The marimo is the aggregative form of the freshwater alga Aegagropila linnaei. In Japan, Lake Akan in Hokkaido is the best known habitat of the marimo. It is so emblematic of the lake and surrounding region that it has been designated as a special national monument. However, due to decades of deforestation and pollution, the marimo populations in Lake Akan have steeply dropped and have not recovered, despite a number of protective measures.