Convoluta thela, Achatz, Johannes G. & Hooge, Matthew D., 2006

Achatz, Johannes G. & Hooge, Matthew D., 2006, Convolutidae (Acoela) from Tanzania, Zootaxa 1362, pp. 1-21 : 6-11

Convoluta thela comb. nov. ( Antonius, 1968)

( Figs. 3–5 )

Conaperta thela ( Antonius 1968, Antonius 1970, Hooge & Tyler 2005)

Material. 3ZooEVV 4648, one set of 1.5-µm-thick serial sagittal sections, and 3ZooEVV 4649, one set of 1.5-µm-thick frontal sagittal sections, living specimens in squeeze preparations, seven sets of serial histological sections, and 10 whole-mount specimens for fluorescence microscopy.

Collection Locality. Bawi Island, Zanzibar, Tanzania (6°8'48.6'' S, 39°7'57.1'' E), from fine-grained sand in tide pools at the low intertidal level, and Bawi Sandbank, Zanzibar, Tanzania (6°8'9.6'' S, 39°7'50.6'' E), from east side of sandbank, fine-grained sand near corals and Thalassia sp. at 2 m water depth.

Description. Mature animals are 700 to 900 µm long, 375 µm wide, and lack enfolded sides ( Figs. 3 A, 4A, 5A). The epidermis is entirely ciliated, the cilia are 5–6 µm long. The epidermal nuclei lie beneath the body-wall musculature, which consists of outer circular, longitudinal cross-over, and inner longitudinal muscles.

A statocyst, 25 µm in diameter, lies ~100 µm behind the anterior tip. Yellow-brown coloration is conferred by zooxanthellae, which are scattered throughout the parenchyma. The number of symbiotic algae varies among specimens. Rhabdoid gland cells are scattered across the entire surface, with especially high concentrations at the anterior end of the body. The refractive needle-shaped rhabdoids are ~1 µm thick and up to 28 µm long, and tightly packed in the rhabdoid gland cells ( Fig. 3 C). The formation of rhabdoids appears to start with cyanophilic vesicles, which fuse progressively. When fixed with glutaraldehyde and stained with toluidine blue, the rhabdoids are preserved and purple, but when fixed with Stefanini’s or Bouin, they dissolve. Greater abundance of the rhabdoid gland cells causes replacement of the zooxanthellae and the margin of the body to look translucent. A well-developed frontal organ composed of cyanophilic mucus gland cells is present anterior to the mouth ( Fig. 5 B).

The nervous system consists of a mass of nerve tissue in front of and lateral to the statocyst, and four pairs of longitudinal nerve cords (ventral, ventro-lateral, dorso-lateral, and dorsal). Two ellipsoid ocelli composed of brownish granules lie lateral to the statocyst ( Fig. 3 D).

The mouth is situated slightly in front of the middle of the body. The digestive central syncytium extends from behind the statocyst to the seminal vesicle and sometimes contains crustaceans.

The paired testes originate behind the statocyst and mature toward the seminal vesicle where sperm sometimes form false seminal vesicles. Mature sperm enter the seminal vesicle through lateral openings ( Figs. 3 B, 5A). The paired ovaries are positioned ventrally near the median line.

The female gonopore lies anterior to the male gonopore, separated from it by a short length of body wall. The female gonopore opens to an unciliated vagina that is surrounded by a sphincter, 35 µm wide, 12 µm thick, and composed of ~8 distinct muscle fibers. The vagina leads to a seminal bursa with a single bursal nozzle that is 60 µm long, directed antero-ventrally and curved dorsally ( Figs. 4 B, 5B).

A well-developed sphincter consisting of body-wall musculature and penis musculature surrounds the male gonopore. The straight, muscular penis is 35 µm long, 25 µm wide, and has a lumen 13 µm wide. It is inserted into a muscular seminal vesicle. The penis musculature consists of longitudinal muscles and an irregular meshwork of circular fibers surrounding them ( Fig. 4 C). Toward the proximal end of the penis the longitudinal muscles branch repeatedly. The branched muscles appear to attach to the cell borders of gland cells described below ( Fig. 4 B). The muscular, spherical seminal vesicle is 100 µm high and 75 µm wide when filled with sperm and glandular secretions. At the distal end it is filled with gland cells with a homogeneous content. The nuclei of these cells lie on the outer wall of the penis, or within the parenchyma. In this case the cells extend through the distal wall of the seminal vesicle. A second type of gland cell, producing erythrophilic vesicles, fills the region of the seminal vesicle that is proximal to the cyanophilic gland cells. The nuclei of these cells lie within the parenchyma, scattered among the muscles of the seminal vesicle or sometimes within the seminal vesicle. Erythrophilic vesicles can also be found free within the seminal vesicle.

