The Joy of Living

© Stan Lupo, CC BY-NC-ND 2.0.

 

In the following extract from D.H. Lawrence’s “Fish”, he imagines the exuberance of a fish’s life:

Quelle joie de vivre [What a joy to live]

Dans l’eau! [In the water!]

Slowly to gape through the waters.

Alone with the element;

To sink, and rise, and go to sleep with the waters;

To speak endless inaudible wavelets into the wave;

To breathe from the flood at the gills,

Fish-blood slowly running next to the flood, extracting fish-fire;

To have the element under one, like a lover;

And to spring away with a curvetting click in the air,

Provocative.

Dropping back with a slap on the face of the flood.

And merging oneself!

To be a fish!

Do you think fish can experience the joy of living? That they could be happy or sad? There is some evidence that they can. Even though there are differences between our brains and theirs, overall there are quite a few physical similarities between us and fish. Even more so than between us and fruit flies, which scientists often use to study medical problems.

Jonathan Balcombe writes in his book What a Fish Knows:

I believe that the main source of our prejudices against fishes is their failure to show expressions that we associate with having feelings. “Fish are always in another element, silent and unsmiling, legless and dead-eyed,” writes Jonathan Safran Foer in Eating Animals. In those flat, glassy eyes we struggle to see anything more than a vacant stare. We hear no screams and see no tears when their mouths are impaled and their bodies pulled from the water. Their unblinking eyes—constantly bathed in water and thus in no need of lids—amplify the illusion that they feel nothing. With a deficit of stimuli that normally trigger our sympathy, we are thus numbed to the fish’s plight.

There is something to this. Scientists have shown that mice express their emotions on their faces, though we don’t usually notice it because we aren’t looking close enough...and they lack eyelids, so what we do notice are their beady little unblinking eyes, but as mammals we are able to recognize more similarities to us than we see with fish. Yet, we are a lot more like fish than you’d think.

For over the 100 millennia or so our ancestors were fish. That was approximately 518 million to 417 million years ago. The first fish resembled a swimming worm that was vertically flattened. The last of our fish ancestors, evolutionary biologist Richard Dawkins calculated, would be our 185-millionth great-grandparents. Twenty million years later, another group of our ancestors, tetrapods, that looked like a cross between a fish and a crocodile, began moving onto land.

During the period fish were our ancestors, they evolved many of the features that we still have today, prompting science writer Natalie Angier write, “everything we can do, fish can do wetter”. While that’s not entirely accurate, as she points out, they did evolve bones, vertebra, spinal cords, skulls, jaws, teeth, tongues, many of our internal organs, arms, legs, wrists and ankles that rotate, and they even gave us opposable thumbs. They also may have given us the ability of internal fertilization, internal pregnancies, and live births.

Fish can also do many things we don’t normally give them credit for. Some can breathe air, walk on land, and climb trees. Others can glide through the air. They stake out territories and defend them from intruders. They keep their homes clean. They have personalities that evolve over time based on their experiences. Some have complex social systems and hierarchies. They can be cooperative and establish reciprocal relationships. Some provide parental care and raise their young. Some have helpers who assist them with this.

You may be sarcastically thinking, “Yeah, right.” We’ll cover some of these later on, but let’s quickly take a quick look at some here.

Joy is a feeling, an emotion. Some consider it to be one of five basic emotions. They arise from hormones that are regulated by the brain’s limbic system and the neuroendocrine system, as determined by external cues, which in this case would be something that generates joy, such as play or chocolate.

Usually evolution doesn’t suddenly give rise to complex systems. It tinkers with and repurposes what is already there. As a result, our neuroendocrine system is very similar that in other mammals and in bony fish, while the limbic system is one of the most ancient parts of our brain. Physiologically, fish have the capacity to feel emotions, but whether they actually do is difficult to confirm.

We know that fish can suffer from depression and the neurochemistry of that is so similar to ours that zebrafish are now being used to develop new treatments for it. Fish that show the signs of depression recover when the antidepressants fluoxetine and diazepam—also known as Prozac and Valium, respectively—are added to their water.

For a number of reasons, zebrafish, like fruit files, are also used in medical research, including for studying arthritis. Fish do get arthritis. And you know how new mothers get “mommy brain” where their behavior and cognitive functions change? Well, small fish called sticklebacks get that too, except that in their case, it’s the males that get it, since they’re the ones who care for their young by circulating water around the eggs, keeping the nest clean, warding off predators, and retrieving stray fry, while the mother goes off to do her own thing.

Zebrafish are also used to study addiction. When given the choice, the fish will repeatedly dose themselves with the opioid hydrocodone, which is usually prescribed to treat pain, and they’ll even enter risky situations to get it. They also show signs of addiction, and withdrawal when going cold turkey. When given naloxone, a drug that counters the effects of opioids, the fish reduced their requests for hydrocodone. This isn’t surprising since they have the same receptor and neurotransmitters as we do for our reward system.

Like us, rats like to play and when they do, their brains release natural opiates and dopamine, a neurotransmitter and hormone that is involved in emotions, mood, motivation, movement, learning, and the reward system, among other things. It’s often called the “feel-good neurotransmitter”. All mammals have dopamine systems, and so do fish. Goldfish actively seek out amphetamines, which cause their brains to release dopamine, and they avoid pentobarbital, which inhibits its release.

In humans, dopamine suppression is associated with depression, stress, anxiety, low motivation, inability to concentrate, feeling hopeless, and being tired, but having difficulty sleeping. Some of these symptoms are detected in mice, fruit flies, and fish, making them good models for studying depression and other chronic ailments.

The New York Times article “Fish Depression Is Not a Joke” quotes biology professor and researcher Julian Pittman, with Troy University in Alabama saying, “The neurochemistry [in us and fish] is so similar that it’s scary”. That same article quotes other researchers who say the leading cause of depression in fish is probably boredom, since they are so naturally curious and crave novelty.

© Elaine Molina Stephens, 2024.

 

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What’s Happening to My Head?

