Contribution to the Themed Section: Scaling from individual plankton to marine ecosystems HORIZONS Planktonic protists: little bugs pose big problems for biodiversity assessments

The view that current species inventories (lists of binomial species) of planktonic protists significantly underestimate true diversity is widespread. However, the existence of synonyms in species lists, due to repeated descriptions of the same form, and polymorphism in which different ``forms'' or life-history stages are but one species, are generally underappreciated. Recent studies have shown that synonyms are probably common, significantly inflating species lists, and taxa previously thought to be distinct species based on morphology have been revealed to be simple variants of single species. Given here are examples from among tintinnid ciliates, dinoflagellates and foraminifera. The commonality of synonyms and the phenomena of both polymorphism and crypticism underscore a need for taxonomic reviews as well as speciesfocused studies involving molecular biologists, taxonomists and ecologists