Dicyemennea anteronucleatum, 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 : 152-154

Dicyemennea anteronucleatum sp. nov.

( Figs 8 , 9 ; Tables 1, 3)

Diagnosis. Medium-sized dicyemid; body length reaching 2,500 µm. Calotte shape conical. Vermiform stages with 23 peripheral cells: 4 propolars+5 metapolars+2 parapolars+12 trunk cells. Infusoriform embryos unknown.

Description. Nematogens ( Figs 8a–e , 9a–f ). Body length 500–2,500 µm and width 60–85 µm; widest in region of parapolars; trunk width mostly uniform. Peripheral cell number 23: 4 propolars+5 metapolars+2 parapolars+10 diapolars+2 uropolars. Calotte conical in shape, bluntly pointed anteriorly; cilia on calotte about 4 µm long, pointed anteriorly. Propolar cells and their nuclei equal or smaller than metapolar cells and their nuclei. Propolar cells occupying anterior 20–25% of calotte length when viewed laterally ( Figs 8a–e , 9a–f ). Cytoplasm of propolar cells more darkly stained by hematoxylin than cytoplasm of other peripheral cells ( Figs 8a–e , 9a–f ). Axial cell cylindrical, rounded anteriorly, extending forward to base of propolar cells. Axial cell nucleus located in anterior part of axial cell ( Figs 8c, e , 9c–e ). About 20 vermiform embryos present per axial cell of large individuals.

Vermiform embryos ( Figs 8f , 9g, h ). Full-grown vermiform embryos length 95–145 µm and width 15–24 µm. Peripheral cell number 23; trunk cells arranged in opposed pairs. Anterior end of calotte bluntly pointed. Axial cell bluntly pointed, extending to base of metapolar cells; nucleus usually located in anterior half of axial cell. Anterior abortive axial cell absent. Axial cell of full-grown embryos with up to 2 agametes.

Rhombogens, infusorigens, and infusoriform embryos. Unknown.

Remarks. The axial cell nucleus of D. anteronucleatum sp. nov. is located consistently in the anterior region of the axial cell of the nematogens. Thereby, the new species can be separated from other congeners by having 23 peripheral cells and a conical calotte. This species is similar to six species ( D. canadensis Furuya, Hochberg and Short 2002 , D. kaikouriensis Short and Hochberg, 1969 , D. littlei Hochberg and Short, 1970 , D. minabense Furuya, 1999 , D. ophioides Furuya, 1999 , and D. ryukyuense Furuya, 2006 ) in the number of peripheral cells of vermiform stages, the shape of the calotte, and lacking the anterior abortive axial cell ( Short and Hochberg 1969; Hochberg and Short 1970; Furuya 1999, 2006c; Furuya et al. 2002). Dicyemennea anteronucleatum sp. nov. is a medium-sized species, with vermiform stages up to 2,500 µm in maximum length. It is easily distinguishable from D. littlei , D. minabense , D. ophioides , and D. ryukyuense , which have a much larger vermiform stage of length up to 5,000 µm.

Dicyemennea anteronucleatum sp. nov. differs from D. kaikouriensis in the maximum number of agametes of vermiform embryos (4 vs. 1) and in the maximum length of vermiform stages (3,550 µm vs. 1,050 µm). Dicyemennea anteronucleatum sp. nov. is distinguishable from D. canadensis in the maximum length of vermiform stages (2,550 µm vs. 600 µm) and more numerous peripheral cells (23 vs. 21).

Dicyemennea anteronucleatum sp. nov. was firstly found from O. longispadiceus off Uozu in Toyama Bay, the Sea of Japan in 2003. The sexual stage, the rhombogen, of the new species was not found in the host. Since then, the author has surveyed the rhombogen and a large number of host individuals collected in several areas in the Sea of Japan. However, it has never been found.

Dicyemennea anteronucleatum sp. nov. occurs with one to four other dicyemids (see Table 4). In such a situation, only the new species remains at its asexual stage, while all the other co-existing species were found at the rhombogen stages. Compared with other co-existing species in renal sacs, the prevalence of D. anteronucleatum sp. nov. was clearly far lower than that of others. The population density of individuals in the renal sac may be a trigger for the change from the asexual to sexual stage ( Lapan and Morowitz 1975). Whatever triggers the stage change are unclear, the situation with D. anteronucleatum sp. nov. indicates that it is probably species-specific.

Etymology. The species name is an adjective composed of 2 Latin roots, anterior and - nucleus meaning “forward” and “-nucleus”, in reference to the characteristic location of the axial cell nucleus.

Taxonomic summary. Type material: a syntype slide (NSMT-Me-50) collected at 28 March 2011; additional syntypes on slide series No. OL2594 (5 slides) in the author’s collection.

Type locality: off Nou (37°09′N, 137°54′E), Niigata Prefecture, Honshu, 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 Bay , Toyama Prefecture, Honshu, Japan, depth 350 m, 6 March 2003; No . OL1684 (5 slides) collected off Hamasaka (35°55′N, 134°25′E), Hyogo Prefecture, Honshu , the Sea of Japan, Japan, depth 200 m, 25 September 2006; No . OL2417 (5 slides) collected off Kanazawa (36°49′N, 136°17′E), Ishikawa Prefecture, Honshu , the Sea of Japan, Japan, depth 200 m, 15 February 2010; Nos . OL2470–2474 (each 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 . OL2511–2516, 2521–2525 (each 5 slides) collected off Karo (35°47′N, 134°14′E), Tottori Prefecture, Honshu , the Sea of Japan, Japan, depth 200 m, 15 March 2010 in the author’s collection .

Host: symbiotype, Octopus longispadiceus ( Sasaki, 1917) (Mollusca: Cephalopoda: Octopoda ), male (mature), 76 mm ML (NSMT-Mo-85865).

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

Prevalence: in 75 of 510 specimens of hosts examined (13.5%).

Figures

Fig. 8. Dicyemennea anteronucleatum sp. nov., photographs of syntype specimens on slide NSMT-Me-50: a, nematogen, entire; b–e, nematogen, anterior region; f, vermiform embryos within axial cell.Scale bars: 10µm in a–f.

Fig. 9. Dicyemennea anteronucleatum sp. nov., drawn from syntype specimens on slide NSMT-Me-50:a, b, nematogen, entire; c–f, nematogen, anterior region; g, h, vermiform embryo within axial cell, cilia omitted (g), optical section (h). Scale bars: 50µm in a, b, 10µm in c–h.