Question: why do some animals get along but others don't? like sharks let some shrimp in their mouth and all over them to clean them, but will eat another in a second. and how do these mutual relationships form over time without them being able to communicate

Ella Mercer answered on 21 Apr 2020:

You got the key phrase – mutual relationships! Mutualism is one of three lifelong animal relationships that can form between animals. Mutualism is where both animals benefit – like your shark/shrimp example. The shrimp will go into the sharks mouth and ‘eat’ dead skin (getting a meal) whilst the shark gets its teeth cleaned. Another example in the ocean is the clownfish (nemo) and the sea anemone: nemo has shelter and food from the anemone and the anemone benefits because nemo will clean it and also provide food. I think one of the reasons some animals have this relationship and others don’t is due to the specific qualities (or traits) of the animals, for example, some shrimp aren’t very good at cleaning so wouldn’t dare go near a sharks mouth. Like you say, how does the shark know not to eat the cleaner shrimp? As evolution has occurred, species have become more dependant on each other (so the shark actually needs the cleaner shrimp to clean its teeth) and scientists think that this is due to natural selection…

There’s a good explanation of ‘How Stuff Works’ (https://science.howstuffworks.com/life/evolution/symbiosis.htm#pt2) “Over many generations, the organisms came to depend more on the symbiosis because natural selection favored those traits and not others. Eventually, the symbiosis became the sole source of the food, shelter, enzyme or whatever else the symbiotes derived from one another.”

I hope that’s at least started to answer your question!