The animal kingdom is chock-full of strange eyeballs, from the free-roaming peepers of a chameleon (which can move each of its eyes independently) to the W-shaped pupils of a cuttlefish. But one of the strangest attributes of any eyeball on Earth belongs to the whale shark (Rhincodon typus). Because these gentle giants of the sea don’t have eyelids to protect their organs of sight, they’ve developed thousands of tiny teeth known as “dermal denticles” to fend off any potentially vision-damaging elements the ocean might throw at them, whether it’s items drifting in the sea or biochemical hazards. In 2020, scientists in Japan discovered the denticles when analyzing both dead and living sharks, and estimated that each eye could have as many as nearly 3,000 of these tiny teeth. When viewed under a microscope, the denticles resemble the shape of an oak leaf, but much smaller.

The whale shark is the biggest fish in the world.

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The whale shark is the largest fish in the world, and it’s not even close. Its closest competition, the basking shark, stretches some 40 feet, but the whale shark has clocked in at a little more than 60 feet, which is about the same length as a bowling lane.

Dermal denticles are commonly found on many shark species’ skin, acting like scales that cut down on turbulence and drag when gliding through the water. However, the whale shark is the only species in whom these denticles congregate around the iris. This scale-like armor, combined with the whale shark’s unique ability to retract its eyes into its sockets, has scientists reexamining the importance of a whale shark’s vision. For years, scientists believed that whale sharks largely navigated the world through their sense of smell, but these two distinct evolutionary defenses could mean that vision is much more important to the species than previously thought. It’s often said that humans know more about the surface of Mars than what’s going on in the world’s oceans, and the surprisingly recent discovery of this fascinating ocular biology hints at how many more mysteries might await us in the deep.

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Eating eel is common around the world, especially in Japan, where it’s often found in sushi. But whether it’s freshwater or marine eel, the animal is always served cooked, because toxins found in its blood can cause extreme muscle cramping if consumed by humans. This cramping can affect your body’s most important muscle — the heart — which is why eating raw eel can be fatal. Luckily, when eels are cooked, those deadly toxins break down and the animal becomes safe to consume. This is good news for chefs, since eel provides a rich taste similar to squid but with a softer texture. 

All animals contain some form of blood.

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Although blood is a common trait among most animals, it’s not necessarily a prerequisite. Flatworms, nematodes, sea anemones, and jellyfish don’t have blood, and animals like starfish instead use seawater to transfer vital nutrients throughout their bodies.

Although eel blood is a particularly dangerous fluid, that didn’t stop French physiologist Charles Richet from experimenting with the stuff in the early 1900s. Inspired by fellow countryman Louis Pasteur and his discoveries in immunology, Richet experimented with a toxin found in eel blood serum and discovered the hypersensitivity reaction known as anaphylaxis. “Phylaxis, a word seldom used, stands in the Greek for protection,” Richet said during a lecture after receiving the Nobel Prize for his work in 1913. “Anaphylaxis will thus stand for the opposite.” So while the everyday eel may be a slippery, slimy, and all-around unappealing animal to some, it holds a distinguished position in the annals of both scientific history and culinary delight.

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In 1943, in a letter to his first wife, Hadley Mowrer, Ernest Hemingway wrote, “One cat just leads to another.” All these years later, the famed writer’s adage rings true, especially at his former estate in the Florida Keys, which is home to about 60 cats. These feline descendants of Hemingway’s original cat colony share a special trait: They’re polydactyl, meaning they have more toes than the average housecat. (According to the museum, about half the cats there display “the physical polydactyl trait but they all carry the polydactyl gene in their DNA.”) Most domesticated felines have 18 toes — five on each front paw and four on each back foot — but polydactyl cats can have as many as eight “toe beans” on each paw.

Key West is home to thousands of street chickens.

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While Key West was called the “Isle of Cats” long before Hemingway’s horde, the city also has a hefty number of roving street chickens. The free-range fowl are both beloved and known for causing trouble, though particularly problematic poultry are relocated out of the city.

Those enlarged feet are considered lucky among sailors, who believe they help boat cats better plant their paws in turbulent waters and catch stowaway rodents. That could partly be why Hemingway, known for his love of sailing, favored polydactyl cats; the first of his colony, named Snow White, was reportedly gifted to him by a sea captain. 

Despite his reputation for machismo, Hemingway had a soft spot for cats. While he was alive, the Florida home where he penned several novels was something of a cat sanctuary, home to as many as 80 cats at once, which were frequently mentioned in his letters to family and friends. More than six decades after Hemingway’s death, his cat clan lives on. Each cat at the author’s residence is born there, given (per tradition) a celebrity name like Lucille Ball, Ginger Rogers, or Alfred Hitchcock, and granted free roam. They’re friendly with the thousands of visitors they meet each year, and may just take after Hemingway’s tough nature — fortunately, the cats safely rode out both Hurricane Irma in 2017 and Hurricane Ian in 2022 with not a hair out of place.

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Humans aren’t the only animals that suffer from motion sickness. For instance, despite being equally comfortable on land or in water, seals can reportedly get seasick if you put them on a boat. Like human beings and many other animals, they rely on their inner ears for balance. The conflict between the signals sent to the brain by their eyes and ears results in the same discomfort experienced by anyone who’s struggled to get their sea legs, but at least the adorable mammals have the option of jumping in the water to alleviate their queasiness.

Crabeater seals don’t eat crabs.

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Despite their name, these seals, which are native to Antarctica, mostly eat krill. In fact, there aren’t many crabs in Antarctica at all. The name is a misnomer attributed to them by early sealers and whalers who misunderstood the animals’ diet.

Aside from this minor impairment, however, seals are highly physically attuned to their environments. Their underwater eyesight is excellent, and their whiskers are also important when hunting. The latter allows them to sense vibrations caused by swimming prey and are so effective that even blind seals can hunt and feed underwater — just don’t ask them to share their catch on your boat afterward.

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