Ever been 40 feet down when a weird question comes to mind? Something like, “Do fish sleep?” If you’ve ever been struck by marine curiosity mid-dive, you’re not alone. The ocean’s filled with strange truths, ancient creatures, and wild adaptations, and the more we explore, the more questions we have.
At Grateful Diver, we’re not marine biologists, but we’ve spent enough time underwater to ask some oddly specific questions, and we’ve chased down the science-backed answers. From brainless jellyfish to gender-bending clownfish, here are answers to 10 curious marine biology questions.
1. Do Fish Sleep?
Believe it or not, fish sleep! Just not the way you might expect. Most fish don’t have eyelids (sharks are one of the few exceptions), so they don’t shut their eyes as we do. Instead, they enter a state of low activity and reduced metabolism, often hovering in place or nestling into coral for protection. Reef fish, like parrotfish, have been observed creating mucus “sleeping bags” to keep parasites and predators away while they rest. Next time you see a fish hovering and looking zoned out, it might just be catching some Zs.
2. Do Jellyfish Have Brains?
Nope. Jellyfish are living proof that you don’t need a brain to survive or to thrive. These gelatinous drifters are among the oldest lifeforms in the ocean, having pulsed through the oceans for over 500 million years. Instead of a centralized brain, jellyfish use a decentralized nerve net that senses changes in light, temperature, and nearby prey or predators. It’s a primitive (yet effective) system.
Fun fact: one species, Turritopsis dohrnii, is capable of reverting back to an earlier life stage, earning it the nickname “the immortal jellyfish.”
3. Why Don’t Sharks Sink?
Sharks stay afloat thanks to giant, oil-rich livers filled with squalene, which gives them natural buoyancy. Unlike most fish, they don’t have swim bladders, so many need to keep swimming to avoid sinking. That said, not all sharks are created equal. Great whites and hammerheads must keep swimming to breathe and maintain buoyancy, but nurse sharks and lemon sharks can rest on the sea floor thanks to special adaptations that let them pump water over their gills.
4. Can Octopuses Feel Pain?
Yes, octopuses can feel pain. They have complex nervous systems and large brains relative to their body size, with two-thirds of their neurons located in their arms. Octopuses show advanced behaviors like problem-solving, learning through observation, and even play. Some have been documented recognizing individual people, escaping from aquariums, and opening jars. Recent research confirms that octopuses not only feel pain but can also experience emotional stress and discomfort. Their intelligence raises important ethical questions about how they’re treated in captivity and fisheries.
5. Are Seahorses Really Monogamous?
Yes… at least, some of them are. Many seahorse species form monogamous pairs during a breeding season, and they strengthen their bond with daily greeting rituals, like dancing. These behaviors help them recognize their mate. What really makes seahorses stand out is that the males carry the babies! The female transfers her eggs into his brood pouch, and he fertilizes, incubates, and eventually gives birth to dozens (sometimes hundreds) of tiny seahorses.
Monogamy isn’t guaranteed across all seahorse species or for life, but in the world of marine relationships, seahorses are about as loyal as it gets.
6. How Do Fish Breathe Underwater?
Fish breathe by pulling water through their mouths and over their gills, where oxygen is absorbed into the blood. This process is super efficient thanks to something called counter-current exchange. It lets fish extract maximum oxygen, even in low-oxygen environments. Basically, gills are nature’s scuba tanks…and way more advanced than they look.
7. What Color Is the Ocean, Really?
The ocean appears blue because water absorbs red and yellow light while scattering blue wavelengths. But its true color can vary depending on depth, clarity, and what’s in the water. Plankton-rich areas might look green; coral reefs can shine turquoise. It’s no surprise that so many artists and designers draw inspiration from its endless palette!
8. Can Sea Stars Regrow Arms?
Absolutely. Sea stars are regenerative superstars. If they lose an arm, many species can grow it back over time. Some can even regenerate an entire new sea star from a single severed limb (so long as it includes part of the central disc). This ability helps them recover from predators, environmental damage, or even accidental injury. Regeneration takes time (often several months or more), but it’s a vital part of their survival strategy.
9. Do Whales Get the Bends?
Humans can get “the bends” if we ascend too quickly after a dive, due to nitrogen bubbles forming in the bloodstream. But whales? They’ve evolved to avoid it entirely. Marine mammals have collapsible lungs that reduce nitrogen absorption at depth, along with slower heart rates and the ability to redirect blood to essential organs during dives. Some whales can dive for over two hours and reach depths of more than 6,000 feet!
10. Why Do Some Fish Change Sex?
Some species are born one sex and change to another as they grow or when the social structure calls for it. This is called sequential hermaphroditism, and it helps fish populations stay balanced. Clownfish, for instance, are all born male. If the dominant female in a group dies, the largest male transforms into a female to take her place. Parrotfish and wrasses do the opposite, starting life as females and turning male when they reach a certain size or status. It’s nature’s way of balancing the population and keeping reproduction going strong.
These quirky facts aren't just for trivia night at the local pub; they’re a reminder of how much there is still to learn and protect. And the more we learn about the ocean, the more we fall in love with it. While you’re out there exploring, make sure you’re protected with Grateful Diver UV long sleeves, neck gaiters, and hats. A portion of every purchase helps support reef conservation through our friends at Reef Relief. And hey, keep asking weird questions!