Large numbers of the tube‐building polychaeteDiopatra leuckartiKinberg form extensive mounds of sand on a Hawaiian reef‐fiat, providing a stable environment with plentiful resources to support a diverse invertebrate community. The vertically oriented tubes reach densities of 21 800 m‐2and the projecting tubes trap drift algae and sediments which are sources of food and tube‐building materials for the associated community. There are 22 polychaete species, six crustacean species and eight of other taxa represented, withLeptochelia dubia(tanaid) andCapitella capitala(capitellid) as the dominant organisms, reaching densities of 32 800 m‐2and 11600 m‐2, respectively. Both species form fine mud tubes on the sediment surface among theDiopatratubes. Other numerically abundant taxa are the deposit feeders comprising orbiniid, capitellid and cirratulid polychaetes, small crustaceans and a holothurian. Crustaceans, fishes and polychaetes are the primary predators. Both low salinities (15–26%, at low tides) due to ground water seepage and fine sediments are probably major factors affecting the species composition of theDiopatramounds. They also act as a buffer between the reef‐flat and the beach, serving to reduc