Remarkable Fish: The Most Extraordinary Species in the Sea
There is a creature in the rivers of South America that can generate enough electricity to stun a horse. In the tropical Pacific, a small orange fish changes its biological sex when social hierarchy demands it. Off the coast of Japan, a tiny pufferfish constructs elaborate geometric patterns on the seafloor -- underwater crop circles -- to attract a mate. In the deep Atlantic, a female anglerfish carries the fused, parasitic remains of her male partner permanently embedded in her flesh. And in the mudflats of Southeast Asia, a fish climbs trees.
Fish are, by an enormous margin, the most diverse group of vertebrates on Earth. With over 35,000 known species -- more than all mammals, birds, reptiles, and amphibians combined -- they have colonized virtually every aquatic environment the planet offers: scalding hydrothermal vents, freezing Antarctic waters, lightless ocean trenches, temporary desert puddles, underground caves, and even the thin film of water on a muddy riverbank. They range in size from the 7.9-millimeter Paedocypris progenetica, a translucent fish from the peat swamps of Sumatra that is the smallest known vertebrate, to the 12-meter whale shark that cruises tropical seas filtering plankton.
Yet despite their extraordinary diversity and ecological dominance, fish remain curiously underappreciated. Mammals get documentaries. Birds get field guides. Fish get fish sticks. This is a guide to some of the most remarkable fish alive today -- species whose adaptations challenge assumptions about what a vertebrate body can do.
Electric Eels: Living Batteries of the Amazon
The electric eel (Electrophorus electricus) is one of the most misnamed animals in biology. It is not an eel. It is a knifefish, more closely related to catfish and carp than to true eels. It breathes air, surfacing every ten minutes or so to gulp oxygen through its heavily vascularized mouth. And it can produce the most powerful electrical discharge of any known animal.
The Voltage
An adult electric eel can generate up to 860 volts in a single discharge -- a figure confirmed by Kenneth Catania, a neuroethologist at Vanderbilt University, whose meticulous research over the past decade has fundamentally reshaped scientific understanding of these animals. The eel's body contains three specialized electric organs -- the main organ, the Hunter's organ, and the Sachs' organ -- composed of thousands of disc-shaped cells called electrocytes arranged in series, like batteries stacked in a flashlight. When the eel's brain sends a signal, all electrocytes fire simultaneously, producing a brief but powerful pulse.
The eel uses three distinct types of electrical discharge. Low-voltage pulses from the Sachs' organ serve as a navigational and communication system, functioning like sonar in murky Amazonian waters where visibility is essentially zero. High-voltage doublets -- pairs of rapid pulses -- are used to locate hidden prey. These doublets cause involuntary muscle twitches in nearby fish, revealing their position through the resulting water disturbance. The full high-voltage discharge is reserved for stunning prey or deterring predators.
The Leap Attack
Catania's most startling discovery, published in the Proceedings of the National Academy of Sciences in 2016, documented a behavior that had been described anecdotally by Alexander von Humboldt in 1800 but dismissed by scientists for over two centuries. When confronted with a large, partially submerged threat, electric eels leap from the water and press their chins against the target, delivering a concentrated electrical shock directly through the body of the perceived attacker. This behavior dramatically increases the effective voltage delivered to the target because the circuit runs through the animal's body rather than dissipating through surrounding water.
"The eels were doing something that had been reported over 200 years ago and dismissed as a myth. They were leaping out of the water to attack." -- Kenneth Catania, Vanderbilt University (2016)
Catania measured the electrical output during these leap attacks and found that the effective power delivered to a target increased as the eel rose higher out of the water, reaching levels sufficient to cause intense pain and involuntary muscle contractions in a human arm. The discovery confirmed Humboldt's account of eels attacking horses during a dramatic collecting expedition in Venezuela -- a story that had been treated as exaggeration for generations.
Clownfish: Sex, Anemones, and the Finding Nemo Problem
The clownfish (genus Amphiprion, approximately 30 species) is among the most recognizable fish on Earth, thanks largely to a 2003 Pixar film. But the biology of real clownfish is far stranger than any animated screenplay.
