What makes the sea turn red and causes thousands of fish to die？As far back as anyone could remember，the blame was placed on the "red tides"．In 1947，scientists finally connected the red tides with a microscopic sea organism （有機(jī)體）（1）

called

called

（call） the dinoflagellate．
    The dinoflagellate is so tiny （2）

that

that

6，000 of these organisms may be contained in （3）

a

a

single drop of water．It stands on the borderline between plant and animal in its classification．It produces its own food，as plants do．But it moves （4）

freely

freely

（free） and eats other organisms，as animals do．
    Dinoflagellates are normally only one of the many kinds of organisms found in plankton．Plankton is the name given to all very small （5）

forms

forms

（form） of sea life．However，when the air and water（6）

are

are

（be） calm and warm，dinoflagellates multiply with （7）

amazing

amazing

（amaze） speed．The surface of the water appears （8）

to be covered

to be covered

（cover） with a red carpet．
    The dinoflagellates give off a poison．Many fish die．Their bodies are washed up on the beach．Beaches are not fit（9）

for

for

any use．Fish that are not killed may become （10）

poisonous

poisonous

（poison） to animals or people who eat them．Commercial fishing comes to a stop．
    As dinoflagellates consume all the food and oxygen in an area，they die．After a time，the sea returns to normal．But when conditions are right，the red tide comes again．