• Sediments;
  • distribution;
  • Lingula;
  • Japan;
  • Hawaii;
  • Moluccas.

Log-probability curves of grain-size distribution have been analysed of sediments collected in three previously described localities occupied by either Lingula anatina or L. reevei (Brachiopoda, Inarticulata). The grain-size population transported by saltation (average about 92–223 μm), generally associated with traction load population, determines Lingula distribution. The two preferential substrates of both studied species (density > 100 individuals/m-2) are compact and stable sediments under moderate water currents: either coarse sands and gravels clogged by fine and very fine sands, or fine, very fine and clay sands (saltation populations more than 60%). As soon as the suspension population and/or traction increase in the sediment to the detriment of the saltation population, the density of Lingula decreases rapidly. Nevertheless, if grain-size distribution and populations have a primordial function for Lingula, other ecological features (i.e. the environmental fauna, high occurrence of digging species, the available nutrients) will affect the distribution and sometimes become prevailing. The absence of important changes in the shell shape of Lingula since its origin suggests that the grain-size requirements remained about the same.