Most aquatic gastropods are benthic and mainly epifaunal but some are planktonic. A few such as the violet snails (Janthinidae) and the sea lizards (Glaucus) drift on the surface of the ocean where they feed on floating siphonophores, while others (heteropods and Gymnosomata) are active predators swimming in the plankton. Some snails (such as the whelk Syrinx aruanus) reach about 600 mm in length. There is also a very large (and poorly known) fauna of microgastropods that live in marine, freshwater and terrestrial environments. It is amongst these tiny snails (0.5-4 mm) where many of the undescribed species lie.