Pseudoposthia hanssoni, Atherton & Jondelius, 2022

Atherton, Sarah & Jondelius, Ulf, 2022, Diversity in the family Isodiametridae (Acoela): New species bring back old problems, Zootaxa 5169 (5), pp. 401-424 : 417-419

Pseudoposthia hanssoni sp. nov.

Figure 11

Zoobank registration. urn:lsid:zoobank.org:act:0E41935C-E677-427A-BA6C-EB5683043453

Materials examined. Digital video and photographs of original living specimens, Holotype ( SMNH Type-9481) and Paratype ( SMNH Type-9482) .

Type locality. Väderöarna naturhamn, Skagerrak, Sweden, 58°34’52 N, 11°03’52 E

Habitat. 4–6 m depth, coarse sand and rocks with some shell hash

Diagnosis. Body length 0.45 mm long and to 0.15 mm wide in squeezed preparation. Frontal organ well-developed. Rhabdoid glands numerous and scattered. Several large vacuoles present in posterior half of the body. Ovaries and testes paired. Bursa absent. Unwalled false seminal vesicle located ventral to the ovaries and anterior to the largest egg, leading directly to the gonopore posterior to the penis. Glandular penis straight, 9–10 µm long. Gonopore large and distinct, surrounded by thick muscular sphincter.

Etymology. This species is named in honor of the late Dr. Hans G. Hansson, expert in North Atlantic marine fauna and long-time staff member at Tjärnö marina laboratorium.

Description. Animal short and wide, body length 0.45 mm and width to 0.15 mm in squeeze-preparation. Specimens widest at midbody with broadly rounded anterior end and pointed rounded posterior end. Body colorless, transparent, except for parenchyma, which varies from bright green or yellowish-brown based on microalgae food. Digestive parenchyma extends from first ¼ body length to just anterior of posterior vacuoles. Mouth present just anterior to body’s midpoint. Several vacuoles present laterally in posterior half of the body.

Frontal organ well-developed. Statocyst present 50 µm from anterior end, 11–12 µm in diameter with 7-8 µm in diameter statolith. Epidermis completely ciliated and ~5 µm wide in longitudinal sections. Cilia ~6–8 µm long. Rhabdoid glands numerous and scattered across the body. Rhabdoids partially subepidermal, 8–10 µm in length in living specimen.

Paired ovaries with numerous eggs. Largest egg medial and slightly posterior to midpoint. Seminal bursa and female gonopore absent.

Seminal vesicle present ventral to the ovaries and anterior of the largest egg. False seminal vesicle globular, 22–25 µm in width and to 40 µm long (25–40 µm), extending anterio-laterally to testes.

Testes located ~⅓ body length from the anterior end. Gonopore large and distinct, present at midbody, surrounded by a large muscular sphincter. Gonopore 25 µm wide and 12–14 µm long. Small, lightly muscular penis present, 9–10 µm long, straight, connecting directly to the gonopore. Penis highly glandular. Two lateral false seminal vesicles connect to a central false seminal vesicle. False seminal vesicles lack walls and are non-muscular, comprising of a mass of sperm or possibly spermatophore. As typical for the genus ( Westblad 1946), the central false seminal vesicle is unconnected to the penis, instead remaining posterior to the penis and directly opening into the gonopore ( Figure 11D ).

Figures

FIGURE 11. Pseudoposthia hanssoni sp. nov. A. micropictograph of the whole body. B. drawing of the whole body C. micropictograph at the midbody focusing on the reproductive structures D. drawing at midbody focused on reproductive structures; dp digestive parenchyma, eg egg, f frontal organ, fsv false seminal vesicle, gp gonopore, p penis, r rhabdoid glands, sph muscular sphincter around gonopore, st statocyst, t testes, v vacuole.