Twait shad are silvery, sometimes with a bronzy tinge, and there are a distinctive row of six to ten large dark spot just behind the gill cover though these may fade when the fish is dead. The adult length is typically . The twait shad is found in most of Europe and all countries bordering the Mediterranean Sea. "Alosa fallax" has a similar lifecycle to "A. alosa" (allis shad). They are known to live in sympatry. Some studies have suggested "A. fallax" and "A. alosa" species can hybridize. They are anadromous species, like many other species in the genus "Alosa". However, some
Subterranean river Subterranean river A subterranean river is a river that runs wholly or partly beneath the ground surface – one where the riverbed does not represent the surface of the Earth (rivers flowing in gorges are not classed as subterranean). It should also not be confused with an aquifer which may flow like a river but is contained within a permeable layer of rock or other unconsolidated materials. Subterranean rivers may be entirely natural, flowing through cave systems. In karst topography, rivers may disappear through sinkholes, continuing underground. In some cases, they may emerge into daylight further downstream. Some fish (popularly