Sequential Hermaphroditism
All clownfish are protandrous sequential hermaphrodites -- every individual is born male and possesses the biological machinery to become female. Within each anemone colony, a strict dominance hierarchy determines who breeds. The largest individual is always female -- the sole breeding female of the group. The second-largest is the breeding male. All remaining fish are smaller, non-reproductive males held in a state of developmental suppression by the dominant pair's hormonal and behavioral signals.
When the breeding female dies, the breeding male undergoes an irreversible physiological transformation. Over a period of weeks, his testes are reabsorbed and replaced by functional ovaries. He grows larger, becomes more aggressive, and assumes the role of dominant female. The largest of the remaining non-breeding males then matures into the new breeding male. This system ensures that the group always has a breeding pair without the energetic cost of searching for a mate in the open ocean -- a critical advantage for a fish that rarely strays more than a few meters from its host anemone.
Anemone Immunity
The mutualistic relationship between clownfish and sea anemones is one of the most studied symbioses in marine biology. Anemone tentacles are armed with nematocysts -- microscopic, spring-loaded stinging cells that inject venom into anything that contacts them. Most fish that touch an anemone are paralyzed and consumed. Clownfish are immune.
The mechanism of this immunity involves a specialized mucus coating on the clownfish's skin. Research has shown that clownfish mucus lacks the specific chemical compounds -- primarily N-acetylneuraminic acid -- that trigger nematocyst discharge. The fish essentially becomes chemically invisible to the anemone's defense system. Newly introduced clownfish perform an elaborate acclimation ritual, gently brushing against the anemone's tentacles with increasing contact over several hours, gradually coating themselves with the anemone's own mucus and adjusting their own mucus chemistry.
The Finding Nemo Effect
The 2003 release of Finding Nemo triggered a surge in demand for wild-caught clownfish for the home aquarium trade. Studies documented increases of 40% or more in clownfish sales in the months following the film's release. In some regions, particularly in the Philippines, Indonesia, and parts of the Indian Ocean, wild clownfish populations declined measurably. The irony -- a film whose central message was that wild animals belong in the ocean, not in captivity -- was not lost on marine biologists. Captive breeding programs have since reduced pressure on wild populations, but the aquarium trade continues to drive collection of clownfish and the anemones they depend upon.
Pufferfish: Poison, Cuisine, and Underwater Architecture
The pufferfish (family Tetraodontidae, over 120 species) are best known for two things: inflating into spiny spheres when threatened, and containing one of the deadliest toxins in the natural world. But one species has recently earned fame for something entirely unexpected -- art.
Tetrodotoxin
Pufferfish carry tetrodotoxin (TTX) -- a neurotoxin that blocks sodium channels in nerve cell membranes, preventing neurons from firing and causing progressive paralysis. Tetrodotoxin is approximately 1,200 times more toxic than cyanide. A single pufferfish contains enough toxin to kill 30 adult humans, and there is no known antidote. Victims remain fully conscious as paralysis spreads from the extremities inward, eventually reaching the diaphragm and causing death by respiratory failure.
The toxin is not produced by the pufferfish itself but is synthesized by symbiotic bacteria -- primarily species of Vibrio and Pseudoalteromonas -- that colonize the fish's liver, ovaries, and skin. Pufferfish raised in captivity without access to these bacteria are non-toxic, confirming the bacterial origin. The fish sequesters the toxin as a chemical defense, concentrating it in the organs most likely to be consumed by a predator.
Fugu: Dining with Death
In Japan, pufferfish is consumed as a luxury delicacy called fugu. The preparation of fugu is legally restricted to chefs who have completed a rigorous training and licensing program lasting two to three years, culminating in a practical examination in which the chef must prepare and eat their own fugu. Licensed fugu chefs learn to remove the toxic organs -- particularly the liver, ovaries, and skin -- with surgical precision, leaving only the non-toxic muscle tissue. Despite these precautions, fugu poisoning incidents still occur, with an average of approximately 30 to 50 cases per year in Japan, resulting in several fatalities annually.
