Amphiscolops

Achatz, Johannes G., 2008, Convolutidae (Acoela) from the Andaman Sea, Zootaxa 1824, pp. 1-16 : 2-3

Amphiscolops sp. ( Fig. 1 )

Material. PMBC 24793, one complete serial sagittal section of an immature specimen, living specimens in squeeze preparations, and three whole mounts for fluorescence microscopy.

Locality. On green algae in the brackish part of a little stream which runs into the northeastern corner of Yon Bay, Phuket Island, Thailand (7°48’54.9” N, 98°23’37.4” E).

Description. Living animals are ~ 1.1 mm long and ~ 0.7 mm wide ( Fig. 1 A). The epidermis is entirely ciliated, with cilia that are ~10 µm long. The nuclei of the epidermis are sunken beneath the body-wall musculature. The dorsal epidermis contains concrements that appear white in reflected light, black in transmitted light. The concrements are arranged in star-shaped patterns ( Fig. 1 C) and form dense aggregations at the anterior end ( Fig. 1 B) and dorsal to the digestive syncytium ( Fig. 1 A).

The brain consists of an anterior arch with lateral thickenings and a central mass, the nuclei lying ventrally. There are five pairs of longitudinal nerve cords: two dorsal pairs, a thick lateral pair, and two weak ventral pairs. Two red, 25 µm-wide ocelli lie at the anterior tip ( Figs. 1 A, B). No statocyst is present.

Numerous, 20–24 µm-wide zooxanthellae are scattered throughout the parenchyma ( Figs. 1 A, B). Nearly translucent specimens with 10 or fewer symbiotic algae are quite common, but most are colored because of zooxanthellae in the parenchyma..

Rhabdoid gland cells and mucous gland cells occur on the entire body surface, most numerous dorsally and at the anterior tip of the body. The rhabdoids are translucent, refractile, occur in bundles of up to 25, and are ~15 µm long ( Fig. 1 C). A “common” frontal organ is absent, but a few gland cells with small vesicles originate posterior to the brain, run to the anterior tip, and protrude separately. Aggregations of black granules appear as black spots up to 10 µm in diameter on the dorsal surface posterior to the eyes ( Fig. 1 ).

The digestive system of all investigated specimens was filled with numerous diatoms and crustaceans.

Remarks. Although all specimens investigated were immature, the species can clearly be placed closest to Amphiscolops blumi Achatz et al., 2007 , and A. langerhansi Graff, 1882 . All share numerous characters, most importantly the star-shaped concrements and their distribution, the possession, kind, and distribution of zooxanthellae, the structure, form and distribution of the rhabdoid gland cells, and the absence of a true frontal organ (see Achatz et al. 2007).

Several specimens exhibited signs of asexual reproduction by transverse fission as described by Hanson (1960) for A. langerhansi . The lack of a statocyst in all investigated specimens is additional evidence that this was an asexually reproducing population.

Figures

FIGURE 1. Amphiscolops sp.; photomicrographs of living specimen. A. Dorsal view of whole specimen. B. Dorsal view of anterior end. C. Star-shaped pattern of concrements in epidermis. Arrows point to the two ocelli, arrowheads to clusters of black granules, double-arrowhead to a bundle of rhabdoids; asterisk marks the aggregation of concrements dorsal to the digestive syncytium.