At first glance, you might think the situation in the above photo looks dire for this cleaner shrimp, which has ventured into the mouth of the giant eel, a voracious predator in its coral reef habitat. With one snap of its jaw, the eel could easily crush the shrimp and swallow it. However, the shrimp is not in danger of becoming this eel’s dinner. The much larger animal remains still, with its mouth open, and allows the smaller shrimp free passage as it picks out and eats tiny parasites living inside the eel’s mouth and on its skin. In this interaction, both organisms benefit: The cleaner shrimp gains access to a supply of food, and its eel client is freed of parasites that might weaken it or spread disease. There are many other examples of such mutually beneficial “cleaner” and “client” relationships in marine habitats. However, other interactions between species are less benign for one of the participants, and still other interactions can negatively affect the reproduction and survival of both species involved.
Urry, Lisa A.. Campbell Biology. Pearson Education. Kindle Edition. https://www.pearson.com/us/higher-education/series/Campbell-Biology-Series/2244849.html