Picola renei, Achatz, Johannes G. & Hooge, Matthew D., 2006

Achatz, Johannes G. & Hooge, Matthew D., 2006, Convolutidae (Acoela) from Tanzania, Zootaxa 1362, pp. 1-21 : 15-20

publication ID	https://doi.org/ 10.5281/zenodo.174702
DOI	https://doi.org/10.5281/zenodo.6262176
persistent identifier	https://treatment.plazi.org/id/03E587C2-F123-FFEF-FE96-FCFD60413820
treatment provided by	Plazi
scientific name	Picola renei
status	sp. nov.

Treatment

Picola renei View in CoL sp. nov.

( Figs. 8–12 View FIGURE 8 View FIGURE 9 View FIGURE 10 View FIGURE 11 View FIGURE 12 )

Type Material. Holotype: 3ZooEVV 4652, one set of 1.5-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue. Paratype: 3ZooEVV 4653, one set of 1.5-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue.

Type Repository. Natural History Museum Vienna ( NHMW), Austria

Type Locality. Bawi Sandbank, Zanzibar, Tanzania (6°8'9.6'' S, 39°7'50.6'' E). Eastern side of sandbank in fine-grained sand near Thalassia sp. and corals at 2 m water depth.

Other Material examined. Living specimen in squeeze preparations, two complete sets of serial sections of epoxy-embedded specimens stained with toluidine blue, 13 whole-mounts for fluorescence microscopy.

Etymology. Species named in honor and memory of Rene Mähr.

Description. Mature animals are 300–400 µm long and up to 190 µm wide ( Figs. 8 View FIGURE 8 A, 9A). The epidermis is ~4 µm thick, its nuclei are not sunken beneath the body-wall musculature. The epidermis is entirely ciliated, with cilia that are 6 µm long. The bodywall musculature consists of outer circular muscles, cross-over muscles, and few inner longitudinal muscles.

On the ventral side, approximately three pairs of longitudinal muscles run along the lateral margins. Two pairs of longitudinal muscles are situated centrally and extend caudally, one originating at the posterior edge of the mouth, the other between mouth and lateral margin. Longitudinal cross-over muscles are present. The outermost branch out at the posterior end. Three to five U-shaped muscles extend longitudinally from the anterior tip of the body before bending around the posterior rim of the mouth ( Fig. 9 View FIGURE 9 A).

A statocyst, 16 µm in diameter, is present ~30 µm behind the anterior tip. Numerous yellow-brown zooxanthellae are scattered throughout the parenchyma. A few rhabdoid gland cells containing tightly packed, refractive, needle-shaped rhabdoids are distributed on the dorsal surface. Numerous frontal gland cells fill the anterior third of the animal, constituting a frontal organ. They merge in front of the statocyst and form a large reservoir before they protrude through the frontal pore. A few accessory gland cells containing vesicles lie ventral to the pore. Due to a fixation artifact, rhabdoid gland cells and cyanophilic mucus glands could not be distinguished but the presence of mucous glands is presumed.

The nervous system comprises nervous tissue around the frontal organ and statocyst; nerve cords could not be distinguished.

The mouth is positioned in the anterior half of the body. The digestive syncytium is inconspicuous, but often contains diatoms.

The diffuse testes are paired, lying dorso-lateral to the ovary, and originate in an unpaired germative zone behind the statocyst. Early spermatids contain fine granules that stain metachromatically pink and are joined in later stages by larger vesicles. Later stages get stained more intensively due to higher concentration of granules and vesicles. Dropletshaped spermatids enter the seminal vesicle dorso-ventrally. Within the seminal vesicle a few mature spermatozoa are among many spermatids ( Figs. 10 View FIGURE 10 A, C, 12A). The spermatozoa have axial microtubules in the cytoplasm and axonemes that lack central microtubules (“9+0” type; Fig. 12 View FIGURE 12 B). Due to serial ultrathin sections we can exclude the possibility that erratically occurring electron-dense material within the axonemes, could be a central microtubule or a central pair of microtubules (data not shown).

The unpaired ovary is positioned ventral to the testes. A female gonopore and vagina are both absent. The well-developed seminal bursa has a distinct wall and a bursal nozzle that is ~16 µm long and directed antero-ventrally in sectioned animals. The sperm within the seminal bursa lack metachromatic granules, but the vesicles described above are present. Within the vestibulum the sperm have a homogenously stained cytoplasm ( Figs. 10 View FIGURE 10 A, B, 11A, B).

The male gonopore lies in a subterminal indentation of the body wall ( Figs. 9 View FIGURE 9 B, C, 10C, 11B). The seminal vesicle consists of four pairs of bridge-like, bilateral muscle pairs that run from the ventral body wall underlying the seminal bursa to the subterminus of the body where they connect to the outermost longitudinal cross-over muscles of the ventral body wall. Muscle fibers of the seminal vesicle are connected to the dorsal body wall by other parenchymal muscles attaching mid-fiber ( Figs. 9 View FIGURE 9 B, C). Special multilayered tissue surrounds the seminal vesicle ( Fig. 12 View FIGURE 12 A). A ~15 µm long, sclerotized penial structure, lies in a glandular bladder at the distal end of the seminal vesicle. Extensions of the glandular bladder connect to the male gonopore ( Figs. 10 View FIGURE 10 C, 11B).

Remarks. The placement of Picola renei in the family Convolutidae is supported by characters of sperm ultrastructure, including the presence of axial cytoplasmic microtubules and the lack of central microtubules within the axonemes ( Petrov et al. 2004). Other features in common with species of Convolutidae include the presence of zooxanthellae, rhabdoids, a frontal organ, a seminal bursa, a single bursal nozzle as well as a seminal vesicle that surrounds the penis. Besides these characters placing P. re n e i within the Convolutidae , no synapomorphies with other described species can be identified to tie it more closely with any given convolutid taxa.

During copulation, the multi-layered tissue sac of the seminal vesicle is probably squeezed by the surrounding seminal vesicle muscles to eject sperm through the penial structure into a recipient. The sclerotization of this apparatus implies it is capable of hypodermal injection of sperm, which could explain the lack of a female gonopore and a vagina.