Named for its large, fan-shaped shell, which can reach up to 40 centimetres in length, this bivalve is also called the horse mussel because of its large size, though it is not a true mussel. It lives partially buried in the sand, with the broad end of its shell pointing upward. This posture allows it to filter water efficiently while keeping its delicate body hidden from predators. Strong byssus threads anchor it in place. These threads are thicker and stronger than those of true mussels, capable of holding the fan shell firmly in the sediment even during storms. The threads are made of a protein that is both flexible and incredibly strong, a natural composite material that scientists have studied for its potential applications in engineering. The fan shell's shell is thin and brittle, easily broken by waves or predators. To protect itself, the fan shell relies on its buried position and its strong anchoring threads. It does not need a thick shell because it is hidden in the sand. The shell is covered in a dark, velvety outer layer called the periostracum, which protects the underlying calcium carbonate from erosion. The Maori names Kupa and Hururoa reflect its large size and its habitat in sheltered bays and estuaries. Its large shell has been used as a tool and as a container. The strong byssus threads were sometimes used as fishing line, a testament to their strength and durability. Populations have declined in many estuaries due to coastal development and water pollution. Fan shells need clean water to feed. Sediment from land clearance smothers their feeding siphons. Pollution from urban runoff poisons their delicate gills. The fan shell is a sensitive species, a canary in the coal mine of our estuaries. When fan shells are thriving, the estuary is healthy. When they are gone, something has gone wrong.