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I'M going to the pet store," my sister said. "Do you need anything for Pepper?"

I checked my pampered canary's supplies. "He needs a new cuttlebone."

"A what?"

"Cuttlebone."

"What on earth is a cuttlebone?"

"It's the bone of a cuttlefish. Well, not a bone really. It's a shell -- but on the inside."

"And a cuttlefish is ...?"

Nuts. I knew she was going to ask that. And I didn't have a good answer.

Cuttlefish are tough to describe because they look and behave like creatures from another planet. Even their name is hard to explain because they are not a fish and aren't related to fish. Because of this, biologists today are calling these animals cuttles, rather than cuttlefish.

Like their closest relatives, the squids, cuttles have eight short arms and two long, slender arms. Cuttle bodies, however, are shorter, broader and flatter than squids. Also, cuttles have delicate fins running completely around the edge of the body, waving in the water like frilly skirts.

INSIDE a cuttle, just under the upper surface, lies its shell, which serves as both skeleton and buoyancy tank. The calcium carbonate shell consists of about 100 separate inner chambers. Blood circulates through the lower chambers, and air fills the upper chambers. This trapped air is what makes cuttles able to hover effortlessly in midwater.

Smaller chambers in the rear of the shell are mostly filled with fluid and determine the cuttle's orientation. When these chambers lose fluid, the rear end of the cuttle rises, thus tilting its head down. This position is handy for hunting crabs and shrimp on the ocean floor.

A cuttle adjusts the amount of air and fluid in its shell to suit its needs. Some species sink to the sand and bury themselves during the day, then emerge at night to hunt for fish and shelled animals.

When cuttles die, their shells float to the surface and sometimes drift ashore. This doesn't happen often in Hawaii because few cuttles live here. One species lives in Kaneohe Bay, but it's extremely hard to find. Besides being nocturnal, it can match the bottom so precisely, it's practically invisible.

ALTHOUGH squids and octopuses can also change their colors, the cuttles' color variations are the most dazzling. A striped cuttle can be a spotted cuttle in a split second. The creatures can turn yellow, white, brown or blue so fast, the colors pass over the body like ripples of water.

When a cuttle spots a potential meal, such as a shrimp, brilliant colors shimmer over its entire body. This may distract the prey. Then, when the cuttle is within range, it shoots its two long tentacles out, pins the shrimp to the sand, then draws it up to the sharp beak that is the cuttle's mouth.

The color variations are especially striking during the mating season. A male might flash a calming courtship pattern to a female on one side of his body and, at the same time, display a strong warning pattern to rival males on its other side.

After mating, the female lays one egg at a time, cementing each to a plant stem or other solid object. Female cuttles leave the eggs unattended to develop and hatch on their own.

You can see these amazing creatures at the Waikiki Aquarium. Be sure to spend some time at the cuttle tanks to watch their amazing color shows.

Last week, I did just that. Now the cuttlebone, a calcium supplement and beak sharpening device hanging in my canary's cage, is as interesting as the canary itself.