Dicyemennea acetabulum, Furuya, 2018

Furuya, Hidetaka, 2018, Eleven New Species of Dicyemids (Phylum Dicyemida) from Octopus longispadiceus and O. tenuicirrus (Mollusca: Cephalopoda: Octopoda) in Japanese Waters, Species Diversity 23, pp. 143-179 : 154-157

Dicyemennea acetabulum sp. nov.

( Figs 10 , 11 ; Tables 1, 3)

Diagnosis. Medium dicyemid; body length reaching 3,200 µm. Calotte disc-shaped. Vermiform stages with 23 peripheral cells: 4 propolars+5 metapolars+2 parapolars+12 trunk cells. Infusoriform embryos with 37 cells; refringent bodies solid; and nucleus present in each urn cell.

Description. Nematogens ( Figs 10a, b , 11a, c, d ). Body length 500–3,050 µm and width 240–450 µm, widest in region of metapolars; trunk width mostly uniform. Peripheral cell number 23 ( Table 3): 4 propolars+5 metapolars+2 parapolars+10 diapolars+2 uropolars. Calotte wheelshaped, cilia on calotte about 6 µm long, oriented anteriorly. Grooves or clefts extending radially across metapolar cells occasionally into propolar cells ( Fig. 10d, e ). Propolar cells and their nuclei larger than metapolar cells and their nuclei ( Fig. 11c ). Cytoplasm of propolar and metapolar cells more darkly stained by hematoxylin than cytoplasm of other peripheral cells. Verruciform cells absent. Axial cell cylindrical, branching radially at anterior part; cell extending forward to propolar cells ( Fig. 11d ). About 40 vermiform embryos present per axial cell of large individuals.

Vermiform embryos ( Figs 10c , 11e, f ). Full-grown vermiform embryos length 120–173µm and width 20–36µm. Peripheral cell number 23 ( Table 3); trunk cells arranged in opposed pairs. Anterior end of calotte rounded. Axial cell rounded anteriorly, extending to propolar cells; nucleus usually located in center of axial cell. Anterior abortive axial cell absent. Axial cell of full-grown embryos with up to 4 agametes.

Rhombogens ( Figs 10f , 11g ). Body similar in length but slightly stockier than nematogens, length 500–3,760µm, and width 200–450 µm. Peripheral cell number 23 ( Table 3). Calotte disc-shaped. Axial cell shape and anterior extent similar to nematogens. Axial cell cylindrical, branching radially at the anterior part; cell extending forward to propolar cells ( Fig. 10f ). Verruciform cells absent. Usually, 1 or 2 infusorigens present in axial cell of each parent individual. About 200 infusoriform embryos present per axial cell of large individuals. Accessory nuclei occasionally present in trunk cells.

Infusorigens ( Figs 10g , 11h; n =20). Mature infusorigens large-sized; composed of 38–127 (mode 72) external cells (oogonia and primary oocytes) +40–112 (mode 61) internal cells (spermatogonia, primary spermatocytes, and secondary spermatocytes)+13–67 (mode 41) spermatozoa. Mean diameter of fertilized eggs 14.1 µm; that of spermatozoa 3.5 µm. Axial cell round or ovoid, diameter 30–70 µm.

Infusoriform embryos ( Figs 10h, i , 11i–k; n =100). Fullgrown embryos large, length 33.5±1.7 µm (mean±SD, excluding cilia); length-width-height ratio 1.0: 0.78: 0.75; ovoid, bluntly rounded and pointed posteriorly; cilia situated at posterior end, 7 µm long. Refringent bodies present, solid, occupying anterior 40% of embryo length when viewed laterally ( Fig. 10i ). Cilia projected from ventral internal cells into urn cavity ( Fig. 11k ). Capsule cells containing small granules. Mature embryos with 37 cells: 33 somatic+4 germinal cells. Somatic cells of several types present: external cells covering large part of anterior and lateral surfaces of embryo (2 enveloping cells); external cells with cilia on external surfaces (2 pairs of dorsal cells+1 median dorsal cell+2 dorsal caudal cells+2 lateral caudal cells+1 ventral caudal cell+2 lateral cells+2 posteroventral lateral cells), external cells with refringent bodies (2 apical cells); external cells without cilia (1 couvercle cell+2 first ventral cells+2 second ventral cells+2 third ventral cells); internal cells with cilia (2 ventral internal cells); and internal cells without cilia (2 dorsal internal cells+2 capsule cells+4 urn cells). Each urn cell containing single nucleus and single germinal cell ( Fig. 11k ). All somatic nuclei appear pycnotic in mature infusoriform embryos.

