eli5: How do fish know where to stay in the ocean? Or all species of fish all together in one big ocean? (e.g do hammerhead sharks and great white sharks stay away from each other or are all of them together and know no boundaries to their “territory”?)
In: Biology
Just like land animals, ocean animals live where their food lives, where temperature allows, at the right depth, and where their migration/breeding areas are.
Whales and great white sharks are both known to cover absolutely huge territories. Whales migrate thousands of miles up and down the oceans, and great whites often wander without a particular home.
Small animals, like fish in a coral reef, don’t travel very far. They have what they need nearby and they encounter lots of other fish who live the same way they do.
Depth is also important. Ocean animals can’t go into water that’s too deep or too shallow than the habitat they evolved in.
Temperature, pressure, light, salinity, availability of food-sources, vibration/sound, presence of predators, presence of shelter from predators…
Each species has its own unique combination of preferences for all of the environmental factors above. They go where it feels best for them to be.
/u/Exeter999’s explanation is excellent, but just to add to it: there *are* reef fishes that wander away from the reef in search of food, mates, whatever. Those fishes have ways of navigating back to the reef. They can sense temperature and ocean currents, remembering where and how those things change along their path away from the reef. They may also remember landmarks if there are any.
One of the most important tools, though, is sound. There is a family of shrimps commonly called [pistol shrimp](https://www.leisurepro.com/blog/wp-content/uploads/2014/02/Alpheidae-pistol-shrimp.jpg) or sometimes trigger shrimp or snapping shrimp. They’re pretty small, but one of their claws is truly amazing. The claws have special chambers that draw in water when they open up. The claw is surrounded by very powerful muscles and tight tendons, and can be “cocked” like the hammer on a gun. When the claw is “triggered” it slams shut and forces the water out with such force that it creates a cavitation bubble – an empty bubble where the water is blasted away. The bubble collapses with enough force to create light, and a very loud popping sound.
From experience working an aquarium shop, you can hear the snap of even a single young shrimp from outside the water, ten feet away, over the noise of many large aquarium pumps and an industrial AC unit. The sound can be well over 200 dB and can stun or even *kill* small fish. These shrimps live in their own individual burrows, but in a small reef the population can number in the *thousands*. For large reefs it may be tens or hundreds of thousands of pistol shrimps.
At night they come out of their burrows to search for food, and to defend their burrows from other pistol shrimps; for both activities they use their pistol claw. With thousands of shrimp all popping and snapping, it creates a cacophony that can be heard underwater for miles. Fish away from the reef use the noise to navigate, either towards the reef or just as a fixed point, like using a compass to point North. They always know which way the reef is.
Fun side fact: pistol shrimp have a mutualistic relationship with a family of goby fish called watchman gobies. They will pair up and share the burrow and food. The goby is faster and can search farther for food, and the shrimp will defend the goby from predators or rivals. [Look at this pair!](https://www.reed.edu/biology/courses/BIO342/2015_syllabus/2014_WEBSITES/sr_jc_website%202/images/goby_shrimp.jpg) Aren’t they just the cutest thing?
Fun fact #2: pistol shrimp, like most arthropods, can grow back limbs as they molt. If they lose their pistol claw, it’s not a problem for long. Their other, smaller claw will immediately start developing into a new pistol claw with each molt, and the claw that they lost will grow back as a new small claw.
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