Diatoms are unicellular golden brown algae, that are characterized by an external box-like skeleton (or frustule) of opaline silica. Diatom frustules are commonly very intricate and varied, and the patterns and types of ornamentation of the frustule form the basis of nearly all diatom taxonomy. The size of diatom frustules ranges from less than one μm (micro-meter) to over 1000 μm, but most frustules occur in the 10–100 μm size range. Thus, diatoms are comparable in size to dinoflagellates but generally larger than calcareous nannofossils. Unlike calcareous nannoplankton and dinoflagellates, however, the vegetative cell of diatoms lacks flagella, although many diatoms produce flagellated gametes during their brief period of sexual reproduction. Consequently, planktonic diatoms are subject to passive current-related dispersal in the surface layers of the ocean. Adaptations that promote flotation include frustule shapes and processes which increase the ratio of surface area to volume, formation of colonies, and storage of fats or oils in the cell which reduce its overall specific gravity.