A flatfish with a sea star. John Butler, NOAA.

As with jellies, fish go through some major transformations. Often their juvenile form looks little like the adults, with some being wrongly classified as separate species. Some sport long appendages, such as the juvenile barbeled dragonfish whose guts exit its belly and trail behind, being as long or longer than its body.

In one of the most amazing examples of adaptation, flatfish have rearranged their bodies to adapt to life on the seafloor. While most animals are symmetric, flatfish are an exception. This includes flounders, sole, plaice, dabs, tonguefish, turbot, and halibut.

In the evolutionary scheme of things, they evolved rapidly over a three million year period, starting about sixty-five million years ago. They are born looking like regular fish, but as they mature they deform with one eye moving to the other side of their head and one side of their mouth rising up on their face so that its crooked. They end up looking like something out of a Picasso painting. Then they start swimming sideways.

These changes didn’t happen one at a time, but were coordinated through related genes. The eye didn’t move and then the mouth—the entire skull changed. This prompted further coordinated changes, such as their body flattening and their fins extending to run down the sides of their body. Their new bottom side—which can be either left or right depending on the species—remains white, while the upper side becomes pigmented for camouflage.

The fish are able to expand and contract the different-colored pigment cells in their skin, so are able to rapidly change their color and shading from light to dark to match their surroundings, confuse predators, and probably to communicate. Tropical flatfish can do this in a couple of seconds, while it can take a couple of days for a cold-water flatfish to change its camouflage. They are very good at this. When placed on a checkerboard, they do a pretty good job of matching the pattern. Well enough so that at a distance in a natural environment, they would blend in.

But fish that evolved to be flat bottom dwellers don’t all do it this way. Some, like rays and skates, flatten vertically without losing their symmetry. Still, these transformations are not nearly as drastic as caterpillars changing into butterflies, but they are striking for a non-insect.

This reformation happens during a stage of rapid growth—somewhat like our pre-teen years—when they suddenly begin leaning to one side when swimming. Then one of its eyes moves up the side of its head and over the top. Or it moves through its head to the other side. In some species this takes five days, in others just one day. And their behavior changes too, from swimming freely to a sedentary life on the bottom.

Some of these fish are large. The European plaice gets up to three feet in length (1 m). They spend their days hiding, buried in the sand, coming out at night to hunt for crustaceans and shellfish that they take into their mouths whole, crushing them with teeth in their throats. Some flatfish even eat lobsters and sand dollars, which don’t have much flesh in them.

Having both eyes on the same side of their head probably gives them 360-degree vision and might give them good depth perception where the visual fields overlap, but since their eyes swivel independently their brains probably process the input from each eye separately.

 

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Attack of the Bobbit Worm

The bobbit worm's pincers are horizontal below its antennae. Jenny Huang, CC by 2.0 (adjusted).

One of the creepiest denizens of the seabed is the bobbit worm, named after the somewhat infamous couple, who—after the husband mistreated his wife—she taught him a rather severe lesson, and by her actions coined the word “Bobbittize”. This just adds to the horror of this worm’s terrifying appearance.

This bristle worm digs a tunnel down into the sand that’s long enough to comfortably fit its body with its hard exoskeleton, which can get up to nearly ten-feet (3 m) in length, although most are about three-feet long (1 m), with a diameter of one inch (2.5 cm).

They have two eyes, but are practically blind, so it uses its five antennae to sense its surroundings. During the day, with just its antennae and two pairs of pincers—retractable mandibles—poking out of its burrow, it waits for an unsuspecting fish, shrimp, snail, sea star, or worm to cruise by, or anything else edible that’s the appropriate size. The antennae move like worms, attracting prey. At night it extends a bit of its body out of the burrow and actively hunts.

When the prey gets close enough, with lightning-fast speed it extends up like a jack-in-the-box and seizes its prey with its sharp pincers, sometimes so strongly that it cuts its victim in half. When the prey remains intact, it injects venom through its teeth that stuns the prey, preventing it from struggling. Then it drags the victim down into its mucus-coated L-shaped burrow. Still, the victim can struggle enough to collapse the burrow’s entrance.

But in spite of all this, these worms aren’t carnivores—they’re omnivores—and they can live on seaweed, or algae and detritus they filter out of the water, or they will scavenge. If they feel threatened, they split themselves into a number of segments, each of which can regrow into a full worm. This is also one of the ways they reproduce, so they don’t mind.

When one of a shoal of fish, such as a bream, spots the worm’s antennae or if one is attacked, it swims vertically above the worm and spits streams of water at it, making the worm retract. Other fish—sometimes including fish of different species—may also join in, firing jets of water at the burrow. This makes them aware of the worm’s location, and might mark the burrow as a danger spot. It’s usually the juvenile breams that do this, not the adults, as they’re a bit more solitary, so it might be an educational lesson for the young.

Such mobbing behavior where prey team up to launch a coordinated attack against a predator is usually seen in birds, but also in some bovines, meerkats, ground squirrels, bees, ants, and freshwater bream called bluegills, which mob turtles, partly to drive them away from their nesting colonies. Humpback whales also mob orcas.

 

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Megalodon

 

We’re not sure how big megalodons were, but this gives you a general idea. Compare it to the more accurate estimates below. Dinosaur Zoo, CC BY-SA 3.0.

The biggest known shark, and fish of any kind, was megalodon. We don’t really know how big it was since so far paleontologists have only teeth and a few vertebrae. It had cartilage instead of bone, and that doesn’t preserve well.

One estimate suggests it was plump and stalky, similar to great white sharks, and could get from fifty to sixty-five feet long (15 to 20 m). At the upper end, it would be roughly as long as a six-story building is high, or longer than an 18-wheeler tractor-trailer truck. Just think about that the next time you pass one on the highway. Imagine that it’s a megalodon swimming beside you, eyeing you with delight at having found a little snack.

