Dicyema bacterocephalum Furuya, 2022

Family Dicyemidae van Beneden, 1882 View in CoL Genus Dicyema von Kölliker, 1849

Dicyema bacterocephalum Furuya , sp. nov. [New Japanese name: Marubō-nihaichū] ( Figs 2 View Fig , 3 View Fig ; Tables 1–3)

Diagnosis. Small sized dicyemid, body length reaching 1120 µm. Calotte cap- or disc-shaped. Vermiform stages with 20, 22 peripheral cells: 4 propolar cells+4 metapolar cells+2 parapolar cells+10 or 12 trunk cells. Infusoriform embryos with 37 cells; refringent bodies solid; and 2 nuclei present in each urn cell.

Description. Nematogens ( Figs 2a–d View Fig , 3a, c–e View Fig ). Body length 510–1110 µm, width 60–75 µm; widest in region of parapolars; trunk width mostly uniform. Peripheral cell number 20 or 22 ( Table 2): 4 propolar cells+4 metapolar cells+2 parapolar cells+8 or 10 diapolar cells+2 uropolar cells. Calotte cap- or disc-shaped, rounded anteriorly; cilia about 4 µm long, oriented anteriorly. Propolar cells equal or larger than metapolar cells, their nuclei equal to or smaller than metapolar cell nuclei. Propolar cells occupy anterior 30–40% of calotte length when viewed laterally ( Fig. 2a, b View Fig ). Cytoplasm of propolar cells more darkly stained by hematoxylin than that of other peripheral cells ( Fig. 2a, b View Fig ). Axial cell cylindrical, pointed anteriorly, extending forward to the base of propolar cells ( Fig. 2a, b View Fig ). About 19 vermiform embryos present per axial cell of large individuals. Accessory nuclei seen in trunk peripheral cells.

Vermiform embryos ( Figs 2c View Fig , 3d, e View Fig ). Full-grown vermiform embryos length 47–72 µm, and 17–20 µm in width. Peripheral cell number 20 or 22 ( Table 2); trunk cells arranged in opposed pairs. Anterior end of calotte rounded. Axial cell rounded anteriorly, extending to the base of propolar cells ( Figs 2c View Fig , 3e View Fig ). Axial cell of full-grown embryos with 2 agametes.

Rhombogens ( Figs 2d View Fig , 3f, g View Fig ). Body similar in length to nematogens, 750–1120 µm in length and 45–120 µm in width. Peripheral cell number typically 20 or 22 ( Table 2). Calotte, axial cell shape and anterior extent similar to nematogens. A maximum of 9, usually 3–5 infusorigens present in the axial cell of each parent individual. About 70 infusoriform embryos present per axial cell of large individuals.

Infusorigens ( Figs 2e View Fig , 3h; n View Fig =10). Mature infusorigens medium-sized; composed of 6–8 (mode 6) external cells (oogonia and primary oocytes)+3–4 (mode 3) internal cells (spermatogonia, primary spermatocytes, and secondary spermatocytes)+4–8 (mode 6) spermatozoa. Mean diameter of fertilized eggs 11.7 µm; that of spermatozoa 2.2 µm. Axial cell round or ovoid, diameter 11–12 µm.

Infusoriform embryos ( Figs 2f, g View Fig , 3i–k; n View Fig =10). Full-grown embryos large, length 26.8±1.8 µm (mean±SD, excluding cilia); length–width–height ratio 1.0: 0.81: 0.73; shape ovoid, bluntly rounded and pointed posteriorly; cilia at posterior end 7 µm long. Refringent bodies present, solid, occupying anterior 30–40% of embryo length when viewed laterally ( Fig. 2g View Fig ). Cilia project from ventral internal cells into urn cavity ( Fig. 3k View Fig ). Capsule cells contain small granules ( Fig. 3k View Fig ). 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 contains 2 nuclei and germinal cell ( Fig. 3k View Fig ). All somatic nuclei pycnotic in mature infusoriform embryos.

