Like Neptune's necklaces swaying to Pacific swells, ropes of greenshell mussels create intriguing (and nutritionally rewarding) alleyways for cruising fish at Port Charles, on the eastern tip of the Coromandel Peninsula. Here, and at hundreds of other coastal sites in the North and South Islands, greenshell mussels are grown in their millions to satisfy a worldwide demand for a unique shellfish. No other green mussel is available on international fish markets, and New Zealand's Perna canaliculus not only looks better, but, say aficianados, tastes better than other mussel varieties. From a fledgling industry in the 1970s (300 tonnes was the total harvest in 1977), mussel farming has become the country's dominant aquaculture industry, with a harvest of close to 50,000 tonnes in 1992. Despite a setback this summer, in which all harvesting was banned following a bloom of toxic algae, the country's mussel farmers are determined to see our local "heroes in a half shell" recognised as the world's best mussel.