Another estimate suggested it may have been longer and thinner, more like a mako shark, which would mean it was slower, less maneuverable, and unable to accelerate as quickly. While the second study refrained from giving an estimated size, one of its authors and their illustration indicated a range from fifty-five to seventy-nine feet (16.5 to 24 m).

It’s thought they weighed around fifty-five tons (50 tonnes)—roughly equal to eighteen large elephants, or about twenty-five great white sharks. And a man could stand on one’s back and would be about as tall as its dorsal fin. It was quite a large shark.

Megalodon means “big tooth” and its teeth were certainly big—about the size of a man’s stretched-open hand. It had five rows of very large teeth—276 of them, most of which were replacements for those that broke off.

Some evidence indicates that they were partly warm-blooded—meaning they kept their body temperature higher than the water they swam in—just as great white sharks, mako sharks, and a number of other sharks do today, but not quite the way we do. Because of their size, they probably had an even higher body temperature than those sharks. This would have made them more active and able to swim faster and for longer distances, but it also would have required them to burn through more food.

Megalodons were swimming around for about twenty million years, which is pretty good considering our species has only been around for about 315 thousand years. They first appeared around 23 million years ago—about 23 million years after the dinosaurs went extinct—and they disappeared 3.6 million years ago. There is no evidence of any megalodons existing since then. If they had, there would definitely be signs of their existence. They were gigantic apex predators that required a huge amount of food to survive.

Now, these are jaws.

Their primary prey were whales, seals, large fish, turtles, and other sharks. They could have eaten an orca in about five bites with their huge mouths, the largest being eleven feet wide by nine feet high (3.4 by 2.7 m). Scientists estimate its stomach could hold a twenty-six-foot-long (8 m), 6.6-ton (6 tonnes) orca. A meal like that could last it for two weeks, but they probably didn’t eat a complete whale.

It looks like they enjoyed eating just the faces of sperm whales—as do mako and great whites—leaving the rest behind for other creatures to eat. Sperm whale noses contain a lot of fat and oils, which sharks used to love devouring. Modern sharks no longer kill sperm whales, but orcas do.

Most whales were smaller back then, although by around seven million years ago the largest sperm whales did get up to the same size as a megalodon, but after megalodons went extinct, whales were free from their primary predator. As a result, they were able to grow in size to become the largest animals on earth. Blue whales evolved after the megalodon’s demise.

It’s not possible for one to still be hiding somewhere. While there are some areas of the ocean that aren’t frequently explored, such as the Mariana Trench, there’s not enough food in the deep ocean to feed such a huge animal, so it would have to live closer to the surface, and there would be some traces if something was killing that many whales. While we do continually discover new creatures in the ocean, nothing is anywhere near megalodon’s size. If they were still around, they probably would have evolved into something much smaller, but so far there’s no evidence of that either.

Several things could have caused megalodons to die out. At that time the earth was getting colder and that could have greatly reduced their food supply. Also there was a large drop in sea level from 5.3 million to 2.6 million years ago. This shrank their coastal habitat, reducing both their prey and their living space. Scientists suspect they may also have suffered from competition with great white sharks, which had appeared on the scene by that time. Sea levels today are even lower than they were then, although they are now rapidly rising again.

There may have also been changes at the locations where megalodons raised their young. So far researchers have found four of these nurseries covering different periods of time. They’re in Spain, Panama, Chesapeake Bay, and Florida. Most of these are now inland. They indicate megalodons devoted time to caring for their young, which weren’t fully grown until the age of twenty-five.

Now, you may be surprised to learn that a toothed whale grew to be just as large as megalodon. A fossil of one was found from twelve million years ago, right in the middle of megalodon’s reign and at fifty-five feet (17 m), it was about the same size. And, being a toothed whale, it had teeth, perhaps for fighting off or eating megalodons. It’s called leviathan and is thought to have looked like a sperm whale, but where sperm whales only have teeth in their lower jaws, leviathan had teeth twice the size in its upper and lower jaws, and unlike sharks, whose teeth are embedded in their gums, leviathan’s teeth were embedded in bone.

 

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Psychedelic Slugs

All of these are nudibranchs. 1.1 (Row 1 picture 1), 2.1, 2.4, 3.1, 4.2, 4.3, 6.2, 6.3, 6.4 Silke Baron, CC BY 2.0 (1.1, 2.4, 3.2, 4.2, 6.3, 6.4 adjusted, cropped); 2.1, 4.3 cropped; 6.2 adjusted). 1.2, 1.3, 2.3, 3.3,4.1, 5.1 5.2, 5.3, 5.4, 6.1 Orangkucing, CC BY-SA 3.0 US (1.2, 1.3, 2.3, 4.1, 5.1, 5.2, 5.3, 5.4 adjusted; 3.3, 6.1 adjusted, cropped). 1.4 Steve Lonhart, NOAA MBNMS. 2.2 Hidekatsu Kajitani, NPS. 2.5 Claire Fackler, CINMS, NOAA, CC BY 2.0. 3.1 NOAA.

Nudibranchs—pronounced new-duh-branks—are some of the most beautiful creatures in the sea. Just by looking at them, most people wouldn’t expect that they’re sea slugs, which is what they’re commonly called. They come in a wide variety of shapes—some with frills along their edges or short tentacles on their backs or the tips of their limbs if they have them—and can be vividly colorful. Some have a rosette of tentacles on their lower back that look like fern fronds, but are actually external gills. Many others have no gills and just absorb oxygen through their skin. They also use some of their frilly tentacles to smell and taste. This makes them appear delicate.

Their undulating form at times resembles a piece of silk slowly floating on the wind, although a nudibranch is much thicker. Sometimes they just look like a torn piece of seaweed. Others are more mound-like, resembling a snail’s foot or like a shell-less abalone. Some have features that look like soft leafless trees on their backs. When found in tide pools, they tend to look like lumps of jelly, but when in water they unfurl in some amazing shapes. And in the tropics their bright and varied colors rival those of butterflies.

Some are so good at camouflage, even disguising themselves as seaweed, that you can look right at them and not know they’re there. You may have to follow the slime trail they leave behind to find them.