Underwater Crop Circles
In 1995, divers off the coast of Amami-Oshima Island, Japan, discovered intricate geometric patterns on the sandy seafloor -- perfectly symmetrical circular structures approximately 2 meters in diameter, with radiating ridges and valleys. The structures were so precise and so unexpected that they were initially dubbed "mystery circles." It took nearly two decades before the architect was identified: a newly described species of white-spotted pufferfish (Torquigener albomaculosus), barely 12 centimeters long.
Males construct these elaborate structures over a period of seven to nine days, working ceaselessly to sculpt the sand using their fins. The ridges and valleys are not random but serve a hydrodynamic function -- they channel water currents toward the center of the circle, where the female will deposit her eggs. Fine sediment particles, carried by these currents, accumulate in the central nest, providing a soft substrate for the eggs. The male also decorates the ridges with fragments of shells and coral. Females inspect multiple circles before choosing a mate, preferring larger, more symmetrically constructed structures. After spawning, the male guards the eggs until they hatch, then abandons the circle and begins constructing a new one from scratch.
Flying Fish: Escaping Predators on Wing-Like Fins
The approximately 70 species of flying fish (family Exocoetidae) have evolved one of the most dramatic predator-evasion strategies in the animal kingdom: they leave the water entirely.
The Mechanics of Flight
Flying fish do not truly fly -- they glide. But their gliding performance is extraordinary by any standard. The process begins underwater, where the fish builds speed using rapid tail beats, reaching velocities of approximately 60 to 70 kilometers per hour (37 to 43 mph). As it approaches the surface, it spreads its greatly enlarged pectoral fins, which function as rigid, cambered airfoils. Upon breaking the surface, the fish becomes airborne, gliding on these wing-like fins at heights of up to 1.2 meters (4 feet) above the water.
| Feature | Flying Fish | Comparable Aircraft |
|---|---|---|
| Glide distance | 200+ meters (650+ feet) | Hang glider: 200-300 meters per 100m altitude |
| Top speed | 70 km/h (43 mph) | Ultralight aircraft takeoff: 65 km/h |
| Flight duration | Up to 45 seconds | -- |
| Wing loading | Similar to birds of comparable mass | -- |
| Lift-to-drag ratio | Approximately 4:1 | Hang glider: 10-15:1 |
Some species, known as four-winged flying fish, possess enlarged pelvic fins in addition to pectoral fins, providing a second pair of "wings" that increases total lift surface and allows for longer, more controlled glides. When a glide begins to lose altitude, the fish can dip its elongated lower tail lobe into the water and vibrate it rapidly -- a behavior called taxiing -- to generate additional thrust and extend the flight without fully re-entering the water. Through successive taxiing sequences, a single flight event can cover distances exceeding 400 meters.
Flying fish are a significant food source for many oceanic predators, including tuna, marlin, swordfish, dolphins, and seabirds. In Barbados, where flying fish are a national symbol and a dietary staple, the species is culturally and economically important -- the island was once known as "The Land of the Flying Fish."
Anglerfish: Nightmares of the Deep
The deep-sea anglerfish (order Lophiiformes, over 200 species) represent some of the most extreme adaptations to life in perpetual darkness. Found at depths of 1,000 to 4,000 meters, where no sunlight penetrates and food is scarce, anglerfish have evolved solutions to the problems of deep-sea existence that border on the grotesque.
The Bioluminescent Lure
Female anglerfish possess a modified dorsal fin spine -- the illicium -- that extends forward over the head like a fishing rod, tipped with a fleshy, bioluminescent bulb called the esca. The light is produced not by the fish itself but by symbiotic bioluminescent bacteria (primarily Photobacterium species) housed within the esca. The fish can control the light's intensity and flashing pattern by regulating blood flow to the organ, effectively switching its lure on and off.
