Dendrocoelopsis vaginata Hyman 1935

Dendrocoelopsis vaginata Hyman, 1935

Dendrocoelopsis vaginatus Hyman, 1935:339.

Dendrocoelopsis vaginata.–Kenk, 1953:177.

HOLOTYPE.–From shores of Flathead Lake, Montana, set of sagittal sections (AMNH 665).

The species was originally described from preserved specimens sent to Dr. Hyman by Dr. R. T. Young of the University of Montana, two of which were sexually mature. Kenk (1953) changed the ending of the specific name to vaginata to agree in gender with the feminine genus Dendrocoelopsis.

Sexually mature specimens are up to 21 mm long and 2.8 mm wide. The anterior end is truncate, the frontal margin, bearing the subterminal adhesive organ, in the quietly gliding animal is either slightly bulging or centrally retracted (Figures 1A, 2A). In locomotion this central portion is elevated somewhat above the substrate. The rounded corners of the head protrude gently laterally so that a slight narrowing or neck appears behind them. The margins of the body then diverge gradually for about one-fourth the body length, become parallel, and converge again in the region of the copulatory complex, forming a bluntly pointed posterior end.

The color of the animal in life is dark chocolate brown dorsally and a little lighter ventrally. A pair of oval fields above the eyes, a spot at the mouth opening, and a circular field around the gonopore are free of pigment. Two indistinct lighter fields extend from the eyes toward the frontal margin, flanking the adhesive organ, and two short, light, longitudinal streaks parallel the body margins at the level of the neck. Occasionally a dark middorsal line is seen, beginning some distance behind the eyes and extending to the region of the pharynx, and a less distinct line, behind the pharynx to the posterior end. In her original description, Hyman (1935:339) reported that the species, collected in Montana, was white and, hence, without body pigment; in a later paper (1963:4) she stated that the dark color seen in another population, from Oregon, was due not to body pigment but to inclusions in the cells of the gastrodermis, presumably originating from ingested food. It appears that Hyman had studied only preserved specimens. The pigment of D. vaginata is diffused in the mesenchymal tissue and is not the granular pigment confined to special mesenchymal cells that occurs in some other triclads and that can be identified in microscopic sections. Furthermore, it bleaches easily in preserved specimens and then cannot be recognized. Live worms from both of Hyman’s localities show clearly that the pigment is not confined to the intestinal system.

The two eyes are situated at a distance from each other equal to about one-third the width of the head and their distance from the frontal margin is slightly greater than that from the lateral margin.

The adhesive organ, located in the center of the frontal margin of the head, appears in the sections as a convex or concave field of infranucleate epithelium lacking cilia or rhabdites and perforated by numerous gland ducts filled with a granular, eosinophilic secretion. The cell bodies of these glands are scattered in the mesenchyme of the anterior third of the prepharyngeal region, predominantly dorsal to the intestinal branches. Muscle fibers attach to the adhesive area, chiefly extensions of the internal longitudinal layer of the ventral integumental musculature. The adhesive organ may be interpreted as a modification of the submarginal adhesive zone, which has a similar infranucleate and rhabdite-free epithelium. The organ and the zone are not continuous but are interrupted on either side at what appears to be the location of the lateral auricular lobes, which are covered with a nucleate, ciliated epithelium, in part lacking rhabdites, presumably a sensory epithelium.

The digestive system shows no peculiarities. The pharynx, measuring in length about one-sixth the length of the body, is inserted at the middle of the body length. Its internal muscle layer consists of intermingled longitudinal and circular fibers as is typical for the family Dendrocoelidae. The anterior intestinal ramus bears five to six pairs of branches and each posterior ramus about 12 to 14 lateral branches.

The reproductive system has been well described by Hyman (1935). The two ovaries are located below or behind the second pair of intestinal branches. The numerous, rather small testicles are ventral, arranged in longitudinal rows extending from some distance behind the ovaries to the level of the copulatory complex. Four rows are developed in the prepharyngeal region, one pair lateral and one pair medial to each of the ventral nerve cords. The medial rows terminate as they approach the level of the pharynx while the lateral rows continue along the sides of the pharynx and the copulatory apparatus.

