Convoluta schuelii

Achatz, Johannes G., 2008, Convolutidae (Acoela) from the Andaman Sea, Zootaxa 1824, pp. 1-16 : 10-13

Convoluta schuelii nov. sp. ( Figs. 8–10 )

Diagnosis. Convoluta with red rhabdoids and endosymbiotic zoochlorellae distributed evenly except at the anterior end where the absence of zoochlorellae within the large lateral thickenings of the brain creates two lateral lighter spots. Male copulatory organ consists of a muscular seminal vesicle filled with sperm proximally and with the distal parts of prostatoid gland cells distally. Female copulatory organ consists of a gonopore, a vagina with a weak sphincter at its distal end, a seminal bursa, and one bursal nozzle. Setting the total body length to 100 units (100 U), the distances from the anterior tip of the body of major parts are as follows: statocyst 13 U, mouth 40 U, female gonopore 79 U, (facultative) male gonopore 83 U.

Type material. Holotype: PMBC 24798, one set of serial sagittal sections. Paratype: PMBC 24799, one set of serial sagittal sections.

Other material examined. Living specimens with a stereo microscope, one set of serial sagittal sections of an immature specimen stained with hematoxylin, two sets of serial sagittal sections of mature specimens stained with Richardson’s stain, and six whole mounts for fluorescence microscopy.

Type locality. From fine grained sand on the south beach of Koluk Kamlok, Thailand (9°30’42.3” N, 98°21’30” E).

Etymology. The species is named after Luca Schueli from Switzerland, president of ECoSwiss, Captain of the Sea Nomad, and leader of the Mergui-Archipelago Excursion 2007.

Description. Mature animals are 650–700 µm long and 200–250 µm wide. The anterior tip is rounded, the posterior tip slightly pointed, and the body is slightly constricted at the level of the mouth ( Fig. 8 B). The anterior end forms a basket ( Fig. 10 B). The coloration is mostly due to the presence of numerous zoochlorellae and red rhabdoid gland cells. The animals slightly enroll their lateral sides ventrally. A pair of more translucent spots at the anterior tip is due to the absence of symbionts there.

The epidermis is completely ciliated; the cilia on the dorsal side are ~5 µm long, on the ventral side ~8 µm long. The nuclei of the epidermis are sunken beneath the body-wall musculature.

The brain consists of an anterior arch with lateral thickenings. Each thickening is about 50 µm long, 35 µm high, and 35 µm wide ( Fig. 8 B). The nuclei of the nerve cells lie in the periphery of the nervous tissue, mostly on the ventral side. There are two dorsal and one ventro-lateral pair of nerve cords ( Fig. 8 B). The statocyst measures 15 µm in diameter.

Red rhabdoid gland cells are distributed in high density and protrude through the body wall on the lateral and dorsal surface but are absent on the ventral side. There is no distinct frontal organ, but frontal gland cells that contain small vesicles protrude across the anterior tip ( Figs. 8 A, B).

The dorsal body-wall musculature consists of circular muscles, dorsal crossover muscles, and longitudinal muscles ( Fig. 10 A). The ventral body-wall musculature consists of circular muscles and longitudinal muscles. Some of the anterior longitudinal muscles run straight from the anterior tip into the mouth, others run around the mouth forming a kind of a sphincter. The other longitudinal muscles cross over each other in front of the mouth. These muscles run caudally, and posterior to the mouth they start to bend laterally away from the midline–the more laterally positioned they are, the stronger they bend ( Fig. 10 B). There are just a few dorso-ventral muscles.

Zoochlorellae are scattered throughout the parenchyma and have a diameter of ~10 µm ( Figs. 8 A, 9A, C).

The mouth is situated 250 µm posterior to the anterior tip ( Figs. 8 A, B, 10B). The digestive syncytium often contains diatoms.

The male follicles originate at the level of the statocyst, the female follicles 50 µm further posterior. The paired ovaries lie more central and ventral than the paired testes. The female gonopore lies 15 µm in front of the male gonopore. The vagina is ~40 µm long and at its entrance some circular muscles constitute a weak sphincter ( Fig. 10 C). Often a plug of mucous material, a remnant of copulation, occurs proximally to the sphincter ( Fig. 9 A). The bursal nozzle measures 40–50 µm in length, is directed frontally, and is slightly Sshaped ( Figs. 8 A, B, 9A).

The male follicles mature caudally and aggregate to form paired false seminal vesicles anterior and lateral to the male copulatory organ. The male gonopore appears to be facultative from histological sections ( Figs. 9 A, B, C), but using the CLSM a pore in the body-wall musculature can be seen ( Fig. 10 B). It lies ~120 µm in front of the terminal tip. The copulatory organ consists of a muscular sheath, which is partially continuous with the ventral body-wall musculature ( Fig. 10 C) and filled with sperm proximally and with the distal part of prostatoid gland cells distally ( Fig 9 A). The gland cells contain basophilic vesicles with a diameter of ~2 µm ( Fig. 9 B). The muscle sheath is egg shaped, 60 µm wide, 80 µm long, and 40 µm high ( Figs. 8 A, B, 9A, 10B, C).

Remarks. Even though a penis is absent in the new species, the architecture of the male copulatory organ, with a muscular sheath enclosing sperm proximally and the distal part of prostate gland cells distally, clearly places this species within the genus Convoluta . The copulatory organ is most similar to that of C. lacrimosa, Achatz et al. 2007 , except for the presence in that species of a long penis, and the sperm are also very similar in these two species.

Figures

FIGURE 8. Convoluta schuelii sp. nov.; reconstructions to show arrangement of organs. A. Sagittal reconstruction of whole specimen. B. Dorsal view. The gonads are paired, but for clarity, just the left testis and right ovary are shown.

FIGURE 9. Convoluta schuelii sp. nov.; photomicrographs of sagittal histological sections stained with Richardson’s staining solution. A. Female and male copulatory organs. B. Glandular part of male copulatory organ. C. Middle part of prostatoid gland cell. Arrows point to muscular sheath of seminal vesicle, arrowheads to prostatoid vesicles; the asterisk marks the plug of male prostatoid mucus in the vagina.

FIGURE 10. Convoluta schuelii sp. nov.; whole mounts stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy. A. Projection of dorsal body-wall musculature. B. Projection of ventral body-wall musculature. C. Projection of male copulatory organ. Arrowheads point to the pair of thicker longitudinal muscles; asterisk marks the male gonopore.