Sphalloplana (Sphalloplana) californica Kenk 1977

Sphalloplana (Sphalloplana) californica

TYPE MATERIAL.—Holotype, set of sagittal sections on 5 slides, USNM 53432. Paratypes, two sets of sagittal and transverse sections on 8 slides, USNM 53433–53434.

EXTERNAL FEATURES (Figures 4, 21).—The largest specimen, when quietly gliding, measured 17 mm in length and 2.5 mm in width. The anterior end is bluntly truncate, with a conspicuous indentation in the center of the frontal margin. The lateral edges of the head are rounded, without auricular projections. No distinct constriction or neck is seen behind the head, at most a very slight transitory narrowing during locomotion. Then the body gradually widens and the lateral margins become parallel up to the postpharyngeal region. The posterior end may appear rounded or bluntly pointed during gliding locomotion. The pharynx is rather short, about one-eighth the body length, and its root lies somewhat posterior to the middle of the body. The motion of the animal is either a smooth gliding or, upon mechanical stimulation, “crawling.” During gliding movement, the central part of the frontal margin is rather sharply elevated above the substrate. The worm is purely white, rather transparent. The intestinal area ends anteriorly with a rounded outline, not showing any V-shaped extensions of the gut branches.

ANATOMY.—The adhesive organ (Figure 32) appears in the sections as a slightly depressed, subterminal field of infranucleate epithelium, pierced by numerous eosinophilic gland ducts and provided with a moderate number of muscle fibers (retractors). There is no invagination of the adhesive epithelium into the mesenchyme, which places the species in the subgenus Sphalloplana.

The testes (Figure 39) are numerous, essentially ventral, some of them extending into the mesenchymal spaces between the intestinal branches or bridging the entire dorsoventral diameter of the body. They occupy, on either side, a longitudinal zone beginning behind the third or fourth pair of lateral intestinal branches and reaching to a level some distance anterior to the insertion of the pharynx, located mainly medially to the ventral nerve cord. The testes are in wide communication with the thin anterior vas deferens that runs along the medial side of the nerve cord. The ovaries are positioned below the second pair of intestinal branches. In the testicular region, the vitellaria or yolk glands lie mainly in the lateral regions of the mesenchyme.

The copulatory apparatus (Figure 49) was studied in two sets of serial sections, one cut longitudinally, the other transversely. The gonopore (gp) leads directly into the male atrium (am) and into the expanded terminal portion of the outlet of the copulatory bursa, the vagina (v). There is no common atrium developed. The lining of the atrium is a flattened epithelium with the usual two muscle layers, a circular and a longitudinal one.

The penis has a large, spherical, highly muscular bulb (bp) and a rather long, conical papilla (pp). The papilla is covered by a flattened epithelium underlaid by a layer of fine circular fibers (f) followed by longitudinal muscles. The fibrous layer is particularly thick at the base of the papilla, where it forms a continuation of the circular muscle layer of the atrial wall. The penis lumen consists of a rounded cavity (pr) in the bulb, lined with tall secretory cells filled with an eosinophilic secretion. From this cavity a narrow canal, the ejaculatory duct (de) runs posteriorly through the penis papilla and opens at its tip. The vasa deferentia (vd) enter the penis bulb posteroventrally, proceed within the bulb anterodorsally, and open, on either side, into a rounded cavity (vs) that empties into the anterior part of the ejaculatory duct. This pair of cavities apparently functions as seminal vesicles, although morphologically it does not correspond to the unpaired seminal vesicle of other species of the genus. The latter is represented in our species by the glandular lumen of the penis bulb which may have the function of a prostate, since no sperm passes through it but it apparently adds some secretions to the sperm fluid.

The two oviducts unite in the space between the male atrium and the bursal duct, the common oviduct (odc) proceeding ventrally and opening into the posterior part of the male atrium, very close to the gonopore. The copulatory bursa (b) is a rounded sac, somewhat compressed in the specimen studied. The bursal duct (bd) runs above the penis and genital atrium as a narrow, straight canal, then bends ventrally and expands considerably into a vagina (v) that histologically, however, does not differ from the narrow part of the duct. The rather thin muscle coat of the duct, not clearly analyzable, appears to consist of an inner circular and an outer longitudinal layer.

All epithelia of the copulatory apparatus are nucleate.

DISTRIBUTION AND ECOLOGY.—The material of S. californica was furnished to me by the courtesy of a group of enthusiastic cave explorers, calling themselves the “Bower Cave Diving Group.” The collections were made in the lake, which fills the greater part of Bower Cave, by scuba diving, chiefly by Messrs. Paul Hara, Bill Kruse, and Steven J. Shimek.

Bower Cave, 8 miles east of Coulterville, Mariposa County, California (type-locality). 16 and 18 July 1971: 4 specimens collected and shipped to me (only one, immature, arrived alive). 22 October 1971: water temperature 52°F (11°C), 3 immature worms collected at a depth of about 70 feet (21 m). 2 April 1972: I sexually mature specimen from about 70 feet (21 m) depth. 10 October 1973: 1 specimen collected at depth of 100 feet (30 m), disintegrated in transit. 11 August 1974: 2 specimens, one of them semimature, collected at depth of 30 feet (9 m).

The species lives in the cave together with amphipods (Stygobromus wengerorum Holsinger), which apparently are its natural food. Mr. Hara informed me that in laboratory cultures the worm does not accept liver as food, but ingests dead amphipods and, very eagerly, crayfish meat. He was able to maintain animals in a culture for over one year and to raise one specimen to maturity.

TAXONOMIC POSITION.—Sphalloplana californica is characterized by a very feebly developed adhesive organ and a somewhat aberrant structure of its copulatory apparatus, particularly the anatomy of the penis. The paired seminal vesicles opening into the ejaculatory duct and the development of a prostatic cavity are unique within the genus as far as we know today. Other characteristics, such as the differentiation of a marginal zone with thickened epithelium and large rhabdites, and the prepharyngeal location of the testes are typical features of the genus Sphalloplana.