The Great Barrier Reef -- a wonderland in a shallow sea

If you like the beach here, perhaps you should thank the parrotfish. He is easy enough to find. He's that bluish-purple fellow with the beaklike teeth who seems to be eating the coral.

In fact, though, he is scraping it. It's in the coral that he finds the algae on which he feeds.

So what does that have to do with the beach? In taking the algae, the parrotfish also takes part of the skeleton of the coral itself; you'll find the scrape marks all over the reef. He digests the algae, but the coral later passes out of his body as a fine sediment . . . sand.

One parrotfish chews on so much coral yearly that he makes some 30 pounds of sand. Multiply that by the number of parrotfish - they are among the most numerous of the species here - and you can pretty well credit them as builders of the beaches.

Still, fine as their handiwork is, it is not the sand, gleaming invitingly in the Tropic of Capricorn sun, that draws most people to this area. It's the reef. The reef is one of the seven natural wonders of the world.

Australia's Great Barrier Reef stretches 1,250 miles, not in one long unbroken chain, but in a series of them, interspersed with shoals, cays, and islands, all formed of coral. It buffers the northeast coast of the continent from the thundering waves rolled toward it when the southeast trade winds blow on the Pacific Ocean.

The reef was officially discovered by Capt. James Cook in 1770, when his ship , Endeavor, foundered on it. (His isn't the only ship to have run aground here. More than 500 have done so since, including, in 1791, the HMS Pandora, which was bound for England from Tahiti with 14 of the Bounty's mutineers aboard.)

Wavy coral, staghorn coral, brain coral, soft coral - the conditions are just right for their growth: water that's 68 degrees F. or warmer and seas shallow enough to allow sunlight to penetrate.

These same conditions make it just right for viewing.

Heron Island is just one of the islands within the Great Barrier Reef area that has been developed for tourism. There is a motel-style resort here, plus some additional lodges; in all there are accommodations for about 200 people. You can rent diving equipment (be sure to bring your proficiency certificate) or you can snorkel. Rides in glass-bottomed boats are available, too.

When you first see it, you will wonder that anyone at all could shelter here. Heron Island is only 40 acres in area - 300 yards across and 11/4 miles in circumference. Even so, it is temporary home not only to tourists but to touring birds by the thousands, including the reef heron, for which it is named; various terns; brown boobies; red-footed boobies; ospreys; lesser frigates; and wedge-tailed shearwaters (muttonbirds).

The latter burrow near the roots of the pisonia trees that shade the island, and they hold nightly concerts of raucous calls. Be careful on any walks that you don't stumble into their nests.

You can walk around the island in little more than half an hour if exercise is what you have in mind. But mostly you'll ramble here, maybe stopping at the marine science research facility that shares the island with the resort or to view the birds in their fishing forays or to admire a beach morning glory.

If you come between October and April and like to do your rambling at night (bring a flashlight), you can watch the green turtles come ashore to lay their eggs. The 500-pound reptiles proceed up the beach with a kind of sand-slowed breast stroke. When they find a spot that suits them, they scoop out a hole with their front flippers, turn around, drop 20 to 50 ping-pong-ball-size eggs in it, cover the eggs with sand propelled by a few swishes of their powerful back flippers, and lumber back into the water.

It's hard to believe, watching them on land, that in the water marine turtles are graceful creatures.

On any walk here you'll want firm-soled footgear - tennis shoes or rubber boots; some of the shore is covered with white coral sand, but much of it is beachrock (cemented sand and debris) and very sharp coral.

It's good to cover your feet in the water, too, since there you will find even more coral.

The coral of the Great Barrier Reef comes in an artist's palette of colors - blue, fuschia, purple, pink. And the fishes it harbors look as if they have been painted specially for exhibit: The clown fish is orange with a white face. With the simple addition of a red bulbous nose, it could perform for Ringling Bros. and Barnum & Bailey. The angelfish has blue and gold bands. The harlequin tuskfish is red striped, and the moorish idol adds splashes of yellow, black, and white to this watery canvas.

The fishes aren't interesting only for their color, though. They seem to come in an infinite variety of shapes and characteristics. The trumpet fish, long and lean, vacuums up its prey. The butterfly fish has an ''eye'' spot near the tail meant to fool predators into grabbing for the wrong end. Painted sweetlips, with orange spots and a protruding mouth - it's obvious how it got its name. The damselfish is tiny but pugnacious. It will come right up and stare into your face mask. If it were any bigger, you'd have to be scared. You are scared of the porcupine fish - ouch! No problem figuring out how it got its name, either.

There are more than 1,000 varieties of fishes to be seen here, and for most you don't have to be a deep diver. A snorkel mask and a gentle patrol of the waters just offshore and the neighboring reefs and islands will be enough. Or there are the glass-bottomed boats (better than nothing, but not as good as snorkeling or diving).

Sometimes the tidepools on the eastern side of the island elicit treasures, as well. The tide recedes with a swirl of foam, leaving behind sea anemones and mollusks and tiny fishes and feathery underwater plant life, all awaiting discovery. (They are treasures for the eye only; the reef is part of Australia's National Heritage and protected by the Great Barrier Reef Marine Park Act.)

Heron Island is 45 miles east of the mainland Australia city of Gladstone, at the southern end of the Great Barrier Reef. A ride in a four- or six-passenger helicopter will bring you here in about 40 minutes.

Don't think of it as 40 wasted minutes, though. One of the best ways to see the Great Barrier Reef is to fly over it, for the water washing the coral here is constantly moving, making a kaleidoscope pattern, rushing away, then rushing back to make another one.

The wave action has a sandpaper effect on the reef, breaking off pieces of coral and pounding them into sand. On second thought, perhaps you don't have to thank the parrotfish for the beach you find here. All you have to do is enjoy it. Practical information

Trans-Australia and Ansett, Australia's internal airlines, fly to Gladstone from Brisbane.