TSU, MIE PREF. – A creature known as “the scavenger of the deep” has surprised its keepers at an aquarium by “fasting” for more than four years.

The giant male isopod, called No. 1 to distinguish it from the two other giant isopods kept at Toba Aquarium in Mie Prefecture, last ate in January 2009, when it was fed a whole horse mackerel.

The isopod devoured the fish, bones and all, in just five minutes. But it has not eaten anything since, with Feb. 10 marking 1,500 days without food.

The crustacean was brought to the aquarium from the Gulf of Mexico in September 2007, measuring 29 cm and weighing 1 kg, and despite its lack of food still looks perfectly healthy, the keepers say.

“We have done all we possibly can,” said Takeya Moritaki, who is responsible for the creature.

Moritaki said he has tried an assortment of food items, including squid tentacles and saury, but the isopod has ignored them all.

Little is known about the biology of the creatures, which live in deep water in the Gulf of Mexico and in the Caribbean and which live off dead creatures and fish.

It remains a mystery why the isopod suddenly stopped eating and why it has remained healthy.

“Giant isopods are always in a state of semihibernation because they don’t know when they can eat, so they limit their energy on breathing and other activities,” said Taeko Kimura, a marine ecologist at Mie University.

“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.”

The isopods are kept in a tank that uses artificial seawater so it is highly unlikely to generate organic substances such as plankton and sea algae that No. 1, which is about the same size as its peers in the tank, can feed on.

The aquarium was previously home to a giant isopod that died of natural causes, despite having a healthy appetite, making No. 1’s good health even more mysterious.