EFFECTS OF TEMPERATURE CONDITIONING ON DEVELOPMENT AND GERMINATION OFGONYAULAX TAMARENSIS(DINOPHYCEAE) HYPNOZYGOTES1

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Abstract
Plankton samples from a dense bloom of the toxic red tide dinoflagellateGonyaulax tamarensisLebour were incubated in the laboratory to study the formation and development of thick‐walled, overwintering cysts. Samples, contained very few cysts, and fusing cells were seen only twice. However, thousands of elongate cysts were formed from large posteriorly biflagellated cells (presumed to be planozygotes) that comprised approximately 50% of the initial motile population. Development of these new cysts (hypnozygotes) was studied under different storage conditions. Each hypnozygote lost pigmentation and formed a thick cell wall during the first several days of storage, producing a starch‐filled cell with a yellow accumulation body. Starch reserves decreased rapidly during storage at warm (22 C) temperatures and more slowly at 5 C (with no apparent effect from light, dark, or added nutrients). Excretion of mucilaginous material was also observed. Periodic germination experiments showed that 22 C temperatures led to rapid development of the hypnozygote, with germination possible 1 mo after formation, whereas 5 C storage retarded development, with the first revivals after nearly 4 mo. Excystment was initiated only after an applied stimulus—either a temperature increase or decrease depending on the previous storage temperature. Cysts formed during the late spring in temperate waters are thus capable of excystment several months later as fall temperatures decrease; those formed in the fall take longer to mature, but are viable after six months of overwintering. Alternating spring and fall blooms seeded by germinating populations of dormant cysts are thus possible, a finding consistent with recent field observations.