MOSCOW (AP) — Some Russian prisons might be mistaken for vacation destinations based on their nicknames, which include animal appellations such as the Black Dolphin and the Polar Owl. But a hunger strike by jailed Russian opposition leader Alexei Navalny has 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. Most operate under the collective colony system that dates back to the Soviet Gulag era.