Sentence:  Japanese fishermen have sometimes observed that sardines hauled up in their nets during a tsunami have enormously swollen stomachs ; the fish have swallowed vast numbers of bottom living <WSD>diatoms</WSD> , raised to the surface by the disturbance . 
Meaning of the word marked with <WSD> tags:  microscopic unicellular marine or freshwater colonial alga having cell walls impregnated with silica