I know dolphins and whales have sonar. But how about fish, jellyfish, giant squid, sea turtles etc? They have no visual points of reference like us land animals, they can literally travel in any direction (vs. our fairly linear plane of movement; although I’m assuming at least that they can sense gravity and therefore know up from down), and yet can navigate so confidently that, for example, animals like sea turtles can return without fail to the same location year after year to lay eggs.
In: Biology
Have you ever seen a fish swim upside down? (Probably rarely). Most fish have buoyancy that favors their upright position that defines the orientation of how they perceive space. Under water is not a zero g environment (as you said). And they do have visual points of reference, they just aren’t grounded to a certain vertical plane.
Most of them still use vision and touch (feeling currents…. like how you can feel a draft of air when someone goes by) for navigation in the near field. For farther navigation, a lot of sea animals either have natal recognition (e.g. salmon which “remember” how to get to the streamheads where they were born, and use olfactory (smell) or magnetic cues to help) or use electromagnetic fields similar to how birds migrate. There’s a lot of different sensory organs that are used in different ways by different animals, but it’s all the same concept, really. Fish swimming in schools use lateral lines which are sensitive to the tiny electric impulses sent out by animal movement to avoid collision and navigate in the school.
Rambled a little… hope that answers your question?
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