and was pierced by many gland ducts filled with an eosinophilic secretion. The other three specimens showed no such marked glandular differentiation, although the epithelium was here also thicker and crowded with tall rhabdites. Only very few gland ducts were seen approaching the area. It may be that the glands develop only in a certain physiological stage of the reproductive cycle and function only a short time. In the typical Dendrocoelum lacteum I have occasionally observed a similar glandular field, and Iijima (1884:367, 371) has noticed a thicker epithelium with numerous long rhabdites in the vicinity of the genital pore. De Beauchamp (1932: 138), however, states that in the genus Dendrocoelum only representatives of the subgenus Neodendrocoelum have glands around the genital aperture. The glandular field of D. lacteum differs from that of other Ohrid species by the presence of rhabdites in the thickened epithelium, while in D. maculatum and relatives the cells of the glandular field lack rhabdite inclusions.

DISTRIBUTION AND ECOLOGY.—Dendrocoelum lacteum is rather common in the sublittoral zone (shell zone) of Lake Ohrid. It was collected in Ohrid Bay at depths ranging from 16–30 m and near Kalište at depths from 7 to 13 m. It also occurs in the profundal region (Ohrid Bay, 47–90 m), but was rarely taken in the littoral zone (near Kalište, above the Chara zone). Cocoons of this species were found repeatedly; they are unstalked, spherical, with a diameter of 3–3.5 mm. Three cocoons that I opened contained 5 to 8 embryos each. The hatching young were rather large (6 mm) and unpigmented. Specimens were kept in laboratory cultures for several weeks and accepted beef liver as food, but did not remain in good condition; they gradually reduced their pigmentation, and in many individuals the head region disintegrated. Apparently the food offered them was not adequate, as the species normally feeds on amphipods, isopods, and oligochaetes (Reynoldson and Davies, 1970). No cocoons were deposited in the aquaria.

TAXONOMIC POSITION.—The lake form of Dendrocoelum lacteum differs from the typical form principally by its pigmentation. Several older authors have described and even named “colored” varieties of D. lacteum in which, however, the color was due to the intestinal contents: Fasciola crenata Müller (1774:64); Planaria lactea var. crocea Baer (1827:728); four color varieties of D. lacteum named by Spoof (1889:7). The only true, genetically stable, colored subspecies known is Dendrocoelum lacteum verbanense, described by Benazzi (1945: 31–33) and again mentioned by Mirolli (1961:976), inhabiting the littoral zone of Lake Maggiore in northern Italy. In the Lake Ohrid form, there is also a true, almost uniform pigmentation present not only in the area covered by the intestine, but also in the head region and the margins of the body. Moreover, a very thin middorsal white line, free of pigment, is seen in starving specimens.

A white form of Dendrocoelum lacteum, D. l. bathycola (Steinmann), is known to occur in the profundal zones of several lakes in the Swiss and Austrian Alps, in a lake (Madüsee) in Pomerania, and possibly in Sweden (Vättern). It is characterized by a reduced size (7 mm), by a tendency to eye reduction, by a small number of intestinal branches, and by a relatively well-developed reproductive system (for a more comprehensive account, see Zschokke, 1911:82–84). It is disputable whether the characters of the subspecies bathycola are genetically fixed or are the result of the profundal environment that differs considerably from the usual littoral habitat of D. lacteum. The discontinuous distribution of the form would rather suggest that it originated from the typical species independently in the individual lakes and that it should be considered merely an ecological modification. The Lake Ohrid form of D. lacteum has little in common with the subspecies bathycola: its size is above the average size of the typical species; the eyes are well developed; and the body is opaque (in bathycola rather translucent).

No transitional specimens that would bridge the gap between the typical D. lacteum and the Ohrid form have been found. Nevertheless, I refrain from considering the lake form to be a separate subspecies until its relation to the typical species can be studied more intimately.

Dendrocoelum jablanicense (Stankovi and Komárek)

Neodendrocoelum jablanicense Stankovi and Kamárek, 1927: 613.