They are mollusks and are related to snails. They crawl like snails, but swim upside-down with their undulating foot upwards. They’re brightly colored as a warning to predators to stay clear or it will get stung.

They are voracious eaters and bad tempered. Most of the time they indiscriminately attack all other nudibranchs, even those of their own species. This sometimes results in fights where they lunge and bite each other. Sometimes one will eat the other. But they prefer eating sea anemones, consuming at each feeding between 50 to 100 percent of their own body weight. Anemones try to avoid this by living high in the intertidal zone. They also have some behavioral defenses, and when damaged they can regrow the tissue and clone themselves.

Some nudibranchs like to eat chunks out of anemones by using their radula—their coiled tongue covered with tiny teeth used for scraping and drilling holes. Researchers aren’t sure why they aren’t stung by the anemones’ tentacles. One hypothesis is that the nudibranch will sneak up and rub itself against the anemone’s trunk, coating itself with the anemone’s mucus so when the anemone’s tentacles touch it, the anemone will sense it as just being a part of itself. Then these sea slugs can eat all they want without being stung. Others think nudibranchs use their own protective mucus. Perhaps different types of nudibranchs use different methods.

Not all sea slugs are nudibranchs. Here, one of the largest sea slugs, which is not a nudibranch, is about to rapidly suck in a hapless nudibranch. Steve Lonhart, NOAA MBNMS.

At least one type appears to hunt in packs. These are white ones with rows of horn-like spurs running down their backs usually found in the Atlantic and Mediterranean. When a nudibranch starts eating a anemone, if the anemone detects something, it leans over in that direction and the attacker is more likely to be stung, but it doesn’t do this when it’s surrounded by three to more than six nudibranchs, so this mobbing behavior might help protect the slugs.

Oddly, nudibranchs also have stinging cells, but aren’t their own—they’re stolen from anemones. Nudibranchs can eat parts of the anemone that contain stinging cells, but the stingers don’t fire. Not only can it swallow the stinging cells without setting them off, while absorbing the nutritional parts, the stinging cells move from its digestive system out to its own tentacles, where it can then use them against anything that touches it by firing off a volley of toxic harpoons.

I find it amazing that they can do this. It’s like if you ate an anemone and then your body moved the venomous cells out to your finger tips where they would sting anyone you touched.

Each species tends to specialize in eating only one type of prey and some obtain their stinging cells by eating corals or jellies. Certain types of nudibranchs obtain toxins from toxic algae—some of which only live in sponges—storing and concentrating these toxins in specialized glands for later use. Others absorb algae that take up residence inside them and contain bacteria that make the toxins they use in a three-way nudibranch/algae/bacteria symbiotic relationship.

Now think about this for a second. They are taking another animal’s weapons and putting them to their own use. It must have taken a long time to evolve that amazing ability.

Some of them can also steal chloroplasts from the algae they eat and put them to use turning sunlight into energy, like in plants, but they’re only able to retain the chloroplasts for up to three months. During that time they can go without eating, but when they have to replace them, they pig out, eating for hours. Still, this essentially turns them into solar-powered sea slugs, or partly solar powered at least.

Blue dragon. Sylke Rohrlach, CC BY-SA 2.0.

One unusual stinging nudibranch that looks like it has fancy wings is called the blue dragon. It lives upside-down floating on the ocean’s surface with its blue side up and its silver side down and contains symbiotic algae that produce food for it. They get their stingers from eating Portuguese man-of-wars, which are among the ocean’s most poisonous creatures.

 

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Snails that Harpoon Fish

Cone snails can be much more dangerous than they look. NOAA.

Some cone snails are armed with deadly projectiles that resemble harpoons, which they use to catch fish and other prey, and also for defense.

These snails are slow moving somewhat shy creatures that tend to hide in holes and crevices, emerging at dawn and dusk to hunt. As they creep along searching for a meal, their siphon can pick up the scent of prey and their eyes on stalks can see it, so they extend their tube-like proboscis, which can reach more than twice the length of their body, towards its potential victim.

When close enough, they use their proboscis like a blowgun, shooting out a venom-loaded harpoon at about four-hundred-miles per hour (650 km/h)—more than half the speed of sound and a whole lot faster than a Lamborghini at maximum speed. The venom paralyzes the fish, while the snail reloads another harpoon from its cache. The cone snail then reels in a cord attached to the harpoon and pulls the fish into its mouth where its radula scrapes away at it. Sometimes the prey is too large for the snail to pull into its shell, so it scrapes away outside its shell. Several hours later it spits out the bones and scales.

There are many different kinds of cone snails and they tend to specialize in one specific prey, such as worms, clams, or other snails. In some, the tip of their proboscis has many tiny tentacles for sensing prey and tiny teeth for boring through the shells of bivalves.

Cone snails can sneak up on fish. Baldomero Olivera, NIH.

Some cone snails that don’t have harpoons will sedate fish by blowing wafts of toxin into the water, or they will sneak up on sleeping fish and envelop it with their tentacle-lined net-like proboscis and smother it. Occasionally they can catch a group of small fish all at once. If the snail has harpoons, it will then shoot one into each of the fish so they can’t escape.

These poisons can vary from species to species and from individual to individual. Sometimes each individual mixes its own poisons for various uses as needed. Their harpoons are filled with venom ahead of time and stored. Each is only used once. They are less than half-an-inch long, but the poisons can be extremely toxic—some of the deadliest in the world—and some can kill you with hours, but their shells are highly valued by collectors.

 

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Clever Fish

A pufferfish. harum.koh, CC BY-SA 2.0 (adjusted, cropped).

Recently scientists have discovered that fish are actually very smart and can be favorably compared to some non-human primates. The problem in the past was that there was a dearth of information about their behavior and social lives, so many of our ideas about fish came more from intuition than evidence.

So if fish are not chowderheaded automatons operating solely on instinct, then how smart are they?

