Cochlodinium polykrikoides Margalef, 1961
Species Overview:
Cochlodinium polykrikoides is an unarmoured, marine, planktonic dinoflagellate species with a distinctive spiral-shaped cingulum. It is a common red tide former associated with fish kills in Japan and Korea.
Taxonomic Description:
Cochlodinium polykrikoides is an athecate species; i.e. without thecal plates. Cells are small, oval and slightly flattened dorso-ventrally (Figs. 1-3). Chains, rarely more than eight cells, are common (Figs. 1-3). An apical groove is present on the apex originating from the anterior end of the cingular and sulcal juncture and extending to the dorsal side of the epitheca. Cells range in size from 30-40 µm in length to 20-30 µm in width (Silva, 1967, Yuki and Yoshimatsu, 1989, Fukuyo et al., 1990, Taylor et al., 1995, Steidinger and Tangen, 1996).
The epitheca is conical and rounded at the apex (Figs. 1-3). The hypotheca is bilobed (Fig. 3). The cingulum is deep and excavated (Figs. 1-3). It is displaced about 0.6 times the cell length, and descends in a distinct left-handed spiral of 1.8-1.9 turns around the cell. The narrow and shallow sulcus nearly runs parallel to the cingulum making 0.8-0.9 turns around the cell between the proximal and distal ends of the cingulum. The sulcus deepens and widens towards the antapex and divides the hypotheca into two asymmetrical lobes (Fig. 3). The right lobe is narrower and slightly longer than the left lobe (Silva, 1967, Yuki and Yoshimatsu, 1989, Fukuyo et al., 1990, Taylor et al., 1995, Steidinger and Tangen, 1996). Trichocysts have been observed in this species, but the number per cell varies, and not all cells bear them. The presence and number of trichocysts increases with cell and culture age (Silva, 1967).
Morphology and Structure:
C. polykrikoides is a photosynthetic species with numerous yellowish-green to brown chloroplasts, rod-shaped or ellipsoid in shape (Fig. 3). The nucleus is situated anteriorly in the epitheca (Figs. 1,2). A red stigma is present dorsally in the epitheca (Silva, 1967, Yuki and Yoshimatsu, 1989, Fukuyo et al., 1990, Taylor et al., 1995).
Reproduction:
C. polykrikoides reproduces asexually by binary fission; plane of fission is oblique (Silva, 1967).
Species Comparisons:
C. polykrikoides closely resembles two other Cochlodinium species: C. helix and C. helicoides. The degree of rotation of the cingulum and sulcus distinguish the former species from the latter two: a. the cingulum in C. polykrikoides makes 1.8-1.9 turns around the cell, while in C. helix it is two turns, and in C. helicoides it is 1.5 turns; and b. the sulcus turns 0.8 times between the proximal and distal ends of the cingulum in C. polykrikoides, whereas it is one time in C. helix and 0.6 times in C. helicoides (Silva, 1967).