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.

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