A few years ago I waded through glassy shallows off Mactan Island, a Philippine map-speck little more than an hour's flight southeast of Manila. The sun was searing that afternoon, although the knee-deep water was kind enough to the skin. Slow-motion steps eased me across a gaudy bottom, sneakers carefully avoiding coral heads, sea urchins, anemones, sponge growths, starfish. Trailing like mermaid tresses, algal strands wound around and between my bare legs.

Four centuries earlier, Ferdinand Magellan had sloshed shoreward through these same biotic shallows, suddenly to be ambushed and slain by hostile natives. Today, only a stone's throw inland, a decaying monument stands to remind the very occasional visitor of the fallen circumnavigator's achievement.

Accompanying me was Evaristo Zambo, from nearby Cebu, who makes a living collecting fancy seashells for export to the souvenir markets of America and Europe. His business prospers, for the Indo-Pacific area in general and the Philippine archipelago in particular comprise the world's most bountiful mollusc habitats, with denizens ranging from tiny cockles and exquisitely patterned cowries, to giant clams of a quarter-ton.