Dendrocoelum (Dendrocoelum) jablanicense.—Kenk, 1930:301.

Dendrocoelum (Eudendrocoelum?) jablanicense.—de Beauchamp, 1931:158.

Dendrocoelum (Neodendrocoelum?) jablanicense.—Gourbault, 1972:68.

Dendrocoelum jablanicense.—Kenk, 1974:17.

MATERIAL DEPOSITED.—Sagittal, transverse, and horizontal serial sections of 4 specimens on 9 slides, USNM 55262–55265.

Dendrocoelum jablanicense was described by Stankovi and Komárek (1927:613–616) from specimens collected in Šum, a large limestone spring west of Struga, on the northern bank of Lake Ohrid, so far the only known habitat of this interesting species. From the same locality, de Beauchamp (1937:357) obtained a single specimen that conformed to some extent with the description by Stankovi and Komárek, as well as several samples of D. adenodactylosum. De Beauchamp considered the specimen to be a somewhat deviate form of D. adenodactylosum. I also collected the species in Šum but have found it to be more common in some tributaries of Lake Ohrid.

EXTERNAL FEATURES (Figure 25).—Because of the very similar outward appearance of Dendrocoelum jablanicense and D. adenodactylosum, I did not differentiate between the two species in the field, nor did I analyze the detailed habit of D. jablanicense in the laboratory. For the external aspect of the living animals we must, therefore, rely on the description given by Stankovi and Komárek. According to these authors, D. jablanicense is somewhat smaller than D. adenodactylosum (the length of mature animals being 10–15 mm and 20 mm, respectively) and has a more slender shape. The truncate head is only little narrower than the body and not separated from it by any constriction. The frontal margin does not show the well-marked lateral lobes seen in D. adenodactylosum; only a slight hint of such lobes is indicated by the wavy outline of the margin. The distance between the two eyes is about one-third the diameter of the head, while in D. adenodactylosum the eyes are farther apart. In my preserved material, the average size of D. jablanicense is smaller than that of D. adenodactylosum, though the difference is certainly not significant. Small mature specimens of D. adenodactylosum may be no larger than large specimens of D. jablanicense.

ANATOMY.—The subterminal adhesive organ is rather weak. It consists of a small area of a median diameter of 40 μm, which has either a shallow concave or a bulging surface in the preserved animal. It connects with numerous eosinophilic gland ducts but apparently has no special muscular differentiations.

The testes are situated ventrally and form a broad zone extending from the level of the ovaries to the tail end on either side of the midline. The zones meet and unite in the midline only behind the copulatory complex.

The arrangement of the organs of the copulatory apparatus is very characteristic for the species, as pointed out by Stankovi and Komárek and again by Stankovi (1969:428). The adenodactyl (ad) lies to the right side of the midline and not to the left side as in all other species of Dendrocoelum from the Ohrid region. The duct of the copulatory bursa (bd) is markedly expanded and has a strictly lateral position to the left of both the penis and the adenodactyl.

The genital aperture (Figure 51, gp) is situated in an area of modified epithelium that is pierced by many gland ducts (gl). It leads into a moderately large chamber, the common atrium, that encloses the papilla of the adenodactyl. Dorsally, this cavity connects with the male atrium (am) by a short, narrow canal. Laterally on the left side, it communicates with the bursal duct by one or two rather small openings (bdo). The male atrium (am) is more or less conical and extends posteriorly beyond its connection with the common atrium, to receive, at its end, the mouth of the common oviduct (odc).

The penis is of moderate size, smaller than the adenodactyl, and consists of a muscular bulb and a rather short papilla. A more or less marked constriction separates the basal part of the papilla from the somewhat expanded distal portion or glans. The outer wall of the papilla has a well-differentiated layer of circular muscles. Beneath these, in the parenchyma of the papilla, are longitudinal muscles fibers. The lumen of the penis is variable in shape, depending on the state of contraction of the muscles of the penis bulb. It may form a voluminous cavity or seminal vesicle within the bulb and open through a narrow canal at the tip of the papilla as shown by Stankovi and Komárek (1927, text-figs. 10, 11) and found in one of my specimens. In other specimens, the cavity may appear constricted, assuming a more tubular shape, and extending throughout the length of the organ. Upon extreme contraction of the penis bulb, the lumen and the tissues surrounding it may be pushed posteriorly into the papilla. The penis is traversed by numerous gland ducts containing a finely granular, weakly eosinophilic secretion and opening into the penial cavity. The two vasa deferentia (vd) enter the penis bulb separately, converge toward the midline of the organ, and unite to form a common vas deferens that connects with the penis lumen from the ventral side. This union of the two sperm ducts is very characteristic, as it has so far been observed in only two other species of the genus Dendrocoelum (D. puteale Kenk and D. kenki de Beauchamp), which in other regards show no close relationship to D. jablanicense.

The two oviducts unite behind the copulatory apparatus, embracing the bursal stalk. The moderately long common oviduct (odc) runs anteriorly and opens into the posterior extension of male atrium. Both the end parts of the paired oviducts and the greater portion of the common oviduct are connected with eosinophilic shell glands. The sacshaped copulatory bursa (b) shows no peculiarities. Its outlet or duct (bd), however, has a remarkable shape and position: it starts as a rather narrow canal, proceeding posteriorly to the left of the penis, then gradually widens to form a laterally compressed sac that occupies about one-half the dorsoventral diameter of the body at the level of the adenodactyl and connects with the common atrium by one or two rather narrow openings (bdo). This double communication between the bursal duct and common atrium is unique among the freshwater triclads. The adenodactyl (ad) is larger than the penis but is not excessive in size. It contains a rather long, tubular lumen.

DISTRIBUTION AND ECOLOGY.—Dendrocoelum jablanicense is an inhabitant of cold springs. All literature records (Stankovi and Komárek, 1927:616; de Beauchamp, 1937:357; Stankovi 1969:428) list the source of Šum, a large spring west of Struga, as the only known habitat of the species. I visited that locality and besides collecting D. jablanicense, I also collected D. adenodactylosum, D. maculatum, Crenobia alpina montenigrina, and Phagocata ochridana. Most of my material of D. jablanicense was taken in the cold springs at Studenište and at Bej-Bunar, where the animals are found on the undersides of stones together with other triclad species. It is possible that its distribution in springs of the Ohrid region is much wider than is known at present, as the animals are easily confused with D. adenodactylosum. Stankovi (1960:179) reported that in Šum the species is found also in the cave from which the spring issues, and he assumed that it might be a troglophile in spite of its well-developed eyes. I made no attempt to rear the species in laboratory culture, as I had not expected to find it among the materials collected in the more eastern springs. It was recognized only when analyzing the anatomy of the preserved specimens.

TAXONOMIC POSITION.—Dendrocoelum jablanicense stands somewhat apart from the other species of Dendrocoelum of the Ohrid region in several anatomical characters. In the field it is easily confused with another white spring planarian, D. adenodactylosum. The principal differences between the two species in life have been pointed out by Stankovi (1969:428): D. jablanicense has a different shape of the head, the eyes are closer together, and the intestinal branches less ramified than in D. adenodactylosum. Among the anatomical characters, the spatial arrangement of the parts of the copulatory complex does not conform with the general scheme observed in the genus: the bursal duct is situated to the left and the adenodactyl to the light of the penis, while in most related species the bursal duct and adenodactyl are placed on the same side, generally left of the penis. The union of the two vasa deferentia before they open into the seminal vesicle is also exceptional. The peculiar communication between the common atrium and the duct of the copulatory bursa, by one or two short canals or narrow openings, is another character not known in other freshwater triclads. Paunovi (1977) found the chromosome number of the species to be 2n = 32.

Dendrocoelum lacteum is a freshwater planarian found in lakes and running waters in Europe, being the most widespread freshwater planarian in this continent.