Apium has more species than Elodea.
Apium (including celery and the marshworts) is a genus of about 20 species of flowering plants in the family Apiaceae, with a subcosmopolitan distribution in Europe, Asia, Africa, South America and Australia. 
Elodea is a genus of 6 species of aquatic plants often called the waterweeds described as a genus in 1803. 
