zooplankton -- Tiny, free-floating organisms in aquatic systems. Unlike phytoplankton, zooplankton cannot produce their own food, and so are consumers.
zooxanthellae -- Symbiotic dinoflagellates in the genus Symbiodinium that live in the tissues of a number of marine invertebrates and protists, notably in many foraminiferans, cnidarians, and some mollusks.
zygote -- The product of gamete fusion. In organisms with a haploid life cycle, the zygote immediately undergoes meiosis, but in organisms with a multicellular diploid stage, the zygote is merely the first stage in the diploid portion of the life cycle.