Postaphanostoma nilssoni, Kånneby, Tobias & Jondelius, Ulf, 2013

Kånneby, Tobias & Jondelius, Ulf, 2013, Four new species of Acoela from Chile, Zootaxa 3736 (5), pp. 471-485 : 475-477

Postaphanostoma nilssoni n. sp. ( Figs. 5–8 )

Type Material: Holotype—SMNH Type-8464 (sagittal sections); Paratype—SMNH Type-8465 (longitudinal sections); Paratype—SMNH Type-8466 (sagittal sections).

Type Locality: Rock pool by Estación Costera de Investigaciones Marinas (ECIM), Las Cruces, Chile (33° 30' 10.548" S, 71° 37' 54.012" W), intertidal in shell gravel.

Other Material Examined: Whole mounts of living specimens.

Etymology: This species is named in honor of Karin Sara Nilsson who helped collecting the material.

Description: Body cylindrical, variable in size and shape. Living free-swimming specimens 4–5 mm in length. Living mounted specimens 2–3 mm and fixed specimens approximately 1 mm in length. Width varies between 0.6–1.25 mm in living highly compressed mounted specimens and is approximately 100 µm in fixed specimens. Anterior and posterior ends rounded with the posterior end usually narrower ( Fig. 5 A). Body colorless, unpigmented. Central syncytium is yellowish and eggs are of a brownish color ( Fig. 5 A). Rhabdoid glands small (appr. 10 µm long), scattered in the anterior end of the body.

Epidermis completely ciliated, approximately 3–4 µm in sagittal sections ( Fig. 6 ). Body wall apparently with well developed dorsoventral musculature. Large vacuoles, up to 300 µm in diameter, confined to posterior part of body ( Figs. 5 A, C). Smaller vacuoles are also present along the lateral sides of the posterior third of body ( Fig. 7 ).

Statocyst 25 µm in diameter and located 160 µm from the anterior end at U 6 in mounted specimens. Statolith 13 µm in diameter. Frontal organ well developed with numerous frontal glands, extending from anterior end along the lateral sides to U18 ( Figs. 5 B; 7).

The position of the mouth could not be determined. Central syncytium, yellowish, extends from U23 to the mature eggs at U64. From U23 to U40 the central syncytium is diffuse but then becomes more compact ( Fig. 5 A).

Ovaries paired situated anterior to male copulatory organs. They extend from the middle of the body at U50 to the anterior parts of the male copulatory organ at U76 ( Figs. 5 A, C; 6). Bursa and female gonopore absent. Testes paired, extending along the lateral sides of the body, from U26 to U51 ( Fig. 7 ). Seminal vesicle absent. Sperm is collected in false seminal vesicles, a collection of sperm not surrounded by a muscular wall (see Westblad 1945; Faubel 1976; Ehlers & Dörjes 1979). The male copulatory organ is formed by an invagination of the ventral epidermis ( Fig. 6 ). The wall of the male copulatory organ is muscular and surrounded by eosinophilic gland cells ( Fig. 6 ). Male gonopore located at U78, between the large posterior vacuoles and the mature eggs.

Diagnosis and taxonomic remarks: Postaphanostoma nilssoni n. sp. can be separated from all other species within the genus based on its large body size. The following morphological characters identifies the new species as a member of Postaphanostoma : (i) a well developed frontal organ with numerous frontal glands; (ii) small poorly developed rhabdoid glands; (iii) seminal vesicle absent; (iv) the male copulatory organ is an invaginated continuation of the epidermis and (v) sperm are collected in false seminal vesicles (Dörjes 1968). The maximum likelihood analysis of the concatenated 18S, 28S and COI sequences shows a sister group relationship between P.

nilssoni and P. glandulosum Dörjes, 1968 ( Fig. 8 ). However, P. glandulosum is the only other species within the genus for which we currently have sequences available.

Postaphanostoma atriomagnum Dörjes, 1968 , Postaphanostoma filum Dörjes 1968 and Postaphanostoma glandulosum are all less than 1.5 mm in total body length and only 0.1–0.2 mm in total body width compared to 4– 5 mm and 0.6–1.25 mm in total body length and width respectively, for living specimens of P. nilssoni n. sp. The other species within the genus all possess a seminal bursa and unpaired ovaries while a seminal bursa is absent and the ovaries paired in P. nilssoni n. sp. Moreover the posterior end of the new species contains more vacuoles than in any of the other species.

Figures

FIGURE 5. Postaphanostoma nilssoni n. sp. Photomicrographs of living specimens A – C. A. Dorsal view of whole specimen. Scale bar: 200 µm. B. Anterior end showing the frontal gland pore and distribution of frontal glands. Scale bar: 100 µm. C. Internal anatomy of posterior end showing the male copulatory organ and position of ovaries. Scale bar: 100 µm.

FIGURES 6 – 7. 6. Postaphanostoma nilssoni n. sp. Photomicrograph of sagittal histological section stained with hematoxylin-eosin. Scale bar: 100 µm. 7. Postaphanostoma nilssoni n. sp. Schematic drawing of specimen (dorsal view). Scale bar: 500 µm.

FIGURE 8. Postaphanostoma nilssoni n. sp. Details of the 50 % consensus tree after bootstrap analysis of the concatenated 18 S, 28 S and COI dataset showing the position of this species.