Acanthozoon Collingwood, 1876

Dixit, Sudhanshu, Bulnes, Verónica N. & Raghunathan, Chelladurai, 2018, Psammoecus trimaculatus Motschulsky 1858, Zoological Studies 57 (45), pp. 1-11 : 2-6

Genus Acanthozoon Collingwood, 1876

Acanthozoon alderi ( Collingwood, 1876)

Thysanozoon alderi Collingwood, 1876: 88 (Type locality: Labuan Island, Borneo); Stummer-Traunfels 1895: 706 ( Ambon Island and Tricomalee, Sri Lanka); Prudhoe, 1975. Label of voucher W 8687 from the Australian Museum in Sydney (Lizard Island, Australia)

Material examined: Neotype. One specimen as serial sections (8 Slides), remainder of animal preserved in 70% ethanol, (ZSI/ANRC-16848), collected on 14th July 2013 at 10 m depth.

New type locality: Sound Island (12°56.167'N 092°58.113'E), Mayabunder, Andaman and Nicobar, India. The specimen was found crawling on rocky substratum in coral reef area.

Additional material: One specimen preserved whole in 70% alcohol (ZSI/ANRC-16849), collected on 16th December 2016 at depth of 12 m off Rutland Island (11°30.119'N 092°37.112'E), Andaman and Nicobar, India. The specimen was found crawling on rocky substratum in coral reef area.

Description: External morphology: Body ovoid, live specimen 5.5 mm long by 3 mm wide; fixed specimen 5 mm long by 4 mm wide ( Fig. 2A ). Background colour light brown, with a marbled blackish pattern, darker towards the margins and the middorsal strip. Middorsal black band with white blotches extending from right after the cerebral region to almost the distal margin without touching it. An inner black marginal band and an external white rim is present. Dorsal body surface and pseudotentacles covered with numerous papillae (3 to 5/mm 2), except around the cerebral eye cluster (devoid of papillae), and on the marginal white rim. The papillae are of various sizes, from 180 µm to 350 µm high ( Fig. 3A ), conical, light brown to white in their bases and yellow tips. Pseudotentacles prominent, pointed, well developed and black with white tips ( Fig. 2A ). Tentacular eyes not visible because of the epidermis pigmentation. Cerebral eyespots cluster horseshoe shaped with about 150 eyes. Pharynx large, ruffled, with 15 folds. Distance between male pore and sucker, 6.1 mm and between male and female gonopore, 1.3 mm ( Fig. 2C ).