Two prostatoid organs, 38 µm long and 20 µm in diameter, are present, mostly positioned at the body midline posterior to the male copulatory organ ( Figs. 3 B, 4A, 5). A prostatoid organ consists of a muscular vesicle surrounding a cone of circular muscles. Several muscle fibers connect the proximal end of the cone to the vesicle. The body-wall musculature is not continuous with the musculature of the prostatoid organs. Gland cells producing erythrophilic vesicles surround and protrude with their distal necks into the prostatoid organs ( Fig. 5 C). These gland cells are identical to those associated with the seminal vesicle. The number of erythrophilic vesicles stored in the prostatoid organs is always positively correlated with the number present in the seminal vesicle–i.e., full before and empty after copulation. We examined the arrangement of prostatoid organs in 19 specimens of Conaperta thela comb. nov. and found the following patterns: in one specimen, one prostatoid organ was positioned at the body midline, anterior to the female gonopore and a second prostatoid organ was present at the body midline posterior to the male copulatory organ; in one specimen, one prostatoid organ was positioned at the body midline anterior to the female gonopore, and a second at the body midline posterior to the female gonopore; in 4 specimens (2 immature) two prostatoid organs were present lateral to the female opening ( Fig. 5 A right); in 11 specimens 2 prostatoid organs were positioned at the body midline posterior to the male copulatory organ ( Fig. 5 A left); in two immature specimens they were entirely absent.

Remarks. On the basis of the new material we could verify the validity of Convoluta thela comb. nov. (see Antonius 1968, page 342), which was described with a single specimen. Antonius (1968) placed the species Convoluta krana comb. nov. and Convoluta thela in the genus Conaperta on the basis of having a common gonopore or better the female and male copulatory organs to open out into a unciliated common genital atrium ( Antonius 1968, page 358). However, our investigation of the holotype of C. thela and two out of four serial sections from the type material of C. krana , revealed that the pores are separated by a short length of ciliated body wall. In freshly collected specimens of C. thela examined by confocal microscopy the pores can be seen to be set apart by circular bodywall muscles and in histological sections the pores are separated and surrounded by a ciliated body wall. Consequently, we transfer both species to the genus Convoluta .

Antonius (1968) reported that his Bouin fixed specimens of Convoluta thela possessed mucoid gland cells throughout the body wall and rhabdoid gland cells that were present solely outside the body-wall musculature. However, the preservation of rhabdoids differs with use of various fixatives ( Martin 1978, Smith et al. 1982, Smith & Tyler 1984). Comparing our Stefanini’s- and glutaraldehyde-fixed specimens of C. thela , it is likely that the rhabdoids in Antonius’ (1968) material dissolved in Bouin’s fixative and lead to a misinterpretation of mucous gland cells. The small rhabdoids described by Antonius (1968) were possibly the distal tips of larger rhabdoid gland cells.

Antonius (1968) described muscles of the male copulatory organ, which insert at the body-wall musculature, penetrate the seminal vesicle, and attach to the penis (Retraktoren des Penis, PR, sensu Antonius 1968). We found no similar muscles in the specimens investigated.

The presence of lateral prostatoid organs in immature animals, and the strict occurrence of two prostatoid organs in all specimens, points to a paired lateral origin of these organs. The variable distributions of the organs could be intermediate stages in the migration towards the median line of the tail. This interpretation is also strengthened by the fact that in all other Convoluta -species possessing prostatoid organs, they occur in a paired, lateral distribution.

In our examination of the type material of C. krana we found in one specimen one prostatoid organ lateral to the female gonopore and a second in a median, posterior position (Rupert Riedl-Collection #12.685). In three others the prostatoid organs were distributed in the common median, posterior pattern.

The differences between the specimen described from the Red Sea and the specimens from Tanzania may indicate the occurrence of subpopulations in C. thela .

Figures

FIGURE 3. Convoluta thela comb. nov.; photomicrographs of living specimen. A. Dorsal view of whole specimen. B. Dorsal view of male copulatory organ. Arrowheads point to prostatoid organs. C. Rhabdoid gland cell. D. Ocellus

FIGURE 4. Convoluta thela comb. nov.; whole mounts stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy. A. Projection of ventral body-wall musculature. B. Projection of male and female copulatory organs. Asterisk marks penis lumen. C. Projection of cross-section through penis. Asterisk marks penis lumen, black arrowheads point to circular muscles, white arrowheads to longitudinal muscles.

FIGURE 5. Convoluta thela comb. nov.; reconstructions to show arrangement of organs. A. Dorsal view showing the two most abundant distributions of prostatoid organs. The gonads are paired but for clarity just the right testes and left ovaries are shown. B. Sagittal reconstruction of whole specimen. C. Sagittal reconstruction of male copulatory organ and prostatoid organ. Insunk nuclei of the epidermis are not illustrated.