Freshwater Eels (Anguilla spp.) have long been prized in many cultures as food, but have long proved impossible to breed in captivity or introduce to new habitats, because of their still-mysterious life history. The American Eel (Anguilla rostrata) lives in East and Gulf Coast rivers and estuaries as an immature 'yellow eel', undergoes metamorphosis to a non-feeding 'silver eel' and migrates to the open Atlantic. Recent research suggests that spawning occurs over the Mid-Atlantic Ridge, east of the Sargasso Sea. Actual spawning has still not been seen, but leaf-shaped transparent leptocephalus larvae are carried by currents to the Continental Shelf, where they metamorphose into 'glass eels' and then into black 'elvers', and migrate into brackish and fresh water, sometimes swimming hundreds of miles up rivers, and residing and growing for several years before metamorphosis. Deliberate releases of American Eels into San Francisco Bay tributaries were made by state and federal agencies were made from 1874 to 1882, before any of the details of this fish's life history were known. Subsequent scattered occurrences of American eels have resulted from the escape or release of eels from fish-markets or aquaria. American Eels have also been seen in surveys of ship's ballast water in San Francisco Bay. None of these releases have resulted in established populations.