Visualization of particle movement near the inhalant siphon of pen shell <i>Atrina</i> spp. using a bivalve resin model

Pen shell Atrina spp. in the Sea of Ariake, have been reported to show a high survival rate when reared while suspended in the water column (e.g., using baskets) compared to the muddy bottom of its natural habitat. We inferred that Atrina spp. take in water with different properties, such as suspended sediment composition, depending on their vertical position in the water column. Such an inference would become more likely, if Atrina spp. can only take in a very thin layer of water near the inhalant opening regardless of their vertical position, as suggested from hydrodynamic studies of other bivalves. To test this hypothesis, a preliminary experiment was conducted by fixing an Atrina-shaped resin model in various vertical positions in a circular flow tank and recording the streamlines of water that entered the quasi-inhalant opening of the model. Results supported our hypothesis: the resin model, whether embedded in the sand at the bottom of the tank, positioned slightly protruding from the sand or suspended in the water column, took in a very thin layer of water near the inhalant opening (distance between the upper edge of inhalant opening and the top of the incoming water < 1.5 cm). This layer became thinner as the mid-layer current speed increased in the tank. Future research should investigate the water streamlines actually taken in by live Atrina spp. and the characteristics of the water such as concentration of particulates.