Dicyemodeca shirarikaense, Furuya, 2025

Dicyemodeca shirarikaense sp. nov. [New Japanese name: Shirarika-nihaichū] ( Figs 3 View Fig , 4 View Fig ; Tables 1, 2)

Diagnosis. Medium-sized dicyemid; body length to 2500 µm. Calotte cap-shaped. Vermiform stages with 24 peripheral cells: 4 propolars + 6 metapolars + 4 parapolars + 10 trunk cells. Infusoriform embryos with 35 cells; refringent bodies liquid; and 2 nuclei present in each urn cell.

Description. Nematogens (n = 20) ( Figs 3a, b, e, f View Fig , 4a, c, d View Fig ). Body length 800–2500 µm, width 90–240 µm, widest in region of metapolars; trunk width mostly uniform. Peripheral cell number 24 ( Table 2): 4 propolars + 6 metapolars + 4 parapolars + 8 diapolars + 2 uropolars. Calotte cap-shaped, round anteriorly; cilia on calotte about 4 µm long, oriented anteriorly. Propolar cells and their nuclei smaller than metapolar cells and their nuclei. Propolar cells occupying anterior 40%–50% of calotte length when viewed laterally ( Fig. 4c, d View Fig ). Cytoplasm of propolar and metapolar cells more darkly stained by hematoxylin than cytoplasm of other peripheral cells ( Fig. 4b, c View Fig ). Verruciform cells present. Axial cell cylindrical in juveniles, swollen anteriorly in large individuals, extending forward to propolar cells ( Figs 3e, f View Fig , 4b View Fig ). About 25 vermiform embryos present per axial cell of large individuals.

Vermiform embryos (n = 20) ( Figs 3c View Fig , 4e, f View Fig ). Full-grown vermiform embryos length 105–180 µm, width 22–35 µm. Peripheral cell number 24 ( Table 2); trunk cells arranged in opposed pairs. Anterior end of calotte rounded or bluntly pointed. Axial cell bluntly pointed anteriorly, extending to the end of propolar cells; nucleus usually located in anterior half of axial cell. Anterior abortive axial cell absent. Axial cell of full-grown embryos with up to 8 agametes.

Rhombogens (n = 20) ( Figs 3d, g View Fig , 4b, g View Fig ). Body similar in length to nematogens, length 950–2050 µm, width 150–310 µm. Peripheral cell number typically 24 ( Table 2). Calotte cap-shaped, rounded anteriorly. Verruciform cells present. Axial cell shape and anterior extent similar to nematogens. One, rarely 2 or 3 infusorigens per axial cell of each parent individual. About 40 infusoriform embryos present per axial cell of large individuals. Accessory nuclei occasionally present in trunk cells.

Infusorigens (n = 20) ( Figs 3h View Fig , 4h View Fig ). Mature infusorigens medium-sized; composed of 6–35 (mode 26) external cells (oogonia and primary oocytes) + 4–24 (mode 6) internal cells (spermatogonia, primary spermatocytes, and secondary spermatocytes) + 4–28 (mode 4) spermatozoa. Mean diameter of fertilized eggs, 12.6 µm; that of spermatozoa, 2.4 µm. Axial cell round, diameter 13–32µm.

Infusoriform embryos (n = 40) ( Figs 3i, j View Fig , 4i–k View Fig ). Full-grown embryos large, length 28.9 ± 1.7 µm (mean ± SD, excluding cilia); length-width-height ratio 1.0: 0.92: 0.88; shape ovoid, bluntly rounded; cilia at posterior end 6 µm long. Refringent bodies present, liquid, occupying anterior 30% of embryo length when viewed laterally ( Fig. 4k View Fig ). Cilia projecting from ventral internal cells into urn cavity ( Fig. 4k View Fig ). Capsule cells containing small granules. Mature embryos with 35 cells: 31 somatic + 4 germinal cells. Somatic cells of several types present: external cells that cover a large part of the anterior and lateral surfaces of the 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 apical micro cells + 2 shield 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 contains a single germinal cell and 2 nuclei ( Fig. 4k View Fig ). All somatic nuclei appear pycnotic in mature infusoriform embryos.

Remarks. Dicyemodeca shirarikaense sp. nov. has a similar peripheral cell number to the other congeners: Dca. dogieli Bogolepova, 1957, Dca. deca (McConnaughy, 1957) , Dca. delamarei ( Nouvel, 1961) , Dca. anthinocephalum Furuya, 1999, and Dca. kukii Furuya, 2018 . However, Dca. shirarikaense sp. nov. differs from Dca. delamarei in calotte shape. The new species has conical or cap-shaped calotte, while Dca. delamarei has a typical calotte shape ( Nouvel 1961).

In infusoriform embryos, Dca. shirarikaense sp. nov. shares two unique cell types, namely the anterior micro cells and shield cells with Dca. anthinocephalum and Dca. kukii ( Furuya 1999, 2018). However, the new species is distinguished from Dca. anthinocephalum by the number of cells of infusoriform embryos (35 vs. 42). Dicyemodeca shirarikaense sp. nov. is distinguished from Dca. dogieli and Dca. deca by lacking solid refringent bodies within the apical cell of infusoriform embryos ( Bogolepova 1957; McConnaughy 1957).

Dicyemodeca shirarikaense sp. nov. shares the most of characters with Dca. kukii . Dicyemennea kukii has been reported from O. tenuicirrus (Sasaki, 1929) View in CoL , in the northwestern Pacific Ocean (Kumano Sea, Mie, Japan; Furuya 2018). In Japanese waters, O tenuicirrus View in CoL is distributed on the continental shelf from the Boso Peninsula to the Kii Peninsula, while O. conispadiceus View in CoL is distributed on the continental shelf from the Hokkaido coast to off the Sanriku coast ( Okutani et al. 1987), thus there is no overlap in habitat between O. tenuicirrus View in CoL and O. conispadiceus View in CoL . Dicyemodeca shirarikaense sp. nov. has a medium-sized infusorigen and the mode of egg number (oogonia and primary oocytes) is 26; while Dca. kukii has a small-sized infusorigen and the mode of eggs is 15 ( Furuya 2018). In addition, the new species is distinguishable by its smaller fertilized egg size than that of Dca. kukii (mean ± SD: 12.6 ± 0.4 vs. 14.0 ± 0.8 µm). The relationship between gonad size and egg size in these two species indicates that Dca. shirarikaense sp. nov. has a strategy of laying a larger number of small eggs, while Dca. kukii lays fewer large eggs.

Etymology. The specific name “ shirarikaense ” is derived from the Ainu language “shirarika”, meaning “near the rocky shore,” which is the origin of the type locality Shiranuka.

Taxonomic summary. Type material. A syntype slide (NSMT-Me-73) collected on 26 February 2019; additional syntypes on slide series No. OC3956 (5 slides) in the author’s collection.

Type locality. Off Shiranuka (42°54′N, 144°12′E), Hokkaido, Japan, depth 100m GoogleMaps .

Other material examined. Slide series Nos OC4149, 4150 (each 5 slide) collected off Shiranuka, depth 100 m, 11 May 2020 in the author’s collection.

Host. Symbiotype, Octopus conispadiceus ( Sasaki, 1917) (Mollusca: Cephalopoda: Octopoda ), male (mature), 155 mm ML (NSMT-Mo-85946).

Site . Anterior ends (calottes) of cap-shaped individuals attaching the surface of renal appendages within the renal sac.

Prevalence. In 43 of 47 specimens of hosts examined (91.5%).