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Many instances of endosymbiosis are obligate ; that is , either the endosymbiont or the host cannot survive without the other , such as the gutless marine worms of the genus " Riftia " , which get nutrition from their endosymbiotic bacteria .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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