Heterochaerus australis Haswell, 1905

Achatz, Johannes G. & Hooge, Matthew D., 2006, Convolutidae (Acoela) from Tanzania, Zootaxa 1362, pp. 1-21 : 12-14

Heterochaerus australis Haswell, 1905

( Figs. 6–7 )

Material. 3ZooEVV 4650 and 3ZooEVV 4651, two sets of 1.5-µm-thick serial sagittal sections, living specimens in squeeze preparations, two partial sets of serial histological sections, and 14 whole-mount specimens for fluorescence microscopy.

Collection Locality. Maruhubi Beach, Zanzibar, Tanzania (6°8'39.7'' S, 39°12'29.4'' E). Samples of green-colored sand were taken in high intertidal zone of a sandflat from small pools of water with many small black snails. Thousands of H. australis occurred in this sediment.

Description. Mature animals are up to 4 mm long, 1.5 mm wide, and their lateral margins are not enfolded. The coloration is variable ( Figs. 6 A–C). Animals typically move with cilia, but also flap their lateral edges when swimming. Cilia, ~8 µm long, occur on the entire body surface but are absent between the male gonopore and female gonopore. The epidermal nuclei are not sunken beneath the body-wall muscles. The body-wall musculature consists of outer circular muscles, longitudinal cross-over muscles, and longitudinal muscles.

A statocyst, 16 µm in diameter, is present ~300 µm behind the anterior tip. Two dark brown ocelli lie lateral to the statocyst. Numerous zooxanthellae, 10–25 µm wide, are scattered throughout the parenchyma. The parenchyma consists of an outer dense peripheral parenchyma and an inner parenchyma with large vacuolated spaces. Cyanophilic rhabdoid gland cells with their nuclei sunken beneath the body wall and containing short, needle-shaped rhabdoids occur on the entire surface. Epidermal concrements, silver colored in reflected light, are variably distributed on the dorsal side ( Figs. 6 A–C). Common mucous gland cells are absent, as is a frontal organ.

Nervous tissue surrounds the statocyst and forms a complex at the anterior tip. Three ventral and two dorsal pairs of nerve cords run caudad from it.

The mouth is positioned at about half of body length.

The paired testes lie in the dorsal peripheral parenchyma, but spermatids and sperm often migrate into the vacuolated parenchyma. Spermatozoa are 35 µm long and sometimes form false seminal vesicles before entering the seminal vesicle dorso-laterally. The paired ovaries lie in the ventral peripheral parenchyma, more medially than the testes, rarely extending into the spaces of the vacuolated parenchyma.

The female gonopore is a 110-µm-wide, transverse slit, and lies ~80 µm in front of the male gonopore. The vagina is dead-ended at its proximal end and lined by high prismatic cells, cells containing cyanophilic vesicles, and erythrophilic gland cells. It opens to a muscular bursa frontally ( Figs. 6 D, 7). The bursa is T-shaped, with a distal unpaired part splitting proximally into two paired lobes. Each bursal lobe contains 1–11 bursal nozzles ( Fig. 6 E). Often the number of nozzles in the lobes is unequal. Each bursal nozzle is ~70 µm long, directed antero-ventrally, and is often bent in a C- or S-shape ( Figs. 6 E, F). The bursal nozzle consists of a stack of short cells, which contain cyanophilic vesicles and are visible even in squeeze preparations ( Fig. 6 F) and some elongate cells that are directed caudally and interconnect with the bursal tissue. The lobes appear somewhat spongy in histological sections where the interconnecting bursal nozzle cells meet them. At the proximal end of the nozzle is a rosette of cyanophilic bursal nozzle cells.

The male gonopore is a 90-µm-wide transverse slit positioned 350 µm in front of the posterior end ( Figs. 6 D, 7). The highly glandular penis is 300 µm long, curled backwards, and invaginated into a muscular seminal vesicle. The nuclei of the glandular cells, containing cyanophilic vesicles, are sunken beneath the penis musculature, into the seminal vesicle. The size and number of vesicles varies highly within the gland cells. The penis musculature consists of inner circular muscles and outer longitudinal muscles. A glandular penis fold is invaginated into the caudal side of the seminal vesicle ( Fig. 7 ). It is lined with the same gland cells as the penis frontally but erythrophilic gland cells caudally.

Remarks. Heterochaerus australis was originally described by Haswell (1905) from shallow rock-pools in Port Jackson, New South Wales, Australia. Later Winsor (1990) reported the species from subtidal sand on Orpheus Island and from Queensland, Australia. Dörjes and Young (1973) found H. australis in small pools on a sandy beach at Mombassa, Kenya, approximately 135 km north of our collection site in Zanzibar. The presence of H. australis at different sites in Australia as well as in Mombassa, Kenya, and Zanzibar, Tanzania, suggests that this species may be widely distributed throughout the Indian Ocean.

According to Haswell (1905), H. australis has three pairs of longitudinal nerve cords, but we found two additional pairs of ventral nerve cords. Haswell (1905) described “small” and “large” types of sperm. We found only the large type of sperm and suspect that his “small type ” represents developing spermatids.

Neither Haswell (1905) nor Winsor (1990) recorded the presence of a penis fold, perhaps because, as fixation causes partial protrusion of the penis, it can be obscured.

A vagina configured like that of H. australis was shown in Polychoerus carmelensis, Costello & Costello 1938 a ( Costello & Costello 1938b) and Amphiscolops japonicus Kato, 1947 .

Figures

FIGURE 6. Heterochaerus australis; photomicrographs of living specimens. A – C. Dorsal view of whole specimens, all with same magnification. D. Ventral view of male and part of female copulatory organs. E. Dorsal view of part of the seminal bursa and bursal nozzles. Arrows point to bursal nozzles. F. Bursal nozzle.

FIGURE 7. Heterochaerus australis; reconstruction to show arrangement of male and female copulatory organs.