A giant isopod has been fasting for more than four years at an aquarium in Japan, according to a report by The Voice of Russia.

The male isopod called No. 1 has been kept at Toba Aquarium in Mie Prefecture, Japan, since its arrival from the Gulf of Mexico in 2007. The crustacean has not ingested any food since its last meal - a whole horse mackerel - in 2009.

On Feb. 10, 2013, the isopod marked the completion of 1,500 days without food. Keepers at the aquarium have tried all means to make the isopod eat. They have tried to provide him food items including squid tentacles and saury, but No. 1 has ignored them and still continues to be on fast.

Despite lack of food, the isopod remains to be healthy. But his keepers said that they are worried about his "weakened state."

Giant isopods (Bathynomus giganteus) are the largest known members of the isopod family. They spend most of their time scavenging the deep sea floor and are commonly known as "scavenger of the deep." They can survive without food for long periods of time.

It is not clear as to why No. 1 stopped eating food and how it has remained healthy for more than four years. But Taeko Kimura, a marine ecologist at Mie University, has a possible explanation. "Giant isopods are always in a state of semi-hibernation because they don't know when they can eat, so they limit their energy on breathing and other activities," Kimura told The Japan Times.

"For that purpose they sometimes keep a large amount of fat in their livers, so maybe No. 1 still has a source of energy in its body, and that's why it still has no appetite," he said.