Comparing intelligence between different types of animals is difficult because they are different. Their environments, priorities, and abilities can vary significantly. Normally we measure human intelligence by asking questions about common knowledge mixed with some specialized knowledge and perhaps some algebra. This doesn’t work well with other cultures and especially not with other animals. With animals we have to look at the basics, like their ability to solve puzzles, and then we compare that with other animals. Doing this with fish has prompted a number of researchers to say that fish compare favorably in some tasks with primates, such as monkeys, which is quite surprising.

Most of the tests have so far only been done with specific species, so our assessments are limited and don’t apply to all fish. There’s bound to be considerable variation between the many different species, just as each species’ abilities will vary between the types of tests. For example, some freshwater fish—galaxias and wild rainbow trout—can learn certain tasks faster than rats. So are they more comparable to rats or monkeys? Well, it depends on what you ask them to do. Pigeons are better than people at spotting someone lost in the open ocean while searching from a helicopter, and at recognizing the shadows of cancer on an X-ray, even though their brains are only the size of a marble. It also depends on what species you’re comparing, and on the animals’ individual personalities, since some individuals in a species will be better at some things than others.

When Israeli researchers studying navigation in fish taught six goldfish to drive a fishmobile—a fish tank on wheels—using their orientation and movements. Two of the goldfish learned faster than others, but all of them soon became particularly good at avoiding obstacles to reach their target, and when allowed to roam freely, one of them took off down a corridor exploring the building.

It’s the differences between individuals that, in fact, define an individual’s personality. Whether they’re shy or bold, aggressive or passive, these are personality traits, and such differences can even be found in bacteria. Personalities do have an effect on intelligence and some individuals are smarter than others within their species and some species are more intelligent than others.

Many people think fish have poor memories or no memory at all, but this is completely wrong. We know that all creatures have some form of memory, including plants and microbes.

An international group of scientists in Texas found that bacteria not only remember things, they’re able to pass this information on to their offspring and then on to theirs for a minimum of four generations. This information tells their descendants whether they should remain in their environment or try to find someplace better to live, which is important since the one they studied splits in half every half hour or so. Even round worms remember things for at least a couple of hours.

But if we find memory in the tiniest of organisms, then it would be a huge surprise if we didn’t find it in fish. And, not to worry, fish do have very good memories.

Fish can remember things for at least a year—the amount of time studied, and for some fish their normal lifespan—and likely much longer depending on the type of fish. A shark, no doubt remembers things far longer than a minnow simply because they live a lot longer. The false idea that fish have a three-second memory stems from long-discredited hypothesis that a fish’s attention span is just a few seconds—that they live totally in the present—but we now know they do remember the past and anticipate the future, just like we do. It’s actually very easy to train fish. Unfortunately people still accept this falsehood without giving it any thought, and they’ve used that lie to justify things that are very harmful to fish.

Their long-term memories can be as good as most other animals, including some people. Here’s one rather impressive example.

Fish that live in tide pools are confined to small pools at low tide. If they discover there’s a predator, like an octopus, hiding in there with them, they need a way to make a rapid escape, so what gobies and other tide pool fish do is, when the tide is high, they memorize a three-dimensional map of the rocky shore’s topography, taking into account where the waterline will be at low tide.

Then when they find themselves in a difficult situation, they blindly leap into the air to land in the next pool, which they’re no longer able to see. They go solely by memory. If they miss, they might be able to flop around until they get in the pool without a hungry bird or crab noticing them, or they might dry up in the sun. A mistake can easily kill them. It’s a dangerous maneuver, but one that could save their life under dire circumstances.

We can make similar cognitive maps, as do many other animals. In the case of fish, it requires planning for the future, while also predicting the water levels at various times and places, but they are very good at it. Fish that were tested were successful ninety-seven percent of the time, while fish who weren’t given a chance to study the terrain made it only fifteen percent of the time. And interestingly, forty days later the knowledgeable fish still recalled their terrain maps and made successful escapes without a refresher course.

There are several types of memory and a number of ways memories can form and be retained, from iron levels in bacteria to RNA in slugs to neuron synapse connections in us, not to suggest that we don’t use all three and many others.

Fish learn from their experiences, of course, but they are very social, so much of their knowledge is gained by watching others or even from watching videos of other fish. They can also use this third-party perspective of watching others to quickly figure out their own place in the hierarchy. In addition, they’re very conscious of who’s watching them. For some fish, if two males are fighting, they’re more aggressive if other males are looking on, since it will increase their status, but they’re less aggressive when females are around when the females see aggression as less desirable in a potential mate.

Here’s a simple experiment involving time, observation, attention, and memory. When presented with a red and a blue plate of identical food, if you start eating from the red plate, the blue one is taken away. If you start with the blue plate, the red one remains there for you to eat later. With no instructions, we’d probably figure out what’s going on after just a few trials, since we’re familiar with tests and experiments, but animals wouldn’t expect such deviousness so it takes them longer.

Cleaner wrasses establish cleaning stations to groom other fish. Here black-and-white-stripped cleaner wrasses tend to two yellow-saddle goatfish. Which of these two species do you think might be smarter: the big one or the smaller one? Silke Baron, CC BY 2.0 (adjusted).

Scientists ran this experiment on orangutans, chimpanzees, capuchin monkeys, and cleaner wrasses. Unfortunately they didn’t include high-school students. All of the cleaner wrasses figured it out the fastest. Two of the chimps caught on soon after, but the rest didn’t. After modifying the test to make it easier, the rest eventually figured it out, except for the other two chimps who never got it. We don’t know how well the high-school students would have done, but one researcher tested his four-year-old daughter and she never caught on.

The researchers then swapped the plate colors so if you start with the blue plate, the red one is taken away, but only the fish and capuchins sussed the change within a hundred trials. On the other hand, while it’s not obvious, the test was designed to fit the sorts of decisions cleaner fish have to make on a regular basis, since they have to judge whether a client fish will hang around or swim away. Their regulars will be patient and more likely to wait their turn, while newbies might just take off.

