Dicyema petalocephalum, 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 : 147-149

Dicyema petalocephalum sp. nov.

( Figs 4 , 5 ; Tables 1, 3)

Diagnosis. Medium dicyemid; body length reaching 2,400 µm. Calotte conical in shape. Vermiform stages with 22 peripheral cells: 4 propolar cells+4 metapolar cells+2 parapolar cells+12 trunk cells. Infusoriform embryos with 37 cells; refringent bodies solid; and nucleus present in each urn cell.

Description. Nematogens ( Figs 4a–c, e–g , 5a, c, e ). Body length 700–2,400 µm and width 40–50 µm; widest in region of parapolars; trunk width mostly uniform. Peripheral cell number 22 ( Table 3): 4 propolar cells+4 metapolar cells+2 parapolar cells+10 diapolar cells+2 uropolar cells. Calotte conical in shape, rounded anteriorly; cilia on calotte about 4 µm long, oriented anteriorly. Propolar cell masses conical in shape, metapolar cell masses a flattened ring shape ( Fig. 5c ). Propolar cells and their nuclei equal or smaller than metapolar cells and their nuclei. Propolar cells occupying anterior 60–70% of calotte length when viewed laterally ( Fig. 4b, c ). Cytoplasm of propolar and parapolar cells contains granules, more darkly stained by hematoxylin than cytoplasm of other peripheral cells. Anterior part of parapolar cell membranes flexible, deformed shape, projecting anteriorly, bifurcating ( Figs 4e–g , 5e ). Axial cell cylindrical, point- ed anteriorly; cell extending forward to base of propolar cells ( Fig. 4b, c ). About 35 vermiform embryos present per axial cell of large individuals. Accessory nuclei seen in trunk peripheral cells. Uropolar cells occasionally verruciform.

Vermiform embryos ( Figs 4d , 5g, h ). Full-grown vermiform embryos length 90–140 µm and width 10–16 µm. Peripheral cell number 22 ( Table 3); trunk cells arranged in opposed pairs. Anterior end of calotte acutely pointed. Axial cell rounded anteriorly, extending to base of propolar cells ( Figs 4d , 5g, h ). Axial cell of full-grown embryos with up to 4 agametes.

Rhombogens ( Figs 4h , 5d ). Body similar in length to nematogens, 700–1,900 µm in length, and width 40–55 µm. Peripheral cell number typically 22 ( Table 3). Calotte shape, axial cell shape, and anterior extent similar to those of nematogens. A maximum of 2 infusorigens present in axial cell of each parent individual. About 50 infusoriform embryos present per axial cell of large individuals.

Infusorigens ( Figs 4i , 5i; n =20). Mature infusorigens medium-sized, composed of 15–21(mode 18) external cells (oogonia and primary oocytes)+4–7 (mode 5) internal cells (spermatogonia, primary spermatocytes, and secondary spermatocytes)+4–7 (mode 4) spermatozoa. Mean diameter of fertilized eggs 12.3 µm; that of spermatozoa 2.2 µm. Axial cell ovoid or round, diameter 12–24 µm.

Infusoriform embryos ( Figs 4j, k , 5j–l; n =50). Full-grown embryos large, length 27.5±2.1 µm (mean±SD, excluding cilia); length-width-height ratio 1.0: 0.82: 0.76; ovoid, bluntly rounded to pointed posteriorly; cilia at posterior end 7 µm long. Refringent bodies present, solid, occupying anterior 30–40% of embryo length when viewed laterally ( Fig. 4j ). Cilia projected from ventral internal cells into urn cavity ( Fig. 5l ). Capsule cells contain small granules ( Fig. 5l ). 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 single nucleus and single germinal cell ( Fig. 5l ). All somatic nuclei pycnotic in mature infusoriform embryos.

Remarks. Dicyema petalocephalum sp. nov. and D. cryptocephalum sp. nov. were found together in the same host specimens ( Table 1). Both two new species share a common feature: bifurcated projections within the parapolar cells. However, D. petalocephalum sp. nov. is easily distinguished from D. cryptocephalum sp. nov. by the number of peripheral cells in vermiform stages (22 vs. 15–17) and the calotte shape in vermiform embryos (acutely pointed vs. rounded). Bifurcated projections in parapolar cells are favorable for fitting flexibly into the surface structures of renal appendages.

Vermiform embryos of D. petalocephalum sp. nov. have an acutely pointed calotte. In the genus Dicyema , such a particular vermiform embryo has been reported in D. oxycephalum Furuya, 2009 , from Sepia longipes Sasaki, 1913 , in Japan ( Furuya 2009). However, D. petalocephalum sp. nov. differs from D. oxycephalum in the number of peripheral cells in vermiform stages (22 vs. 28–34) and cells of infusoriform embryos (37 vs. 39).

Etymology. The species name is an adjective composed of 2 Greek roots, pétalo and - kephalos, meaning “petal” and “-headed” in reference to the characteristic anterior part with petal-like projections.

Taxonomic summary. Type material: a syntype slide (NSMT-Me-48) collected at 26 February 2014; additional syntypes on slide series No. OL3196 (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 Nos . OL870, 871 (each 5 slides) collected off Iwase (36°48′N, 137°15′E), Toyama Bay , Toyama Prefecture, Honshu, Japan, depth 350 m, 6 March 2003; Nos . OL2153–2156 (each 5 slides) collected off Karo (35°47′N, 134°14′E), Tottori Prefecture, Honshu , the Sea of Japan, Japan, depth 200 m, 28 January 2009; Nos . OL2364–2368, 2379–2381 (each 5 slides) collect- ed off Ohda-shi (35°23′N, 132°19′E), Shimane Prefecture, Honshu , the Sea of Japan, Japan, depth 200 m, 28 January 2010 in the author’s collection .

Host: symbiotype, Octopus longispadiceus ( Sasaki, 1917) (Mollusca: Cephalopoda: Octopoda ), female (mature), 52 mm ML (NSMT-Mo-85866).

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

Prevalence: in 39 of 510 specimens of hosts examined (7.0%).

Figures

Fig. 4. Dicyema petalocephalum sp. nov., photographs of syntype specimens on slide NSMT-Me-48: a, nematogen, entire; b, c, nematogen, anterior region; d, vermiform embryos within axial cell; e–g, nematogen, anterior region; h, rhombogen, anterior region; i, infusorigen; j, k, infusoriform embryos, sagittal section (j), horizontal section (k). Scale bars: 50µm in a, 5µm in b– k.

Fig. 5. Dicyema petalocephalum sp. nov., drawn from syntype specimens on slide NSMT-Me-48: a, nematogen, entire; b, rhombogen, entire; c, d, nematogen, anterior region; e, f, rhombogen, anterior region; g, h, vermiform embryo within axial cell, cilia omitted (g), optical section (h); i, infusorigen; j–l, infusoriform embryos, dorsal view (j; cilia omitted), ventral view (k; cilia omitted), sagittal section (l). Scale bars: 200µm in a, b, 10µm in c–k.