Anyone who has seen goldfish will notice a phenomenon: no matter when you see them, they always maintain the same posture and expression, seemingly never tired or resting, even when they die, their eyes are open. Is this really the case? Today, I'll share why fish sleep with their eyes open. The reasons why fish sleep with their eyes open.
Fish move forward by undulating their tails and bodies, aided by their fins. As vertebrates, their nervous systems and muscles also experience fatigue, requiring sleep to alleviate it. Fish sleep in a variety of postures; some prefer to burrow into the sand for a sense of security and to avoid predators. Others like to sleep motionless behind aquatic plants or artificial rock formations. Parrotfish secrete a gelatinous substance that they blow bubbles while sleeping. This gelatinous substance hardens upon contact with water, and the parrotfish then burrows into the bubble to sleep, leaving a small hole at the tip. Tuna swim while sleeping, making it difficult to observe when they fall asleep.
In fact, the structure of a fish's eye is basically similar to that of a human eye, the difference being that fish lack tear glands and eyelids. The lens of a fish's eye is spherical, and visual accommodation relies on the forward and backward movement of the lens, not its convexity. Therefore, fish are extremely nearsighted, rarely able to see objects more than 12 meters away. However, because their eyeballs protrude, their field of vision is extremely wide; a single eye can see a horizontal range of 60-180 degrees and a vertical range of 150 degrees. In contrast, a human's single eye can only see a horizontal field of vision of 160 degrees and a vertical field of vision of only 80 degrees.
Why do fish sleep with their eyes open? Because they don't have eyelids.