Of course, primates are smart and inventive problem solvers, but they seemed to get frustrated with this task and may have just given up on it until the key suddenly occurred to them.

 

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Slime Fish

Here’s an unusual defensive weapon—slime.

Hagfish have four rows of teeth—two rows on each side of its mouth—that they use to rip into dead or dying animals. Here they are eating a shark, accompanied by an anemone and several brittle stars. Ryan Somma, CC BY-SA 2.0, (adjusted and cropped).

Hagfish are eel-like jaw-less fish that live on the seabed, searching for worms, small invertebrates, fish, and carrion to eat. On finding a carcass or dying animal, it tears open a hole, burrows its way in, and begins eating on the inside, while also absorbing nutrients through its soft, scale-less skin. In captivity, using skin absorption alone they’ve go eleven months without eating. Invertebrates, such as sea stars, corals, and jellies, can also eat by absorption, but hagfish are the only known vertebrate-like animal that can do it. This causes some to suspect hagfish are an intermediate animal between the vertebrates and invertebrates—those with spines and those without—but it’s also possible they could be vertebrates who lost their spines.

Even though they have no jaws and are unable to bite, they do have two rows of teeth made of cartilage, which they use to tear off chunks of meat by twisting. Otherwise they use their teeth to scrape off bits.

A hagfish with a yellow crinoid, an orange sponge, and two lobsters. Peter Southwood, CC BY-SA 4.0.

But when hagfish come under attack, they have an interesting and very effective defense. Slime.

Many fish produce slime. Some cover themselves in it to protect them from toxins. This is how the clownfish is able to hide in sea anemone tentacles. Others cover themselves in a blanket of it to hide their smell while they’re sleeping.

Hagfish, on the other hand, add threads to their slime, which entangle and turn into a sort of net. Along their flanks are more than a hundred microscopic glands that are in spots along its sides from which they eject slime and the long, thin tightly wound threads. When the hagfish is disturbed it shoots this out and when the threads hit the water, in less than half a second that small amount expands a hundred thousand times in size to more than six gallons (24 L), creating a large cloud of mucus right in the attacker’s face.

This slime can clog their attacker’s gills. Thrashing around only makes it worse. While the attacker tries to escape, the hagfish ties itself in a knot near its head and slides its body through, wiping away the slime and sneezing off any that gets into its single nostril. It doesn’t need to escape since the predator is too preoccupied to repeat their attack.

When the predator, which is often a bony fish or shark, tries to bite the hagfish, the hagfish’s skin is so loose that its body inside slips out of the way unharmed, then there’s the explosion of slime. There are around six thousand mucus threads per cup of slime. Once it’s slimed, it immediately starts coughing and shaking its head, while swimming away.

So far biologists have only seen attacks—not the aftermath—so we don’t know whether the slime kills the predators or just eventually dissipates in water after teaching them a very severe lesson. If they do die from suffocation, then the hagfish could go to feed on the attacker’s body.

The ultimate result is that fish, including sharks, don’t eat hagfish, but predators without gills do. This includes seals, dolphins, and seabirds, who don’t seem to mind getting slimed.

 

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The Smartest Bivalve in the World

A scallop sitting on a rock. Benjamin Hollis, CC BY 2.0.

Similarly, scallops sometimes live under the sand, but they also rise to the surface and swim away. Next to the squid, octopuses, and the other cephalopods, scallops are the “cleverest” mollusk...and they don’t have brains. But they do have neuron clusters called ganglia. They have three pairs of these, each pair roughly corresponding to parts of a brain. One usually fused pair controls the viscera and gills, another usually fused pair controls movement and touch sensations in their foot, while the cerebral pair controls decision making, such as when to swim. This pair also controls the mouth and the sense of where they are in their environment—their position and orientation.

Altogether their ganglia are sort of like a primitive brain that’s divided into either four or six pieces, but they’re interconnected and are able to work together. Unlike other bivalves, scallops also have additional ganglion associated with vision and their sense of smell.

We have ganglia too, but they act as relay stations for signals going to and from our brain. Some of ours are visceral, motor, and cerebral, as are those of a scallop, and we also use some of the same neurotransmitters, such as dopamine, serotonin, and norepinephrine. We have additional ganglia that scallops don’t have, but those aren’t of interest here.

A scallop with blue eyes and a close-up from another scallop. Top: Rachael Norris and Marina Freudzon. Bottom: Matthew Krummins, CC BY 2.0.

Depending on the species, scallops have between thirty and a couple hundred of eyes scattered around their mantle fringe, along with their tentacles, near the edge of their shells. Each eye is at the end of its own tentacle and they’re usually, but not always, bright blue. Each has a pupil, which is coordinated with the others so that they all open and close at the same time.

Their eyes give the scallop a minimum 270-degree horizontal view, but whether they see a combined panoramic view like we do, or whether they process the input from each eye separately, we don’t yet know. The latter offers faster response times with less processing, so having a panoramic vision might not be as useful to a bivalve. Still, they can see a wide-angle view, with a blind area behind them of a quarter of the full visual field. In addition, a few of the eyes in the top side are curved to look up, while some on the bottom side look down, which extend their view vertically.

Each eye uses a curved mirror made up of millions of square and rectangular reflective crystal plates that fit together like tiles. This is unusual since these crystals don’t naturally form into squares. Somehow the scallops get them to take that shape. The mirror focuses light on their two retinas. There are some other animals with mirrors in their eyes, but rarely achieve as clear an image as scallops do.

One of the two retinas detects peripheral movement in dim light and the other distinguishes movement in brighter light in the center of its field of view, along with variations in light intensity. Both would be useful in spotting predators, while the latter could help them when seeking a good spot to settle. They are also able to spot drifting food that is only six one-hundredths of an inch wide (1.5 mm). They usually rely on their well-developed sense of smell to detect food, but vision seems to let them know when to investigate, which caused one scientist to suspect they experience curiosity.

Swimming scallops. NOAA, CC BY 2.0.

