Sentence:  Japanese fishermen have sometimes observed that sardines hauled up in their nets during a <WSD>tsunami</WSD> have enormously swollen stomachs ; the fish have swallowed vast numbers of bottom living diatoms , raised to the surface by the disturbance . 
Meaning of the word marked with <WSD> tags:  a cataclysm resulting from a destructive sea wave caused by an earthquake or volcanic eruption