Internal morphology: Dorsal body wall 43 to 120 µm high, epidermis cellular, ciliated, with presence of few rhabdites dorsally ( Fig. 3A, B ). The epidermal cells of varying heights, almost cuboidal on the surface of the body (30 µm), up to cylindrical on papillae tips (100 µm), loaded with a thin basophilic granular secretion. Big eosinophilic droplets in the distal cells of the papillae. Beneath the epidermis, there is a distinct basement membrane, somehow creased, and between the folds, the circular muscle fibres, followed inwards by a thin longitudinal muscle layer ( Fig. 3A, B ). Ventral body wall 40 to 54 µm high, epidermis ciliated, with scattered rhabdites. The epidermal cells are more homogenous in height than on dorsal surface. The basement membrane, the circular muscle layer and longitudinal muscle layer more developed than dorsal side. The parenchymatic tissue with numerous short muscular fibres arranged in all directions, being the dorso-ventral fibres, the longest. The intestine comprise one central branch and numerous anastomosing lateral branches constituting a net ( Fig. 3A, B ). While over the pharynx and reproductive system, the branches are only dorsal, beyond this point, the branches are dorsal and ventral oriented. The main branch of the intestine is immersed in a well-developed layer of longitudinal muscle fibres. The muscular fibres are densely packed and sometimes the height of this layer exceeds the height of the inner intestinal tissue. In the secondary intestinal branches, the muscular layer is not apparent. Papilla without intestinal branches. The intestinal tissue is completely ciliated. The epithelium lining the main intestinal branch is higher than the epithelium of the lateral branches. In the first place, the cells are cylindrical, with basal nuclei, alternating highly vacuolated cells; with cells filled with eosinophilous granules. The inner epithelium of the lateral branches is lower, consisting in cuboidal cells, with central nuclei and rich in eosinophilous secretions. Uteri ramified, arranged dorsal to testes and seminal ducts, and extending from the beginning of the second quarter up to the end of the second quarter of the body. The male copulatory organs consist in a true seminal vesicle, a prostatic vesicle arranged free, and an armed penis ( Fig. 3C, D ). In their course, the seminal ducts combine forming a net, and join in a single duct on each side, just before entering the seminal vesicle from postero-dorsal. The seminal vesicle is elongate, 414 µm by 181 µm and is arranged more or less vertical dorsal to the prostatic vesicle. The prostatic vesicle is ovoid and small, 128 µm by 81 µm, immersed in a surrounding secretory tissue. The ejaculatory duct and the prostatic duct joins before entering the penis papilla. The penis papilla is armed with a short sclerotized tubular stylet, 150 µm long. The armed penis is surrounded by an epithelial fold, the penis sheath, housed in a well-developed male atrium. The epithelium of the penis sheath as well as of the male atrium completely ciliated. Scattered rhabdites opens in the male atrium and the ventral epidermis. The oviducts join before entering the vagina from postero-dorsal. The vagina oriented vertical ( Fig. 3E, F ). Proximally the inner epithelium is cuboidal, distal columnar, completely ciliated. The cement glands surround the female canal and opens distally in the vagina. The vagina opens to an extremely shallow female atrium, and to the female gonopore, provided with muscular ring. Scattered rhabdites in the atrium epithelium and the neighbouring ventral epidermis.

Additional observations: Some morphological variation was registered. The specimen, ZSI/ ANRC-16849 was bigger ( Fig. 2F ), 7.7 × 3.5 mm living and 7 × 4 mm fixed. The colour pattern agrees with the above mentioned, but the middorsal band reach the hindermost part of the body, and the papillae were yellow towards the margin and orange to the centre of the body.

Distribution: Labuan Island, Borneo (former type locality); Ambon, Indonesia; Trincomalee, Sri Lanka, Lizard Island, Australia; Sound Island, Andaman and Nicobar, India (new type locality).

Figures

Fig. 2. Acanthozoon alderi. A, dorsal view of living specimen ZSI/ANRC-16848 (neotype). B, diagram of dorsal surface. C, ventral view of fixed specimen ZSI/ANRC-16849 (additional material). D, diagram of ventral view. E, dorsal view of living specimen ZSI/ANRC-16849 (additional material). F, dorsal view of fixed specimen ZSI/ANRC-16849 (additional material). fg, female gonopore; m, mouth; mg, male gonopore; mr, marginal rim; ph, pharynx; sd, seminal duct; smb, submarginal band; su, sucker. Scale bars: A, B and F, 20 mm.

Fig. 3. Acanthozoon alderi (ZSI/ANRC-16848). A, sagittal section showing the dorsal papillae. B, sagittal section showing dorsal epidermis devoid of papillae. C, sagittal section of the male copulatory system. D, sagittal diagram of the male copulatory system. E, sagittal diagram of the female system. F, sagittal section of the female system. cg, cement glands; ed, ejaculatory duct; eg, extravesicular glands; eo, eosinophilous droplets; fa, female atrium; fg, female gonopore; ib, intestinal branch; im, intestinal muscle layer; ma, male atrium; mi, main intestine branch; ov, oviduct; pa, papilla; pm, parenchymal muscle fibres; ps, penis sheath; pv, prostatic vesicle; rh, rhabdites; sv, seminal vesicle; tm, transversal muscles; u, uteri; v, vagina. Scale bars = 200 µm.