6349
On the other hand , the river is also under attack from hardy , competitive , alien critters , such as the European green crab , which is considered to be one of the world 's ten worst invasive species , and is present in the river .
 None
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 None