Amphiscolops potocani

Amphiscolops potocani nov. sp. (Figs. 2, 3)

Diagnosis. Amphiscolops with three lobes at the terminal end. The male gonopore lies subterminal and leads to a ciliated penis, which is invaginated into a dense plug of tissue and muscles. The T-shaped vagina opens to a pair of seminal bursae, which have one bursal nozzle each. Setting the total body length to 100 units (100 U), the distances from the anterior tip of the body for major features are as follows: statocyst at 13 U, mouth at 63 U, female gonopore at 77 U, male gonopore at 93 U.

Type material. Holotype: PMBC 24794, one complete set of serial sagittal sections. Paratype: PMBC 24795, one complete set of serial sagittal sections.

Type locality. In coarse sand at the waterline on Poni Island, Bo Yar Nyunt, Mergui-Archipelago, Myanmar (10°28’01” N, 98°13’34” E).

Other material examined. One partial set of serial cross sections of a mature specimen and living specimens.

Etymology. The species is named after my friend Christian Potocan who initiated my participation in the Mergui-Archipelago-Expedition.

Description. Mature animals are ~ 1.1 mm long and ~ 0.5 mm wide. The anterior tip is slightly pointed. The width of the body broadens towards the middle of the body and becomes smaller towards the posterior end, where the body terminates in three lobes – two lateral and one medial, all of the same length (Fig. 2 A). The animals appear greenish due to a variable numbers of endosymbiotic zoochlorellae. Aggregations of endosymbiotic zooxanthellae cause the animals to have variable brown spots. There are specks of white refractive concrements, which are also very variable in size, form, and distribution. While the worms adhere to and glide over the substrate, they elevate the middle of the body, leaving the lateral sides in contact with the substrate. Occasionally the worms swim into the water column.

The epidermis is entirely ciliated; the cilia are ~10 µm long. The nuclei of the epidermis are sunken beneath the body-wall musculature. Beneath the terminal web on the dorsal side occurs a layer of fluid-filled vacuoles, which seem to be inside parenchyma cells that extend through the body-wall musculature (Figs. 3 A, B). Concrements occur within the dorsal epidermis; their density and pattern vary greatly between different specimens.

Two paired thickenings of the anterior nervous arch are positioned laterally and slightly anterior to the statocyst. They are connected by strong neuropil, which is pervaded by frontal gland cells. One pair of dorsal nerve cords branches off laterally from the thickenings, and more laterally the anterior arc of nervous tissue divides into a dorsal and a ventral pair of nerve cords (Fig. 2 A). Posterior to the statocyst lie a few basophilic gland cells (Fig. 3 A).

Cyanophilic mucous gland cells are densely distributed all over the dorsal and lateral body walls but are completely lacking on the ventral side. A bundle of ~20 cells originates posterior to the brain and forms the frontal organ (Figs. 2 A, B, 3A, B).

Two types of symbiotic algae occur in various densities within different specimens. Zoochlorellae are distributed in the periphery of the parenchyma and within the epidermis, especially dorsally. Zooxanthellae occur exclusively more centrally around the ovaries (Fig. 3 C).

The mouth lies at about 2/3 of the body length. The digestive syncytium extends from the caudal end of the frontal gland cells to the male copulatory organ.

The ovaries are paired, lie medial to the testes, and extend from the frontal gland cells to the anterior tip of the bursal tissue. At about a third of the body length, the oocytes separate to lie one above the other, forming two layers, the more ventral one of young oocytes, and the dorsal one of maturing oocytes (Fig. 3 C). The maturing oocytes initially have elaborated lobes (indicating they are undergoing meiosis) and finally become rounded; and they bear many yolk granules and lipid vesicles. The vagina is ~130 µm long, not ciliated, slightly T-shaped, and lined with a thick epithelium (Figs. 2 B, 3A). On each proximolateral side it opens to a seminal bursa with a bursal nozzle (Fig. 2 A); each of these two bursal nozzles is 70–80 µm long, slightly curved, and directed antero-ventrally (Fig. 2 A).

The male germinal follicles originate dorsolateral to the oocytes, the germative zone extending from the statocyst to the level of the male gonopore. They mature while moving caudally, dorsal and lateral to the digestive syncytium but ventral to the mucous gland cells, not touching the dorsal body wall. Mature sperm aggregate to form paired false seminal vesicles anterior and lateral to the male copulatory organ. The male copulatory organ consists of a tissue plug, muscles, and a penis. The plug consists of dense, peripheral parenchyma and parenchymal muscles, which surround and run through this parenchyma, especially in a circular manner, but do not form a distinct muscular bulb. The penis is invaginated into the plug and is up to 150 µm long (Figs. 2 A, B, 3D). The penis epithelium is ciliated with cilia that are ∼ 10 µm long, and the nuclei lie within the epithelium. Numerous gland cells containing basophilic vesicles protrude into the proximal and dorsal part of the lumen of the penis. Underneath the epithelium are the circular and, deeper, the longitudinal penis muscles (Fig. 3 D).

Remarks. In the shape of its body, the new species resembles Amphiscolops trifurcatus (Beltagi, 1983) – the only difference being that the middle lobe is longer and wider than the lateral lobes in A. trifurcatus, whereas in A. potocani all lobes have approximately the same length. However, of all members of the genus, only these two species have three caudal lobes.

The paired occurrence of parts of the female system (gonopores, vagina, bursal nozzles) is common within the genus Amphiscolops, but the species’ paired seminal bursae, unconnected by bursal tissue, are unique (cf. Winsor 1990).

The occurence of two different species of endosymbionts is also described in the genus Waminoa Winsor, 1990, both being dinoflagelattes (Ogunlana et al. 2005, Winsor 1990) and in Amphiscolops marinelliensis Beltagi & Khafaji, 1984, which also possesses zoochlorellae and zooxanthellae. Among the flatworms, only acoels have both zooxanthellae and zoochlorellae within the same host.

Genus Convoluta Ø rsted, 1843