Remarks. Dicyemennea acetabulum sp. nov. is very similar to Dicyemennea trochocephalum Furuya, 1999 , in having a wheel-shaped calotte ( Furuya 1999). However, the new species is easily distinguishable from D. trochocephalum in the number of peripheral cells (23 vs. 25–29) and the cell number of infusoriform embryos (37 vs. 39). The new species is very similar to D. rostrata Short and Hochberg, 1969 , in having branching axial cells and grooves in propolar and metapolar cells ( Short and Hochberg 1969). However, the new species is easily distinguished from D. rostrata in the cell number of infusoriform embryos (37 vs. 39) and the absence of the anterior abortive axial cell in vermiform embryos.

Dicyemennea acetabulum sp. nov. is similar to D. discocephala Hochberg and Short, 1983 , D. rostrata , D. pileum Furuya, 2008 , and D. umbracurum Furuya, 2009 , in the number of peripheral cells and the shape of the calotte ( Short and Hochberg 1969; Hochberg and Short 1983; Furuya 2008, 2009). However, D. acetabulum sp. nov. differs from the species in the size ratio of the propolar to the metapolar cell. In the above four species, the propolar cells are smaller than the metapolar cells, while that of D. acetabulum sp. nov. are larger.

Etymology. The specific name acetabulum is derived from the form of the polyphysacean green algae, Acetabularia spp.

Taxonomic summary. Type material: a syntype slide (NSMT-Me-51) collected at 6 February 2015; additional syntypes on slide series No. OL3223 (5 slides) in the author’s collection.

Type locality: off Karo (35°47′N, 134°14′ E), Tottori Prefecture, the Sea of Japan, Japan, depth 200 m .

Other materials examined: slide series No. OL2467 (5 slides) collected off Ohda-shi (35°23′N, 132°19′E), Shimane Prefecture, Honshu, the Sea of Japan, Japan, depth 200 m, 2 March 2010; Nos . OL2647–2652 (each 5 slides) collected off Nou (37°09′N, 137°54′E), Niigata Prefecture, Honshu, the Sea of Japan, Japan, depth 200 m, 19 May 2011 in the author’s collection .

Host: symbiotype, Octopus longispadiceus ( Sasaki, 1917) (Mollusca: Cephalopoda: Octopoda ), male (mature), 72 mm ML (NSMT-Mo-85868).

Site : anterior ends (calottes) attach to surfaces of the renal appendages within the renal sacs.

Prevalence: in 29 of 510 specimens of hosts examined (5.2%).

Figures

Fig. 10. Dicyemennea acetabulum sp. nov., photographs of syntype specimens on slide NSMT-Me-51: a, nematogen, entire; b, nematogen, anterior region; c, vermiform embryos within axial cell; d–f, rhombogen, anterior region; g, infusorigen; h, i, infusoriform embryos, horizontal section (h), sagittal section (i). Scale bars: 50µm in a, 5µm in b–k.

Fig. 11. Dicyemennea acetabulum sp. nov., drawn from syntype specimens on slide NSMT-Me-51: a, nematogen, entire; b, rhombogen, entire; c, d, nematogen, anterior region; e, f, vermiform embryo within axial cell, cilia omitted (e), optical section (f); g, rhombogen, anterior region; h, infusorigen; i–k, infusoriform embryos, dorsal view (i; cilia omitted), ventral view (j; cilia omitted), sagittal section (k). Scale bars: 100µm in a, b, 50µm in c, g, 10µm in d–f, h–k.