Dispersal and biogeography of silica-scaled chrysophytes

The silica-scaled chrysophytes-here mainly represented by the freshwater genera Mallomonas and Synura-have special problems in dispersal from one habitat to another because they cannot tolerate desiccation. Their dispersal is limited by the fragile construction and aquatic habit. Dispersal from one water body to another involves dangerous changes of the environment, and the ability to avoid desiccation during transport is crucial. So, air-borne and ectozoic dispersal by birds or mammals can only work at short distances. This danger may be avoided by endozoic dispersal of thick-walled cysts; as far as they can tolerate the digestion fluids in the intestine. In spite of these difficulties, Chrysophytes have been dispersed worldwide, but they display various distinct distribution patterns, e.g., cosmopolitan, arctic-northern temperate, bipolar, and tropical. Quite a large proportion may be considered endemic, occurring only within a restricted area. Even if the exact dispersal methods are elusive, the distribution of chrysophytes around the world proves their ability for dispersal. On the other hand, the different degree of distribution shows the varying success of the individual species. The distribution of a species at a given time depends on several factors: dispersal capacity-available vectors-suitable available habitats-and most important: sufficient time for dispersal. It is remarkable that the chrysophytes-in spite of their fragile cell construction and apparently low dispersal capacity-show distribution types comparable to those found in, e.g., blue-greens and desmids, whose cell construction appears much better adapted for dispersal.