On noticing a predator, scallops are quick to respond. They’re the only bivalve that can swim and they do this in two ways. By clapping their shells together they produce jets of water that shoot out of either the front or the back next to one side of the hinge or the other. When they squirt it out the front, they shoot backwards hinge first a few feet in a straight line. Repeated claps can move them pretty quickly. When they jet water out the back on each side of their hinge, they can go straight forward, or they can alternate jets on each side of the hinge. This produces a forward-moving zigzag motion, going to the left, then to the right, then the left again. In general, they can swim at a rate of about fourteen inches (36 cm) per second.

Unlike other bivalves, scallops don’t have siphons, but by changing the shape of their mantle to direct the jet of water, they can move in almost any direction. They can shoot upward off the sand and can quickly change direction. No matter which way they go, their movements resemble fleeing pairs of clapping castanets.

Sea stars are one of their primary predators and scallops don’t have to see or touch one to know it’s there—they can smell it. And the slightest whiff will send the scallop jetting away. Their other predators include fish, crabs, and lobsters.

By the way, since starfish aren’t fish, marine biologists and naturalists instead call them sea stars, just as jellyfish are now called jellies, but they haven’t gotten around to changing the names of most of the other things that are similarly misnamed.

For some unknown reason, most bivalves—such as oysters and mussels—don’t have eyes, and once they’re adults, they can’t swim either, but they can hear and smell.

Clams are also unable to swim, so they dig down to escape, or if it’s too late, they can clamp tightly shut for long periods of time, hoping the predator gives up and goes away. Scallops have one powerful muscle for closing their shells, where clams and mussels have two, but unlike those two animals, scallops can’t completely close their shells, so they’re much less protected if a predator catches them.

 

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Shark Attack

A smalltooth sand tiger shark can get up to thirteen feet (4.1 m) in length. NOAA.

Shark attacks—that is, attacks by people on sharks kill more than 100 million sharks a year, most of them just for their fins. More than one hundred million. Each year. That’s a lower estimate. It could be closer to three hundred million. Since 1970 overfishing has wiped out more than half of the world’s shark, ray, and chimera populations, knocking marine ecosystems off of balance worldwide.

Sharks, on the other hand, kill about five people a year worldwide. And it’s estimated they’ve killed less than five hundred people in the past five hundred years. Researchers estimate jellies kill about a hundred people a year. I don’t know of any killer jellyfish films even though they kill about twenty times more people than sharks do, but there are more than 180 killer-shark movies. Jellies just aren’t scary and they don’t have a mouth full of teeth.

A mako shark is also known as a sharp-nosed mackerel shark, and in Australia as a blue pointer. They can reach near fifteen feet (4.5 m) in length and weigh more than half a ton (half a tonne). NOAA.

There are no shark species that target people as prey. Most researchers are convinced that when a shark bites someone, it’s almost always by mistake. A starving shark might eat someone out of desperation, but apparently we don’t taste good to them since they usually spit people out and rarely try to take another bite. We don’t really have much in the way of blubber like seals do.

This prompts researchers to believe that attacks are actually cases of mistaken identity, where the shark thinks the person might be a seal, sea lion, or something else edible, especially when the suspected prey is on a surfboard, so it takes a bite to find out. From below, a surfer on a surfboard looks like a seal and a boogie boarder looks like a turtle, so a shark will investigate.

It appears that many attacks are by juvenile sharks who might still be learning what constitutes prey. In addition, many bites are on surfboards and kayaks that people just happen to be using. One of the primary preventative measures shark experts now recommend is to wear a brightly colored wetsuit—not blue or black, which makes you resemble a seal.

Sharks prefer to approach an animal from its blind side, sometimes waiting just outside the visual area until the animal turns away. They can detect which way a creature is facing without using sight, so they don’t actually see their prey until they’re sneaking up on it.

Some sharks, like the great white, prefer to come up from below. When looking up from underneath, researchers found the silhouette of a surfer on a board does resemble a fat seal or a sea lion. Even a person swimming horizontally can look like a seal. In addition, shark eyes aren’t as sharp as ours, so things look blurrier to them. They mainly rely on their other senses, and incidents often happen when there’s poor visibility. Bull sharks are good at hunting under murky conditions. People and prey also sound similar when moving in the water.

Sharks and other sea creatures that are horizontal and roughly shaped like torpedoes with faces on the front end must find us vertical creatures very odd with our long, spindly, waving arms and legs, topped by a round head with a strange-looking face on the side, instead of the end. The closest thing they’ll probably see to us is a polar bear, if they get that far north.

Swimmers and scuba divers can at least be more graceful in the water, but most of us playing in the surf or frolicking in the swells are rather awkward and clearly out of our element. To sea creatures we probably look rather like how a spider monkey trying to swim for the first time would look to us. In spite of our awkwardness, we’re clearly drawn to the water, perhaps to the puzzlement of the sharks. Still, we no doubt look more like a seal to them when we’re on a surfboard or in a kayak since our flailing appendages are less prominent. Also their vision isn’t that great.

There are no rogue sharks that are out to get people. It was an Australian surgeon named Sir Victor Coppleson who came up with this idea in his 1958 book, Shark Attack, but the idea didn’t really catch on until the 1975 movie Jaws came out.

Before Jaws, people didn’t think much about sharks. That movie created a massive panic that persists to this day and has resulted in the killing of a tremendous amount of sharks just because they’re thought to be deadly and many people falsely think the world would be better off without them. One study found that in the media, sixty percent of the time sharks are portrayed negatively, while only ten percent discuss their conservation.

Both Stephen Speilberg and Peter Benchley, the author of the book Jaws, deeply regretted the role their work played in the devastation of shark populations, and until he died, Benchley actively worked for their conservation. Benchley insisted his book was fiction, adding, “Sharks don’t target human beings, and they certainly don’t hold grudges.”

A great white shark can reach nineteen feet (5.8 m) in length and weigh more than two tons (2 tonnes). Hermanus Backpackers, CC BY 2.0 (adjusted).

