Praeaphanostoma diatomovorum, 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 : 413-414

Praeaphanostoma diatomovorum sp. nov.

Figure 8

Zoobank registration. urn:lsid:zoobank.org:act:091F3968-6642-44CD-9799-37A65EFB701C

Materials examined. Serial sectioned specimen, Holotype ( SMNH Type-9453); Digital video and photographs of original living specimens and serial sectioned specimens, Paratypes ( SMNH Type-9454– SMNH Type-9467) .

Type locality. Klubban beach, Gullmarsfjorden, Sweden, 58°15’05 N, 11°27’52 E ; Kvarnbukten , Bohuslän, Sweden, 58°15’31 N, 11°28’28 E

Habitat. 1–2.5 m depth, marine, silty sand with organic content; 1–1.5 m depth, medium sand

Diagnosis. Body length 0.5–0.9 mm. Body pigmentation absent. Frontal organ moderate in size. Rhabdoid glands sparse in rows. Vacuoles absent. Testes paired; ovary unpaired. Seminal bursa present without ornamentation. Short vagina connects to common antrum and gonopore. Male copulatory organ subterminal without penis.

Etymology. This species is named in reference to the numerous diatoms present in the digestive parenchyma of the type specimens.

Description. Living specimen 0.5–0.9 mm in total body length depending on state of elongation and 0.12–0.2 mm in maximum width at point of largest egg. Specimens in squeeze-preparation drop-shaped; free-swimming specimens more elongate with rounded anterior and posterior ends. Body colorless, transparent, except for parenchyma, which varies from green or yellowish-brown based on microalgae food. Digestive parenchyma extends from ¼ body length from the anterior to level of posterior egg. Mouth present just past ¼ body length from anterior end, ~22 µm in width.

Frontal organ extending from anterior end to just posterior to statocyst. Statocyst present 90–110 µm from anterior end, 11 µm in diameter with 8 µm in diameter statolith. Epidermis completely ciliated and ~4 µm wide in longitudinal sections. Cilia 4–6 µm long. Rhabdoid glands present in longitudinal rows, sparse and best visible at the posterior end. Posterior rhabdoids partially subepidermal, colorless, 9–10 µm in length.

Ovary unpaired with up to four eggs. Seminal bursa present just posterior to largest egg. Bursa globular or eggshaped with narrower, tapering proximal part; width to 36 µm (24–36 µm) and length to 65 µm (46–65 µm). Bursa typically filled with evident sperm. Bursal nozzles or appendages absent. A short unciliated vagina connects the bursa ventrally to a common antrum and gonopore.

Testes paired, located just posterior of the midbody and anterio-lateral to the ovary. Seminal vesicle subterminal in posterior end, spherical or globular with a width of 23–38 µm and a length of 25–38 µm. Penis absent.

Figures

FIGURE 8. Praeaphanostoma diatomovorum sp. nov. A. micropictograph of the whole body. B. drawing of the whole body C. micropictograph of the bursa D. micropictograph of the posterior focusing on the reproductive structures; b bursa, dp digestive parenchyma, eg egg, f frontal organ, gp gonopore, m mouth, r rhabdoid glands, st statocyst, sv seminal vesicle, t testes