Dicyemennea megalosomum, 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 : 157-159

Dicyemennea megalosomum sp. nov.

( Figs 12 , 13 ; Tables 1, 3)

Diagnosis. Large dicyemid; body length up to

15,200 µm. Calotte wheel-shaped. Vermiform stages with 32–38 peripheral cells: 4 propolars+5 metapolars+2 parapolars+21–27 trunk cells. Infusoriform embryos with 37 cells; refringent bodies solid; and nucleus present in each urn cell.

Description. Nematogens ( Figs12a–c , 13a, c, d ). Body length 500–3,450 µm and width 340–690 µm; widest in region of metapolars; trunk narrow to posterior. Peripheral cell number 32–38 ( Table 3): 4 propolars+5 metapolars+2 parapolars+19–25 diapolars+2 uropolars. Calotte wheelshaped, cilia on calotte about 6 µm long, oriented anteriorly. Propolar cells and their nuclei equal in size to metapolar cells and their nuclei ( Figs 12a, b ; 13c, d ). Cytoplasm of propolar and metapolar cells more darkly stained by hematoxylin than cytoplasm of other peripheral cells. Verruciform cells absent. Axial cell cylindrical, enlarging at anteriorly; cell extending forward to propolar cells ( Figs 12a–c , 13c, d ). About 50 vermiform embryos present per axial cell of large individuals.

Vermiform embryos ( Figs 12f , 13f, g ). Full-grown vermiform embryos range length 105–230 µm and width 37–69µm. Peripheral cell number 32–38 ( Table 3); trunk cells arranged in opposed pairs. Anterior end of calotte rounded. Axial cell rounded anteriorly, extending to propolar cells. Anterior abortive axial cell present ( Fig. 12f ). Axial cell of full-grown embryos with up to 36 agametes.

Rhombogens ( Figs 12d, e , 13e ). Body similar in length but stockier than nematogens, length 500–15,200 µm and width 340–710 µm. Peripheral cell number 32–38 ( Table 3). Calotte disc-shaped. Axial cell shape and anterior extent similar to nematogens. Verruciform cells absent. Usually, 20 (maximum 52) infusorigens present per axial cell of each parent individual. Usually, 100–500 (maximum 1,420) infusoriform embryos present per axial cell of large individuals. Accessory nuclei occasionally present in trunk cells.

Infusorigens ( Figs 12g , 13h; n =20). Mature infusorigens large-sized; composed of 30–190 (mode 76) external cells (oogonia and primary oocytes)+21–131 (mode 44) internal cells (spermatogonia, primary spermatocytes, and secondary spermatocytes)+25–149 (mode 57) spermatozoa. Four to 16 nuclei present within axial cell. Mean diameter of fertilized eggs 13.4 µm, that of spermatozoa 4.0 µm. Axial cell round or ovoid, diameter 35–94 µm.

Infusoriform embryos ( Figs 12h, i , 13i–k; n =100). Full-grown embryos large, average length 36.7±1.7 µm (mean±SD, excluding cilia); length-width-height ratio 1.0: 0.85: 0.82; shape ovoid, bluntly rounded to pointed posteriorly; cilia at posterior end 6 µm long. Refringent bodies present, solid, occupying anterior 30% of embryo length when viewed laterally ( Fig. 12i ). Cilia projecting from ventral internal cells into urn cavity ( Fig. 13k ). Capsule cells containing small granules. Mature embryos with 37 cells: 33 somatic+4 germinal cells. Somatic cells of several types present: external cells covering most 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 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 ( Figs 12i , 13k ). All somatic nuclei appear pycnotic in mature infusoriform embryos.

Remarks. Dicyemennea megalosomum sp. nov. is very similar to D. acetabulum sp. nov. and D. trochocephalum in having the wheel-shaped calotte. The new species was found together with D. acetabulum sp. nov. in O. longispadiceus but is clearly distinguishable from D. acetabulum sp. nov. in the number of peripheral cells (32–38 vs. 23). The new species is easily distinguished from D. trochocephalum in the number of peripheral cells (32–38 vs. 25–29) and the cell number of infusoriform embryos (37 vs. 39) ( Furuya 1999). In addition, Dicyemennea megalosomum sp. nov. differs from the other congeners principally on the basis of its distinctly huge body.

Etymology. The species name megalosomum is an adjective composed of 2 Greek roots, megálo and - sóma meaning “big” and “body”, in reference to the characteristic large body of the adult.

Taxonomic summary. Type material: a syntype slide (NSMT-Me-52) 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 . OL871 (5 slides) collected off Iwase (36°48′N, 137°15′E), Toyama Prefecture, Honshu , Toyama Bay, Japan, depth 350 m, 6 March 2003; No . OL2474 (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, 2648 (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) attached to surfaces of renal appendages within renal sacs.

Prevalence: in 27 of 510 specimens of hosts (4.9%).

Figures

Fig. 12. Dicyemennea megalosomum sp. nov., photographs of syntype specimens on slide NSMT-Me-52: a–c, nematogen, anterior region; d, e, rhombogen, anterior region; f, vermiform embryos within axial cell; g, infusorigen; h, i, infusoriform embryos, horizontal section (h), sagittal section (i). Scale bars: 50µm in (d), 10µm in a–c, e–i.

Fig. 13. Dicyemennea megalosomum sp. nov., drawn from syntype specimens on slide NSMT-Me-52: a, nematogen, entire; b, rhombogen, entire; c, d, nematogen, anterior region; e, rhombogen, anterior region; f, g, vermiform embryo within axial cell, cilia omitted (f), optical section (g); h, ifusorigen; i–k, infusoriform embryos, ventral view (i; cilia omitted), dorsal view (j; cilia omitted), sagittal section (k). Scale bars: 200µm in a, b, 50µm in d–e, 10µm in f–k.