Notes on Alexandrium population dynamics

We review within-year and between-year survival strategies of the meroplanktonic dinoflagellate Alexandrium, with special attention to the role of cyst beds and extended dormancy. Some of the constraints on the evolution of cyst bed dynamics are discussed in the framework of a model borrowed from desert seed ecology, in which Q, the annual germination rate, is selected by p, the probability that the vegetative phase will be successful on decadal time scales. Since Alexandrium, and the closely related Pyrodinium, undergo gametogenesis at relatively low cell concentrations, specialized traits must have evolved to achieve syngamy. It is suggested that motility and the use of chemical signals promote mating, and that the toxins act as pheromones. It is also proposed that toxins in cysts are used as signals to influence planozygote settlement so as to control dispersal of this stage, and ensure that cyst beds are sufficiently stocked to inoculate the water column adequately at the appro priate time of year.