Remarks. Dicyema bacterocephalum sp. nov. is the first species of the genus found in Sepia kobiensis and is similar to D. balamuthi McConnaughey, 1949 , D. clavatum Furuya and Koshida, 1992 , D. colurum Furuya, 1999 , D. hadrum Furuya, 1999 , and D. schulzianum van Beneden, 1876 in the calotte shape of vermiform stages and peripheral cell numbers ( Beneden 1876; McConnaughey 1949; Furuya et al. 1992b; Furuya 1999). However, D. bacterocephalum sp. nov. is distinguishable from D. balamuthi , D. clavatum , D. colurum , and D. hadrum in the cell number of infusoriform embryos (37 vs. 39). Dicyema bacterocephalum sp. nov. shares the cellular composition and cell number of infusoriform embryos with D. schulzianum but there is a marked difference in the number of peripheral cells; D. bacterocephalum sp. nov. has 20 or 22 peripheral cells, while D. schulzianum is consistently 22. Both individuals having 20 and 22 peripheral cells are found in a single axial cell in D. bacterocephalum sp. nov.

Etymology. The species name is an adjective composed of two Ancient Greek roots, bact and - kephalos, meaning “rod” and “-headed” in reference to the characteristic shape of the body of vermiform stages.

Taxonomic summary. Type material: a syntype slide (NSMT-Me-58) collected on 10 April 2016; additional syntypes on slide series No. SK3517 (5 slides) in the author’s collection.

Type locality: off Minami-Ise (34°08′N, 136°40′E), Mie Prefecture, Honshu, the Kumano Sea, Japan, depth 150 m GoogleMaps .

Other materials examined: None.

Host: symbiotype, Sepia kobiensis Hoyle, 1855 (Mollusca: Cephalopoda: Sepiida ), female (immature), 64 mm ML (NSMT-Mo-85901).

Collector of host: T. Moritaki.

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

Prevalence: in 1 of 30 host specimens examined (3.3%).

Dicyema conocephalum Furuya , sp. nov. [New Japanese name: Kono-nihaichū] ( Figs 4 View Fig , 5 View Fig ; Tables 1–3)

Diagnosis. Medium-sized dicyemid; body length reaching 1540 µm. Calotte conical in shape. Vermiform stages with 28–32 peripheral cells: 4 propolar cells+4 metapolar cells+2 parapolar cells+18–22 trunk cells. Infusoriform embryos with 37 cells; refringent bodies solid; and two nuclei present in each urn cell.

Description. Nematogens ( Figs 4a, b View Fig , 5a View Fig ). Body length 700–1540 µm, width 45–60 µm; widest in region of parapolars; trunk width mostly uniform. Peripheral cell number 28–32 ( Table 2): 4 propolar cells+4 metapolar cells+ 2 parapolar cells+16–20 diapolar cells+2 uropolar cells. Calotte conical in shape, rounded anteriorly; cilia on calotte about 4 µm long, oriented anteriorly. Propolar cells and their nuclei equal to or smaller than metapolar cells and their nuclei. Propolar cells occupy anterior 40–50% of calotte length when viewed laterally ( Fig. 4a, b View Fig ). Cytoplasm of propolar and parapolar cells contains fibrous structure, more darkly stained by hematoxylin than cytoplasm of other peripheral cells ( Fig. 4a, b View Fig ). Axial cell cylindrical, pointed anteriorly; cell extending forward to base of metapolar cells ( Fig. 5a View Fig ). About 30 vermiform embryos present per axial cell of large individuals. Accessory nuclei seen in trunk peripheral cells.

Vermiform embryos ( Figs 4c View Fig , 5d, e View Fig ). Full-grown vermiform embryos length 54–70 µm, width 13–15 µm. Peripheral cell number 28–32 ( Table 2); trunk cells arranged in opposed pairs. Anterior end of calotte pointed acutely. Axial cell is pointed anteriorly, extending to the base of metapolar cells ( Figs 4c View Fig , 5d, e View Fig ). Axial cell of full-grown embryos with one agamete.

