Efficient crushing of hard benthic diatoms in the gut of the Manila clam Ruditapes philippinarum – Experimental and observational evidence

Clarification of whether suspension-feeding infaunal bivalves are able to utilize benthic diatoms as food is essential for understanding coastal food webs. Although many benthic diatoms possess hard thickly siliceous cell walls (frustules), no study has examined if bivalves can effectively break their frustules to digest and assimilate the cell contents. In the present study, feeding rates and digestion efficiencies of the Manila clam fed on three benthic and a planktonic diatom species with different mechanical strengths (hardness) of frustules were compared to determine if the clam can efficiently break benthic diatoms with relatively hard frustules. Fragments of diatom frustules in the gut were observed using a scanning electron microscope . Although digestion efficiencies of the Manila clam were significantly different among the diatom species, the efficiencies were all above 90% for all four diatom species indicating that the Manila clam can efficiently crush hard diatom frustules of benthic species. The mechanism for crushing diatom cells in the gut appears to rely on the density of diatom cells ingested into the clam's stomach. By the rotation of the crystalline style, densely packed diatom cells in the stomach are agitated leading to attrition with each other, and subsequently the frustules are ruptured. The results of the present study indicate that when benthic diatoms are suspended in the water column the Manila clam can effectively utilize benthic diatoms as a major food source irrespective of the hardness of the diatom's frustules.