Definition: A taxon is Critically Endangered when the best available evidence indicates that it meets \r\nany of the criteria A to E for Critically Endangered, and it is therefore considered to be facing an extremely high risk of extinction in the wild
Europe: Black, Azov, Caspian and Aral Sea, ascending some rivers (Danube up to Bratislava, Volga up to Kazan, Ural up to Chkalov), unknown or very rare in others
Definition: Gravel is an environmental material which is composed of pieces of rock that are at least two millimeters (2mm) in its largest dimension and no more than 75 millimeters.
Comment: "Large" is ambiguous. For details on "Large rivers" (e.g. the Mekong river) see http://worldwildlife.org/biomes/large-river-ecosystems This class will be replaced with a less ambiguous class.
Definition: Anadromous fish live in the sea mostly, breed in fresh water; the best-known are salmon, which hatch in small freshwater streams, go down to the sea and live there for several years, then return to the same streams where they were hatched, spawn, and die shortly thereafter.
Definition: Solitary animals are those that spend a majority of their lives without others of their species, with possible exceptions for mating and raising their young. Antonyms for a solitary animal include a social animal or a colonial animal.
Definition: Lecithotrophic larvae are provided with a source of nutrition to use during their dispersal, usually a yolk sac. They generally have greater dispersal potential than direct developers, but short pelagic larval durations relative to planktotrophic larvae.
