Small Cell and Intermediate Cell Formation in Species Ofdinophysis(dinophyceae, Dinophysiales)

Observations of two distinct size classes with similar shape in natural populations ofDinophysisEhrenberg were first reported by Jorgensen in 1923 and intermediate forms exhibiting a continuum between the typical vegetative cell and a putative small cell by Wood in 1954. Focused attention onDinophysisspp. associated with diarrhetic shellfish intoxications in the last decade has provided new examples of small cells in the genus, sometimes with contours dissimilar from the corresponding vegetative cells; dimorphic individuals; and large/small cell couplets. This work was based onin situobservations during intensive sampling for cell cycle studies ofDinophysis acuminataClaparéde et Lachmann,Dinophysis acutaEhrenberg,Dinophysis caudataSaville‐Kent, andDinophysis triposGourret; on laboratory incubations ofD. acuminata;and on a thorough search of documented information on morphological variability ofDinophysisspp. Duringin situdivision, most dividing cells exhibit a normal longitudinal fission, but some (1%–10%) undergo a “depauperating” fission, leading to pairs of dimorphic cells with dissimilar moieties. After separation and sulcal list regeneration, these dimorphic cells becomeD. skagiiPaulsen,D. densPavillard,D. diegensisKofoid, andD. diegensisKofoid var.curvata‐like individuals, which can also be observed forming coupletsD. acuminata/D. skagii,D. acuta/D. dens, andD. caudata/D. diegensisattached by their ventral margins. Small cells can grow again to large size, as shown in laboratory incubations ofD. acuminata, thus partly explaining observations of thecal intercalary bands, and intermediate forms. The sexual nature of the small cells will not be unequivocally demonstrated until controlled germination of the alleged cyst forms is achieved, and some intermediate forms may correspond to undescribed stages after cyst germination. These observations suggest common patterns in the life cycle ofDinophysisspp. Intraspecific morphological variability ofDinophysisspp. in a given geographic area can largely be attributed to small cell formation, as a response to changing environmental conditions, and may be a part of the sexual cycle of these species. Small cells seem to be able to enlarge, leading to intermediate cell and further vegetative cell formation as part of a three‐looped life history pattern inDinophysis.