He wrote that once while diving:

I raised my eyes and found myself face to face with, and not five feet away from, a great white shark.[...] I froze,[...] But the shark froze, too. And then abruptly, frantically, implausibly, the great white wheeled around, voided its bowels, and disappeared in a nasty brown cloud.[...] Could the most fearsome predator on earth, the largest carnivorous fish in the sea, have fled from a puny human, from me, like a startled rabbit?

If I have one hope, it is that we will come to appreciate and protect these wonderful animals before we manage, through ignorance, stupidity and greed, to wipe them out altogether.

Sharks have come close to extinction in the distant past, while humans have also barely made it through bottlenecks of near extinction. There’s evidence that humans nearly went extinct at least twice, maybe more times. Despite our current population, there are many ways it could happen again to both us and sharks.

Contrary to the popular false view, sharks are not cruel, single-minded predators intent on ripping people apart in furious feeding frenzies; neither are they torpedoes of death and destruction; nor are they emotion-less psychopathic killers.

Now that scientists are tagging sharks and can track them in real time, we’re learning a lot more about what they’re actually doing.

Researchers have found that great whites usually show little interest in humans. Using drones along Southern California beaches, they saw people and great white sharks in close proximity quite often, with sharks sometimes right next to or below the people. The people appeared to be unaware and the sharks only seemed curious. These were juvenile sharks, which can be up to nine feet long (2.7 m). The adults grow to twenty feet (6 m), or about the length of a full-sized school bus.

On one holiday in Australia there were thousands of people in the water of Sydney Harbor, along with a major swim event. No sharks were spotted that day, but seven adult bull sharks were there and didn’t bother anyone. Without tracking, no one would have known they were in the harbor that day.

This happens often. Researchers regularly discover sharks cruising among swimmers without the swimmers or those on shore ever being aware of their presence, since they keep a low profile. Many sharks don’t like being around people and will flee the area, perhaps because by far we’re their number one predator.

Still, that doesn’t mean it’s safe. Military diver Paul de Gelder survived being bitten by a bull shark in Sydney Harbor, losing his right hand and part of his leg. A few years later, a documentary crew took him to Fiji so they could film him hand-feeding bull sharks without a cage, giving him a chance to confront his fear. He was quickly surrounded by 150 of them. He also swam with great whites without a cage. He later told The Guardian, “Seeing them in their natural environment, changed everything for me.[...] My preconceptions vanished.” He’s now a shark expert, conservationist, and is working hard to protect sharks, explaining, “They’re not vicious, man-eating monsters.”

He has since taken a number of celebrities down to feed the sharks. They can probably get away with it since in those situations the shark would have no doubt that it’s dealing with a human. They approach out of curiosity and to get handouts, but don’t seem to have any interest in biting anyone. De Gelder said they had to take baby steps to get Mike Tyson down there, but once Tyson was there, he kept trying to pet all of the sharks.

People should be afraid of sharks, just as they would be of a lion or bear. They are wild animals that can be dangerous and unpredictable, especially when they feel threatened. But we have to realize that they are not actively hunting us and for the most part would rather leave us alone. But not always.

There are accounts of sharks and rays that seem to enjoy swimming with divers and especially like feeling the bubbles from the diver’s regulators rise up and tickle them from below.

Many divers develop personal relationships with sharks. They quickly learn to recognize individuals and each has its own personality. The sharks have sensitive skin and seem to enjoy being petted. They even seek it out from the divers they know.

A couple of fish keep an eye on a blacktip reef shark. They usually hunt in packs and generally eat small fish, crustaceans, and mollusks. © Elaine Molina Stephens, 2017.

Marine biologist Frauke Bagusche, on her dives in the Maldives, off the southern tip of India, would find herself surrounded by as many as fifteen blacktip reef sharks. Even though they seemed curious and showed no signs of aggression, they could still be startling. In her book The Blue Wonder she wrote, “On a number of occasions their excellent camouflage has given me near heart attacks when I would suddenly discover a shark swimming beside me. It is very difficult to describe the feeling of a shark, with its intelligent eyes, sizing you up.”

In Jonathan Balcombe’s What a Fish Knows, professional diver Cristina Zenato, who has been diving with sharks for twenty years, tells of how she’s often greeted by her favorite shark, Grandma, an eight-foot reef shark, saying, “She has a soft nature, and a way of approaching me with the desire to be petted and touched. She is usually very keen to come to me. Even when somebody else is down there with food and I am some distance away she will approach me before anybody else. Sometimes when I let her go she quickly turns and comes back into my lap.”

Fish—and sharks are fish—are also very sensitive to touch and some enjoy being touched. They can also recognize human faces. Balcombe and others cite anecdotes of numerous fish that rush over to their caretakers or divers to be touched, held, or gently stroked. Some large groupers squirt water at people they don’t like and one does this to anyone who doesn’t rub its head.

Most experts even dislike the term “shark attack”, since in many cases the shark is defending itself, such as when a shark is dragged aboard a boat, or the bites are made out of curiosity. These are not attacks. Calling them attacks implies an intention on the shark’s part, which may not be there.

Some sharks are territorial and when protecting their territory, they’re reluctant to back down. They will give warning signals and displays, which make other creatures retreat, but inexperienced people probably won’t understand these, and that can lead to a dangerous situation.

If you take the number of people in the world and the number of deaths by sharks, you can easily calculate the odds of a person dying from a shark, and these odds are very small. You’re more likely to be killed by a cow than a shark. You’re more likely to be killed by most everything—including vending machines—than a shark. On the other hand, if you’re a surfer, your odds go up quite a bit, since it’s mostly surfers who get bitten or die from sharks.

If you trace your ancestors back 440 million years, you’d find they looked a lot like sharks, because they were the last common ancestor of all jawed vertebrates. In other words, every vertebrate with a jaw, including you, chickens, and Tyrannosaurus rexes, descended from these shark-like fish. These are the most recent ancestors that we also share with sharks and which make sharks our distant cousins. They were called acanthodians, in case you want to add them to your family tree.

 

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