Rhombogens ( Figs 4d View Fig , 5b, c View Fig ). Body length 700–1100 µm, similar to that of nematogens, width 60–72 µm. Peripheral cell number typically 28–32 ( Table 2). Calotte shape, axial cell shape, and anterior extent similar to those of nematogens. A maximum of 2 infusorigens present in the axial cell of each parent individual. About 30 infusoriform embryos present per axial cell of large individuals.

Infusorigens ( Figs 4e View Fig , 5h; n View Fig =10). Mature infusorigens medium-sized, composed of 6–15 (mode 7) external cells (oogonia and primary oocytes)+3–6 (mode 4) internal cells (spermatogonia, primary spermatocytes, and secondary spermatocytes)+4–26 (mode 8) spermatozoa. Mean diameter of fertilized eggs 13.7 µm; that of spermatozoa 2.5 µm. Axial cell ovoid or round, diameter 12–17µm.

Infusoriform embryos ( Figs 4f, g View Fig , 5i–k; n View Fig =20). Full-grown embryos large, length 26.7±1.8 µm (mean±SD, excluding cilia); length–width–height ratio 1.0: 0.87: 0.81; ovoid, bluntly rounded to pointed posteriorly; cilia at posterior end 7 µm long. Refringent bodies, solid, occupying anterior 30–40% of embryo length when viewed laterally ( Fig. 4g View Fig ). Cilia project from ventral internal cells into urn cavity ( Fig. 5k View Fig ). Capsule cells contain small granules ( Fig. 5k View Fig ). 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 embryos (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 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 contains two nuclei and a single germinal cell ( Fig. 5k View Fig ). All somatic nuclei pycnotic in mature infusoriform embryos.

Remarks. Dicyema conocephalum sp. nov. is the first species of the genus found in Sepia kobiensis . It is characterized by an acutely pointed calotte of vermiform embryos, a large number of peripheral cells variable number from 28 to 32 ( Table 1). Dicyema ganapatti Kalavati, Narasimhamurti, and Suseela, 1984 and D. oxycephalum Furuya, 2009 , are very similar to D. conocephalum sp. nov. in the shape of the calotte in vermiform stages and the number of peripheral cells ( Kalavati et al. 1984; Furuya 2009). However, D. conocephalum sp. nov. differs from D. oxycephalum in the cell number of infusoriform embryos (37 vs. 39; cf. Furuya 2009). The axial cells of D. conocephalum sp. nov. extend forward to the middle of the metapolar cells, whereas those of D. ganapatii end forward to the middle of metapolar cells. In addition, D. conocephalum sp. nov. can be distinguished from D. ganapatii based on the typical number of agametes (1 vs. 2) in full-grown vermiform embryos ( Kalavati et al. 1984).

Etymology. The species name “ conocephalum ” is an adjective composed of two Ancient Greek roots, konikós and - kephalos, meaning “conical” and “-headed” in reference to the characteristic anterior part of vermiform embryos.

Taxonomic summary. Type material: a slide of syntypes (NSMT-Me-61) collected on 30 November 2015; additional syntypes on slide series No. ST3303 (5 slides) in the author’s collection.

Type locality: off Minami-Ise (34°08′N, 136°40′E), Mie Prefecture, Honshu, the Kumano Sea, Japan, depth 260 m GoogleMaps .

Other materials examined: slide series No. ST3505 (5 slides) collected off Minami-Ise (34°08′N, 136°40′E), Mie Prefecture, Honshu, the Kumano Sea, Japan, depth 260 m, 10 April 2016, in the author’s collection GoogleMaps .

Host: symbiotype, Sepia tenuipes Sasaki, 1929 (Mollusca: Cephalopoda: Sepiida ), male (mature), 107 mm ML (NSMT-Mo-85904).

Collector of host: T. Moritaki.

Site : anterior ends (calottes) inserted into crypts of the renal appendages within the renal sacs.

Prevalence: in 28 of 91 host specimens examined (30.8%).