MOSCOW (AP) — Some Russian prisons might be mistaken for vacation destinations based on their nicknames, with animal appellations that include the Black Dolphin and the Polar Owl. But a hunger strike by jailed Russian opposition leader Alexei Navalny cast a spotlight on the fear and torment that critics say are the signatures of Russia’s prison system. Amid reports about his declining health, Navalny was transferred Sunday from a penal colony known for its particularly strict treatment of inmates to a hospital unit in another prison. Russia’s penal institutions house nearly 520,000 inmates, by far the largest number in Europe though a slightly smaller proportion of the general population than prisoners represent in Turkey. Most of the country’s prisons are collective colonies, a system dating back to the Soviet Gulag era, with inmates sleeping in dormitories and working in production facilities.