In spite of their name, Osmotic Eels are actually starfish-like organisms. They are small aquatic creatures that are used in medicine for cleaning and healing wounds. They can also be used to remove certain neural parasites.
The Osmotic Eel is apparently omnivorous. They can eat Vulcan root leaves - and presumably other kinds of floral matter - and they usually ingest small quantities of blood while healing an injured artery. According to Dr. Phlox, an Osmotic Eel only needs a nutrient broth every three or four days.