In the lightless deep ocean, this glowing lure attracts prey -- small fish, crustaceans, and other organisms drawn to any source of light in an otherwise black environment. When prey approaches within striking distance, the anglerfish engulfs it with a rapid expansion of its enormous jaws and highly distensible stomach, capable of swallowing prey up to twice the anglerfish's own body length.
Sexual Dimorphism and Male Parasitism
The reproductive biology of the ceratioid anglerfish (suborder Ceratioidei) is among the most extreme in the vertebrate world. Females are the large, predatory individuals most people picture when they think of anglerfish -- bulbous, dark, jawed, and lured. Males are something else entirely.
Male ceratioid anglerfish are tiny -- often less than one-tenth the size of females, and in some species barely a centimeter long. They possess enormous nostrils and well-developed olfactory systems, adapted for a single purpose: locating a female by following species-specific pheromone trails through the vast darkness. When a male finds a female, he bites into her flesh and releases enzymes that dissolve the skin of his mouth and her body, fusing the two organisms together. The male's circulatory system merges with the female's. His eyes, fins, and most internal organs degenerate. He becomes, in essence, a permanently attached parasitic sperm-producing organ, nourished by the female's blood supply and providing sperm on hormonal demand.
A single female may carry multiple fused males simultaneously. This system, while alien to human sensibility, is a rational adaptation to a fundamental problem of deep-sea life: in a vast, dark, sparsely populated environment, finding a mate is so unlikely that once you do, you should never let go.
Ocean Sunfish: The Heaviest Bony Fish on Earth
The ocean sunfish (Mola mola) looks like an evolutionary accident -- a massive, disc-shaped fish that appears to be missing its back half. It is, in fact, one of the most unusual vertebrates alive, and the heaviest bony fish in the world.
Adult ocean sunfish reach weights of up to 2,300 kilograms (5,070 pounds) and lengths of 3.3 meters (10.8 feet) from dorsal fin tip to anal fin tip. The body is laterally compressed into a flattened oval, and the tail fin -- present in most fish as the primary propulsive structure -- has been evolutionarily replaced by a stiff, rudder-like structure called the clavus, formed by the fusion of the dorsal and anal fins. Ocean sunfish swim by sculling their tall dorsal and anal fins from side to side, a locomotion method that is slow but surprisingly efficient for open-ocean cruising.
Reproductive Output
Female ocean sunfish produce more eggs than any other known vertebrate -- an estimated 300 million eggs in a single spawning event. Each egg is tiny, approximately 1.3 millimeters in diameter, and the vast majority will be consumed by predators or fail to develop. A newly hatched ocean sunfish larva weighs less than a gram and must increase its body mass by over 60 million times to reach adult size -- the greatest size increase of any vertebrate from birth to maturity.
Despite their enormous size, ocean sunfish feed primarily on jellyfish, salps, and other gelatinous zooplankton -- a nutritionally poor diet that requires the fish to consume large quantities. They are frequently observed basking at the ocean surface, lying on their sides -- a behavior that may serve to warm their bodies after deep dives into cold water, or to attract seabirds that pick parasites from their skin.
Archer Fish: Physics in the Mangroves
The archer fish (genus Toxotes, approximately seven species) of Southeast Asia and northern Australia have evolved a hunting technique that requires an intuitive understanding of optics. They shoot down insects from overhanging vegetation by spitting precisely aimed jets of water from their mouths.
Compensating for Refraction
The physics of this feat is more complex than it appears. Light bends when it passes between water and air -- a phenomenon called refraction -- which means that an insect viewed from underwater appears to be in a different position than it actually is. Archer fish must compensate for this optical distortion to hit their targets accurately. Experimental research has demonstrated that archer fish learn to correct for refraction from different angles, adjusting their aim based on the apparent versus actual position of prey. They can hit targets at distances of up to 3 meters (10 feet) above the water surface with remarkable accuracy.
The water jet itself is shaped by the fish pressing its tongue against a groove in the roof of its mouth, creating a tube through which water is expelled by a rapid compression of the gill covers. The fish modulates the speed of the water during the spit so that the rear of the jet travels faster than the front, causing the water to coalesce into a single large droplet just before impact. This focusing effect delivers maximum force to the target -- enough to knock an insect off a leaf and into the water below.
