The emerald treasure of the rocky shore. The green-lipped mussel is a large, wedge-shaped bivalve with a smooth, dark brown to greenish-black shell. The inside of the shell is a pearly, iridescent blue-grey, and the edge of the shell is a distinctive bright green, hence the name. They attach themselves to rocks and other hard surfaces using strong, silky threads called byssus, better known as beards. These animals are the filter-feeders of the intertidal zone. They sit with their shells slightly open, drawing water in through their siphon, filtering out plankton and tiny particles of food, and expelling the clean water back out. A single adult mussel can filter several litres of water an hour, cleaning the sea and feeding itself at the same time. They live in dense, crowded beds, with thousands of individuals stacked on top of each other, all filtering away. This crowding provides protection from predators. An oystercatcher or starfish can only eat so many mussels before moving on. The ones in the middle of the bed survive. The life cycle begins with spawning. Females release eggs into the water, males release sperm, and fertilisation happens in the open sea. The resulting larvae drift with the currents for several weeks, feeding on plankton, before settling onto a hard surface and attaching permanently with their byssus threads. Once attached, they never move again. A mussel that picks the wrong spot dies. One that picks a spot with good water flow and plenty of food can live for decades, growing slowly, filtering water day and night. Green-lipped mussels are a Kiwi icon, both on the plate and in the water. They are farmed by the millions in the Marlborough Sounds, exported around the world and eaten steamed, fried, grilled or raw. They are also the source of a famous anti-inflammatory extract, used in supplements for arthritis and joint pain. To eat a green-lipped mussel is to eat a piece of New Zealand aquaculture. A green-edged, pearly-shelled, delicious bivalve that cleans the sea as it feeds.