Most low-latitude species of Monogenea produce large numbers of ciliated larvae. However, at high latitudes, species of the entirely viviparous family Gyrodactylidae, which produce few nonciliated offspring and are very rare at low latitudes, represent the majority of gill Monogenea, i.e., about 80–90% of all species at high northern latitudes, and about one third of all species in Antarctic and sub-Antarctic waters, against less than 1% in tropical waters. Data compiled by A.V. Gusev in 1978 indicates that Gyrodactylidae may also be more common in cold than tropical freshwater systems, suggesting that Thorson's rule may apply to freshwater invertebrates.