"The archer fish has essentially solved a ballistics problem that involves fluid dynamics, optics, and gravity -- and it does so with a brain smaller than a pea." -- Stefan Schuster, University of Bayreuth (2006)
Young archer fish are poor shots and improve their accuracy through practice and, remarkably, through social learning -- juvenile fish that observe experienced adults shooting at targets learn to aim correctly faster than those that must discover the technique independently.
Mudskippers: Fish That Climb Trees
The mudskippers (subfamily Oxudercinae, approximately 32 species) are perhaps the most compelling living illustration of how vertebrates may have first colonized land. These small gobies of tropical and subtropical tidal flats spend the majority of their active lives out of water, walking, climbing, and even jumping across mud, rocks, and mangrove roots.
Amphibious Adaptations
Mudskippers breathe through multiple mechanisms simultaneously. They retain water in enlarged gill chambers that function as primitive lungs, exchanging gas across the moistened gill surfaces. They also absorb oxygen directly through their skin -- a process called cutaneous respiration -- provided the skin remains moist. Some species can absorb up to 50% of their required oxygen through the skin alone.
Their pectoral fins have evolved into muscular, arm-like limbs capable of supporting the fish's weight and propelling it across solid surfaces in a crutching gait. Mudskippers can also perform powerful jumps by curling their bodies and snapping their muscular tails against the ground, launching themselves up to 60 centimeters (2 feet) into the air. Several species are accomplished climbers, ascending mangrove roots, rocks, and even tree trunks using their pectoral fins and a pelvic sucker that functions as an adhesive disc.
Mudskippers are highly territorial. Males defend mudflat territories by engaging in aggressive displays that include raising their brightly colored dorsal fins, performing rapid push-ups on their pectoral fins, and engaging in mouth-wrestling combat with rivals. They construct elaborate burrows in the mud that serve as shelters during high tide and as nesting chambers -- males carry mouthfuls of air into submerged burrows to create air pockets where eggs can develop in an oxygen-rich environment even when the burrow is flooded.
Seahorses: Male Pregnancy and Pair Bonding
The seahorses (genus Hippocampus, approximately 46 species) are among the most anatomically unusual fish alive. With their horse-shaped heads, prehensile tails, upright swimming posture, and independently moving eyes, they look more like chess pieces than fish. But their most extraordinary feature is reproductive.
Male Pregnancy
Seahorses are the only animals in which the male becomes truly pregnant. The female deposits her eggs into the male's brood pouch -- a vascularized, enclosed structure on his ventral surface -- where he fertilizes them internally. The pouch then seals shut and functions as a womb. The male controls the internal environment, regulating oxygen levels, salinity, and nutrient delivery through a network of blood vessels in the pouch lining. Over a gestation period of two to four weeks depending on species, the embryos develop fully within the male's body. He then undergoes muscular contractions -- labor -- and expels fully formed miniature seahorses, sometimes numbering in the hundreds or even over a thousand in a single brood.
Pair Bonding and Daily Rituals
Many seahorse species form long-term pair bonds that are reinforced through elaborate daily greeting rituals. Each morning, bonded pairs meet and perform a synchronized dance, changing colors, intertwining tails, and swimming side by side for several minutes. These daily rituals are believed to synchronize reproductive cycles and reinforce the pair bond. If a partner dies, the surviving seahorse may take an extended period before bonding with a new mate.
Camouflage
Seahorses are masters of concealment. Many species can change color to match their surroundings, and some have taken camouflage to an extreme. The pygmy seahorses (Hippocampus bargibanti and related species), which measure barely 2 centimeters in length, live exclusively on gorgonian sea fans and have evolved body shapes, colors, and skin textures so perfectly matched to their host coral that they were only discovered when a scientist collected a sea fan and found seahorses clinging to it in the laboratory.