In the copulatory apparatus (Figure 4) the genital atrium may be divided into a large anterior male atrium (am) and a smaller common atrium (ac), but frequently there is no constriction visible between the two cavities. Apparently the shape of the atrium is subject to deformation by muscular contraction at the moment of killing. The wall of the male atrium is lined with an epithelium of tall, glandular cells, that of the common atrium with a cuboidal epithelium. Below the lining of both divisions is a layer of circular muscle fibers, followed by a layer of longitudinal fibers.

The penis consists of a large spherical bulb and an elongated conical papilla. The bulb, which is formed by the usual concentric layers of muscle fibers, contains a voluminous cavity, the seminal vesicle (vs). The cavity is lined with a secretory epithelium and has villus-like extensions projecting inward. The cavity continues into the penis papilla as a narrower canal, the ejaculatory duct (de), running in the axis of the papilla and opening at its tip. The lining of this canal is a cuboidal epithelium. Below the flattened external epithelium of the penial papilla is a thick layer of fine circular fibers which do not stain like muscle fibers, the fibrous layer (f); presumably this is of an elastic nature and seems to impart a certain rigidity to the papilla. This layer is followed by a sheet of longitudinal muscles which in part attach to the fibrous layer. The two sperm ducts or vasa deferentia (vd), which at the sides of the pharynx had enlarged to form the tortuous false seminal vesicles or spermiductal vesicles, approach the penis bulb, enter it from the sides, and open separately into the seminal vesicle. Numerous gland ducts, with a granular, faintly eosinophilic secretion, penetrate the penis bulb from the surrounding mesenchyme and empty into the posterior part of the seminal vesicle and the ejaculatory duct.

The two oviducts or ovovitelline ducts converge toward the midline and unite in the space between the atrium and the bursal stalk to form a common oviduct (ode) which opens into the atrium at the transition between the male and common atria. The terminal parts of the paired oviducts and the common oviduct receive many outlets of strongly eosinophilic glands, the so-called shell glands.

The sac-shaped copulatory bursa (b) is well-developed. Its outlet or stalk (bd) originates from its posterior wall, first ascends dorsally, proceeds above the penis and the genital atrium in the midline or somewhat to the right side, and finally bends ventrally to open into the atrium close to the genital aperture (gp). In its proximal part, above the penis, the duct is rather narrow and smooth, but it progressively widens distally, and is thrown into irregular folds. Its muscular coat consists of interlaced circular and longitudinal fibers and gradually increases in thickness toward its posterior opening.

Dendrocoelopsis vaginata is an inhabitant of cool streams and lakes in some of the northwestern states. So far it is known only from a few localities, but it will probably be found to be more widely distributed upon further collecting. The following are the records that have come to my attention:

MONTANA. GALLATIN COUNTY: (1) Spring north of Belgrade, in outlet crossing the county road about 4 miles north of junction with U. S. Highway 10. 2 September 1967: water almost stagnant, with much vegetation, temperature 10.4° C, pH 7.5; 5 immature specimens under stones. (2) Tributary of Thompson Creek, crossing county road north of Belgrade, 4.3 miles from junction with U. S. Highway 10. 2 September 1967: water with fast current, 108° C, pH 7.6; about 15 specimens under stones, 5 of them mature. 4 September 1967: 10 specimens of various sizes, 4 of them mature, under stones. 4 June 1970: 14.0° C, 4 specimens under stones, 2 of them mature (Polycelis sp. also occurs in this locality). (3) Tributary of Gallatin River, opening into the river above the bridge on U. S. Highway 10, 5.3 road miles northwest of Belgrade. 2 September 1967: water fast, clear, 16.6° C, pH 8.8; 2 Dendrocoelopsis under stones, one of them mature. LAKE COUNTY: (1) Along shore of Flathead Lake and in tributary of the lake, type-locality of D. vaginata (Hyman, 1935:344). (2) Yellow Bay Creek, about 10 yards from the mouth of the creek in Flathead Lake (possibly the same locality as that mentioned by Hyman). 10 August 1969: several specimens collected by Mr. Frank P. Crowley and shipped to me alive [Polycelis coronata coronata (Girard) also present], MISSOULA COUNTY: Greenough Park, Missoula, feeder spring and creek for Rattlesnake Creek (1 mile from Clark Fork River). 21 May 1968: water slightly muddy, 10° C, 8 specimens collected by Mr. F. P. Crowley, most of them sexually mature.

