Dicyemennea tsunekii, Furuya, 2025

Dicyemennea tsunekii sp. nov. [New Japanese name: Tsuneki-nihaichū] ( Figs 1 View Fig , 2 View Fig ; Tables 1, 2)

Diagnosis. Large-sized dicyemid; body length to 1.0mm. Calotte conical, rounded anteriorly. Vermiform stages with 33–35 peripheral cells: 4 propolars + 5 metapolars + 2 parapolars + 22–24 trunk cells. Infusoriform embryos with 39 cells; refringent bodies liquid; and 2 nuclei present in each urn cell.

Description. Nematogens (n = 20) ( Figs 1a, c, d View Fig , 2a, c, d View Fig ). Body length 3400–10200 µm and width 120–210 µm, widest in region of metapolars; trunk width mostly uniform. Peripheral cell number 33 ( Table 2): 4 propolars + 5 metapolars + 2 parapolars + 20–22 diapolars + 2 uropolars. Calotte conical, rounded anteriorly; cilia on calotte about 4 µm long, oriented anteriorly. Propolar cells and their nuclei smaller than metapolar cells and their nuclei. Propolar cells occupy anterior 35%–40% of calotte length when viewed laterally ( Figs 1c View Fig , 2c, d View Fig ). Cytoplasm of propolar and metapolar cells more darkly stained by hematoxylin than cytoplasm of other peripheral cells. Verruciform cell absent in peripheral cells. Axial cell cylindrical, rounded anteriorly and extending forward to base of metapolar cells ( Figs 1c View Fig , 2d View Fig ). About 20 vermiform embryos present in an axial cell of large individuals. Agametes occasionally fusiform in shape ( Fig. 1d View Fig ).

Vermiform embryos (n = 20) ( Figs 1e View Fig , 2e, f View Fig ). Full-grown vermiform embryo length 190–340 µm, width 20–32 µm. Peripheral cell number 33–35 ( Table 2); trunk cells arranged in opposed pairs. Anterior end of calotte bluntly pointed. Axial cell rounded anteriorly, extending to base of propolar cells; nucleus usually located in middle of axial cell. Anterior abortive axial cell present ( Figs 1e View Fig , 2f View Fig ). Axial cell of full-grown embryos often with up to 8 agametes.

Rhombogens (n = 20) ( Figs 1b, f View Fig , 2b, g View Fig ). Body similar in length to nematogens, length 3700–10600 µm, width 120– 210 µm. Peripheral cell number typically 33–35 ( Table 2). Calotte conical, rounded anteriorly. Verruciform cell absent. Axial cell shape and anterior extent similar to nematogens. One to 9 infusorigens present in axial cell of each parent individual. About 180 infusoriform embryos present per axial cell of large individuals. Accessory nuclei occasionally present in trunk cells.

Infusorigens (n = 20) ( Figs 1g View Fig , 2h View Fig ). Mature infusorigens large in size, composed of 30–105 (mode 67) external cells (oogonia and primary oocytes)+ 13–93 (mode 26) internal cells (spermatogonia, primary spermatocytes, and secondary spermatocytes) +4–122 (mode 14) spermatozoa. Mean diameter of fertilized eggs and spermatozoa 13.2 µm and 3.0µm, respectively. Axial cell round or ovoid, diameter 23–84 µm.

Infusoriform embryos (n = 40) ( Figs 1h, i View Fig , 2i–k View Fig ;). Full-grown embryos large, length 32.2 ± 3.7 µm (mean ± SD, excluding cilia); length–width–height ratio 1.0: 0.80: 0.76; shape ovoid, bluntly rounded to pointed posteriorly; cilia at posterior end 8 µm long. Refringent bodies present, solid, occupying anterior 35% of embryo length when viewed laterally ( Figs 1i View Fig , 2k View Fig ). Cilia projecting from ventral internal cells into urn cavity ( Fig. 2k View Fig ). Capsule cells containing large granules ( Fig. 2k View Fig ). Mature embryos with 39 cells: 35 somatic + 4 germinal cells. Somatic cells of several types present: external cells covering a large part of anterior and lateral surfaces of embryo (2 enveloping cells); external cells with cilia on external surfaces (2 paired 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 anterior lateral cells+ 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 germinal cell and 2 nuclei ( Fig. 2k View Fig ). All somatic nuclei pycnotic in mature infusoriform embryos.

Remarks. Dicyemennea tsunekii sp. nov. is the first species of the genus described from O. conispadiceus and is very similar to D. californica McConnaughey, 1941 , D. granularis McConnaughey, 1949 , D. mcconnaugheyi Furuya, 2018 , and D. nouveli McConnaughey, 1959 , in peripheral cell numbers, the calotte shape of vermiform stages, and the cellular constitution and cell number of infusoriform embryos ( McConnaughey 1941, 1949, 1959; Furuya et al. 2004; Furuya 2007, 2008, 2018). The accumulation of eosinophilic granules within diapolar cells, forming verruciform cells, is characteristic of D. granularis and D. mcconnaugheyi ( McConnaughey 1949; Furuya 2018). In this feature, D. tsunekii sp. nov. can be distinguished from these latter two species because it does not accumulate conspicuous granules or form verruciform cells. In other characteristics, D. tsunekii sp. nov. differs from D. granularis in the length of infusoriform embryos (nearly 32 µm vs. 35 µm) and maximum agamete number of vermiform embryos (8 vs. 2) ( McConnaughey 1949). It can also be distinguished from D. mcconnaugheyi in the number of infusorigens (9 vs. 3 rarely 6) and maximum number of agamete of vermiform embryos (8 vs. 6) ( Furuya 2018).

Dicyemennea californica shares the most characters with D. granularis except for the absent of granules in diapolar cells, thus D. tsunekii sp. nov. is also distinguishable from D. californica in the length of infusoriform embryos (nearly 32 µm vs. 35 µm) and maximum agamete number of vermiform embryos (8 vs. 2) ( McConnaughey 1941).

Dicyemennea nouveli View in CoL has been described from the enteroctopodid octopus, E. dofleini View in CoL , in the northeastern Pacific Ocean (Monterey Bay, California), the northwestern Pacific Ocean (off northern and eastern areas of Cape Shiriya, Aomori, Japan), and the northern region of the Sea of Japan ( Russia) ( McConnaughey 1959; Bogolepova-Dobrokhotova 1963; Furuya 2008). In Japanese waters, the habitat of O. conispadiceus View in CoL overlaps with that of E. dofleini View in CoL . However, D. tsunekii sp. nov. is distinguishable from D. nouveli View in CoL in the length of infusoriform embryos (nearly 32 µm vs. 40 µm), the number of infusorigens (9 vs. 5), and maximum number of agamete of vermiform embryos (8 vs. 4).

Etymology. The specific name “ tsunekii ” is in honor of Dr. Kazuhiko Tsuneki who opened the door to biology for the author.

Taxonomic summary. Type material. A syntype slide (NSMT-Me-72) collected on 9 April 2019; additional syntypes on slide series No. OC3970 (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 OC3956, 3959 (each 5 slide) collected off Shiranuka, depth 100 m, 26 February 2019 in the author’s collection.

Host. Symbiotype, Octopus conispadiceus ( Sasaki, 1917) (Mollusca: Cephalopoda: Octopoda ), male (mature), 115 mm ML (NSMT-Mo-85945).

Site of attachment. Anterior ends (calottes) inserted into crypts of the renal appendages within the renal sacs.

Prevalence. Found in 10 of 47 host specimens examined (21.3%).