For Myrtle the turtle, Snaggletooth the shark and a lot of other marine creatures accustomed to better things, yesterday was moving.

And for the New England Aquarium it was a major undertaking: moving its large collection of sealife into temporary holding tanks so that the main aquarium, which has mysteriously cracked, can be repaired. A delicate task - one slip and it could have been fish and chips.

The job involved rounding up a lot of big and/or mean and/or slippery sea animals, hoisting them out of the acquarium with a crane, loading them onto stretchrs, taking the elevator down four floors and dumping them into the holding tanks.

By day's end, Myrtle, a 462-pound green sea turtle, was resting resignedly in her tank; Snaggletooth, a 9 1/2-foot sand tiger shark, was glowering unhappily from his. Today officials will put a tranquilizing chemical into the aquarium's water and remove the smaller fish with nets.

"Our biggest problem is making sure no one gets hurt - people or fish," said aquarium curator Louis Garibaldi.

"You want to handle them with great care so you don't damage their scales," said John Prescott, executive director of the aquarium on Boston's waterfront.

The unusual circular tank, the largest in the world and considered one of the finest for viewing aquatic life, measures 50 feet in diameter and holds about 250,000 gallons of water.

The unexplained cracks, spiderwebbing through one of the 67 glass panels near the bottom of the tank, was noticed New Year's Day. Vandalism is suspected, and repairs are expected to cost $40,000.

The transfer of Myrtle, Snaggletooth and the other "big guys" was carried out by a team of divers working much like cowhands at a roundup. "Many of the animals are complacent and they don't expect to be grabbed. Hell, if you were a 9-foot shark, you'd be pretty surprised if someone grabbed you around the waist," said Garibaldi.

Head diver George Nally coaxed Snaggletooth over to him with some food, grabbing the 166-pound fish by the head to keep its jaws shut white a second diver held the tail.

"We almost lost him a couple of times and he started coming back at us . . . I just had to hold him in a hammerlock real hard and let him fight all he wanted until he tired out," Nally said.

Myrtle, on the other hand, "was very well-behaved, just excellent," he said. "She just gave up totally."

After the divers corraled them, the larger fish and sea turtles were hoisted over the edge of the tank by an electric crane to a waiting stretcher. While the animals were brought to the first floor by elevator, the divers raced down the four flights of stairs to help unlod them into a tray surrounding the giant ocean tank - all within three to five minutes.

Said Nally of Snaggletoogh, the first hand-fed shark in the country. "He was totally shocked. Now he's just lying on the bottom down there wondering, What did I do?"