How many hydrozoan species are there?

A list of all living hydrozoan species was compiled and analysed. The number of supposedly valid species was found to be around 3200. Considerable uncertainties exist due to the possibly high number of invalid nominal species. The total number of existing hydrozoan species cannot be estimated because the accumulation curve for new species shows no sign of convergence. Only in the Thecata, Narcomedusae and Trachymedusae is an estimate of the maximal number feasible. However, application of new methods, notably a molecular systematic approach, could drastically alter these predictions.