Convoluta enelitta Antonius, 1968

Achatz, Johannes G. & Hooge, Matthew D., 2006, Convolutidae (Acoela) from Tanzania, Zootaxa 1362, pp. 1-21 : 3-6

Convoluta enelitta Antonius, 1968

( Figs. 1–2 )

Material. 3ZooEVV 4646 and 3ZooEVV 4647, two sets of 1.5-µm-thick serial sagittal sections, living specimens in squeeze preparations, eight complete and four partially complete sets of serial histological sections, and twenty-one whole mount specimens for fluorescence microscopy.

Collection Locality. Kunduchi Beach (6°39’51.3” S, 39°48’22.6” E), Dar es Salaam, Tanzania. Numerous specimens collected from medium-grained sediment and green sheetlike algae in small pools from high and mid intertidal.

Description. Mature specimens are droplet shaped, 600–850 µm long and ~340 µm wide, with ventrally enfolded sides ( Figs. 1 A, 2A). The epidermis is entirely ciliated, the cilia are 5–6 µm long. The nuclei of the epidermal cells are sunk beneath the body-wall musculature.

The body-wall musculature is stronger on the ventral than on the dorsal side, and consists of outer circular muscles, inner longitudinal muscles running parallel to the body margin, and pore muscles originating at the mouth ( Fig. 2 A). The body-wall musculature is not continuous with that of the penis or vagina.

A statocyst, 25 µm in diameter, is present ~100 µm behind the anterior tip. Green coloration is conferred by dense zoochlorellae, which are scattered throughout the parenchyma but are absent from the tissue of the seminal vesicle, seminal bursa, and bursal nozzle ( Figs. 1 A, B). Approximately 150 rhabdoid gland cells are scattered over the dorsal surface, each containing ~25 orange rhabdoids that are ~10 µm long and ~1 µm wide. Mucus gland cells, ~13 µm long and ~8 µm wide, with homogenous cyanophilic content, occur on the dorsal side most numerous in the anterior third of the animal and at the posterior tip. Smaller mucus gland cells with cyanophilic granules are distributed ventrally at the lateral margins. At the anterior tip, a small frontal organ is present. Refractive concrements are scattered in the epidermis on the dorsal surface, most abundant behind the eyes and over the seminal bursa.

The nervous system consists of a pair of ganglia lying anterior to the statocyst and one ventral and two dorsal longitudinal nerve cords. Two rectangular or star-shaped ocelli composed of orange-colored granules are located lateral to and slightly in front of the statocyst (data not shown).

The mouth is situated 250–300 µm behind the anterior tip. The margin of the mouth is strengthened by parenchymal muscles, which are connected to the body wall. The digestive central syncytium extends from behind the statocyst to the seminal vesicle and often contains small crustaceans.

The diffuse paired testes originate at the level of the statocyst and lie dorsal and lateral to the paired ovaries, which originate slightly behind the statocyst. At the level of the bursal nozzle mature sperm migrate to the ventral side of the body, reaching the paired antero-lateral openings of the seminal vesicle, which are ~20 µm long canals surrounded by glandular tissue. Sperm are stored at the periphery of the seminal vesicle.

The female gonopore is positioned anterior to the male gonopore. The unciliated vagina is ~130 µm long, lined with cyanophilic gland cells, is weakly muscular and connects to the seminal bursa caudally. The bursal nozzle is 30 µm long, directed ventrally, and has a “sorting apparatus” consisting of ~15 cells. In a third of the specimens we examined, the distal end of the vagina possessed paired lateral extensions ( Figs. 1 B, 2B).

The male gonopore opens directly to the male copulatory organ, which consists of a muscular seminal vesicle with an invaginated penis. The seminal vesicle is connected to the body wall by parenchymal muscles. The penis is ~170 µm long, curled backwards, and composed of outer circular, intermediate longitudinal, and thin inner circular muscles. The distal end of the penis is lined with the same cyanophilic gland cells as the vagina. Gland cells, also visible in live observation ( Fig. 1 B), fill the lumen of the penis. They have a polygonal surface, and their nuclei are sunken beneath the penis musculature into the vesicle. Caudally, within the seminal vesicle, these cells form a cellular bladder with extrusions positioned beneath the penis. At its proximal end, the penis is broader, its ventral epithelium is thicker, and its inner circular muscles terminate ( Figs. 2 C, D). Necks of erythrophilic gland cells penetrate the ventral epithelium of the penis, their cell bodies lie within the parenchyma outside the seminal vesicle. Few of these erythrophilic cells protrude into the distal part of the penis.

Remarks. We confirmed the conspecificity of our specimens with Convoluta enelitta through comparison of the type material and found only minor differences with Antonius’s (1968) description. The “high-prismatic erythrophilic cells” in the ventro-proximal penis epithelium described by Antonius (1968) are the distal necks of gland cells that lie outside the seminal vesicle, the paired sperm openings of the seminal vesicle are situated ventrofrontally, the penis has an additional inner layer of circular muscles, and no body-wall musculature is continuous with that of the penis.

While most of the specimens we examined had a straight vagina, approximately one third of the mature animals possessed a T-shaped vagina. The variation in shape could be a temporary change caused by recent copulation. This explanation is supported by the apparent lock-and-key correspondence between the shape of the penis of C. enelitta and the T-shaped vagina. The differences mentioned above between specimens from the Red Sea and from Tanzania could indicate the existence of subpopulations of C. enelitta .

Figures

FIGURE 1. Convoluta enelitta; photomicrographs of living specimen. A. Dorsal view of whole specimen. Eyes are not seen in this focal plane. B. Dorsal view of male and female copulatory organs. Arrowheads point to the gland cells in the lateral extensions of the vagina.

FIGURE 2. Convoluta enelitta; whole mounts stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy. A. Projection of ventral body-wall musculature. Asterisk marks the mouth. B. Projection of male and female copulatory organs. Arrows point to unspecific stained lateral extensions of vagina. C. Projection of male copulatory organ (in line with, but at a shallower depth than B). D. Lateral projection of male copulatory organ. Arrowhead points to erythrophilic cells in proximo-ventral part of penis.