The Overfishing Crisis: Emptying the Oceans
The extraordinary fish described in this article exist within an increasingly threatened global ecosystem. Industrial fishing has transformed the world's oceans over the past century, and the numbers are stark.
A landmark study led by Ransom Myers and Boris Worm, published in Nature in 2003, estimated that 90% of all large predatory fish -- tuna, swordfish, marlin, sharks, and large groundfish such as cod and halibut -- had been removed from the world's oceans since the onset of industrialized fishing in the 1950s. Subsequent research has refined but not fundamentally altered this conclusion. The Food and Agriculture Organization of the United Nations reported in 2022 that 35.4% of global fish stocks are overfished -- exploited beyond sustainable levels -- while an additional 57.3% are fished at maximum sustainable capacity, leaving virtually no room for increased exploitation.
The consequences extend far beyond the targeted species. Bottom trawling -- dragging heavy nets across the seafloor -- destroys benthic habitats including coral reefs, sponge gardens, and seagrass beds. Bycatch -- the unintentional capture of non-target species -- kills an estimated 38 million tonnes of marine animals annually, including sea turtles, dolphins, seabirds, and juvenile fish. Ghost nets -- abandoned or lost fishing gear -- continue to entangle and kill marine life for decades.
The collapse of the Atlantic cod fishery off Newfoundland in 1992 remains the most dramatic example of what happens when extraction exceeds regeneration. A fish population that had sustained communities for 500 years was reduced to less than 1% of its historical biomass in a matter of decades. The Canadian government imposed a moratorium on cod fishing in 1992 -- it has never been fully lifted, and the cod have never fully recovered.
Climate change compounds the crisis. Rising ocean temperatures are shifting species distributions poleward, disrupting food webs, and causing mass coral bleaching events that destroy the reef habitats upon which thousands of fish species depend. Ocean acidification -- caused by the absorption of atmospheric carbon dioxide -- threatens the ability of marine organisms to build calcium carbonate shells and skeletons, with cascading effects throughout marine food webs.
Conclusion
The fish described in these pages represent a fraction of the diversity and ingenuity contained within the world's waters. Electric eels that weaponize bioelectricity. Clownfish that rewrite their own sex. Pufferfish that carry enough poison to kill a classroom and build sand castles to impress a mate. Anglerfish whose males dissolve into their partners. Sunfish that produce 300 million eggs and start life smaller than a fingernail. Archer fish that solve physics problems. Mudskippers that walk and climb and breathe through their skin. Seahorses in which fathers give birth.
These are not marginal curiosities. They are the products of hundreds of millions of years of evolution in the most competitive and dynamic environment on Earth. They are evidence that the vertebrate body plan is far more flexible, far more inventive, and far more strange than terrestrial life alone would suggest.
Whether they persist depends entirely on what happens in the next few decades. The ocean is not inexhaustible. The fish within it are not replaceable. And the loss of any one of these species -- each one a unique solution to the problem of being alive in water -- would be a loss not just for marine biology, but for the full catalog of what life on this planet has managed to become.
References
Catania, K. C. (2016). "Leaping eels electrify threats, supporting Humboldt's account of a battle with horses." Proceedings of the National Academy of Sciences, 113(25), 6979-6984.
Buston, P. M. (2004). "Territory inheritance in clownfish." Proceedings of the Royal Society B: Biological Sciences, 271(Suppl 4), S252-S254.
Kawase, H., Okata, Y., & Ito, K. (2013). "Role of huge geometric circular structures in the reproduction of a marine pufferfish." Scientific Reports, 3, 2106.
Myers, R. A., & Worm, B. (2003). "Rapid worldwide depletion of predatory fish communities." Nature, 423(6937), 280-283.
Schuster, S., Wohl, S., Griebsch, M., & Klostermeier, I. (2006). "Animal cognition: How archer fish learn to down rapidly moving targets." Current Biology, 16(4), 378-383.
Food and Agriculture Organization of the United Nations. (2022). The State of World Fisheries and Aquaculture 2022. FAO, