Small cell and intermediate cell formation in species of Dinophysis (Dinophyceae, Dinophysiales)

Observations of two distinct size classes with similar shape in natural pop ulations of Dinophysis Ehrenberg were first reported by Jorgensen in 1923 a nd intermediate forms exhibiting a continuum between the typical vegetative cell and a putative small cell by Wood in 1954. Focused attention on Dinop hysis spp, associated with diarrhetic shellfish intoxications in the last d ecade has provided new examples of small cells ill the genus, sometimes wit h contours dissimilar from the corresponding vegetative cells; dimorphic in dividuals; and large/small cell couplets, This work was based on in situ ob servations during intensive sampling for cell cycle studies of Dinophysis a cuminata Claparede ct Lachmann, Dinophysis acuta Ehrenberg, Dinophysis caud ata Saville-Kent, and Dinophysis tripos Gourret; on laboratory incubations of D. acuminata; and on a thorough search of documented information on morp hological variability of Dinophysis spp, During ill situ division, most div iding cells exhibit a normal longitudinal fission, but some (1%-10%) underg o a "depauperating" fission, leading to pairs of dimorphic cells with dissi milar moieties, After separation and sulcal list regeneration, these dimorp hic cells become D. skagii Paulsen, D, dens Pavillard, D. diegensis Kofoid, and D, diegensis Kofoid var. curvata-like individuals, which can also be o bserved forming couplets D, acuminata/D. skagii, D. acuta/D. dens, and D. c audata/D. diegensis attached by their ventral margins. Small cells can grow again to large size, as shown in laboratory incubations of D, acuminata, t hus partly explaining observations of thecal intercalary bands, and interme diate forms, The sexual nature of the small cells will not be unequivocally demonstrated until controlled germination of the alleged cyst forms is ach ieved, and some intermediate forms may correspond to undescribed stages aft er cyst germination. These observations suggest common patterns in the life cycle of Dinophysis spp, Intraspecific morphological variability of Dinoph ysis spp, in a given geographic area can largely be attributed to small cel l 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 t o enlarge, leading to intermediate cell and further vegetative cell formati on as part of a three-looped life history pattern in Dinophysis.