OREGON. MULTNOMAH COUNTY: Woodbury Spring in Crystal Springs area of Eastmoreland District of Portland, one-half block south of Holgate on 28th Avenue. 12 September 1967: water almost stagnant, clear, 11.2° C, pH 6.3; many D. vaginata taken on liver bait, both young and mature (the spring contains also Phagocata oregonensis Hyman and Polycelis sp.). This is possibly the same locality as that mentioned by Hyman (1963:1).

WASHINGTON. WHITMAN COUNTY: Nigger Creek, 18 miles south of Cheney. 1 August 1970, 5 D. vaginata collected by Mrs. Virginia Erickson and sent to me alive, one of them mature.

Sexually mature specimens of D. vaginata were taken in the field at all seasons, when collections were made, from March to September; no data are available for the winter months. It is possible that the species breeds the year round. No indications of asexual reproduction were observed. The temperature of the water varied from 10° to 16.6° C, with an average of 12.2° C (6 measurements), the pH from 6.3 to 8.8. Further observations on the physicochemical parameters of the habitats are lacking.

Specimens were kept in the laboratory in cold-water cultures (14° C) for several months, fed beef liver or Tubifex worms, but could not be maintained indefinitely. Three cocoons were deposited in the aquaria, all within the first weeks after the collections. The cocoons were unstalked, spherical, with a diameter of 1.6 to 1.8 mm, attached to the substrate by a copious amount of colorless jelly. Two cocoons hatched several weeks after deposition and yielded seven and five young. They were of different sizes, the largest being about 3.5 mm long and 0.7 mm wide. They showed the adhesive organ as a transverse, opaque field in the center of the frontal margin (Figure 3A). On both sides of the organ the margin sloped backward to the region of the feebly developed rounded auricles, behind which a slight constriction or neck was visible. The young were initially pigmentless, white, but had acquired body pigment by the time they were four weeks old.

The three pigmented species of the genus Dendrocoelopsis, i.e., D. vaginata, D. piriformis Kenk (1953) from Alaska, and D. ezensis Ichikawa and Okugawa (1958) from Hokkaido, Japan, resemble each other rather closely in their external appearance. Dendrocoelopsis vaginata may be distinguished from D. piriformis by having a slightly different outline of the body, the lateral margins running parallel when the animal is gliding quietly, while in D. piriformis the greatest width is in the last third of the body. Furthermore, the pigmentation of the two species is different, D. vaginata lacking the accumulation of dark pigment between the eyes and on the sides of the adhesive organ and not showing any pattern of longitudinal stripes, such as are seen frequently in D. piriformis. Dendrocoelopsis ezensis (Figure 2B), which in its pigmentation appears to be close to D. vaginata, has the eyes set farther apart (about one-half the width of the head) than D. vaginata. Anatomically the three species may be separated by the location of the testes: in D. vaginata they are ventral, extending to the copulatory apparatus; in D. piriformis, dorsal, extending to the posterior end; and in D. ezensis, ventral and dorsal, extending to the posterior end of the body. The anatomy of the penis also furnishes good distinguishing characteristics: D. piriformis lacks a fibrous layer in the wall of the penis papilla; and D. ezensis has a plug-shaped penis papilla and a thick muscular layer surrounding the seminal vesicle and the ejaculatory duct.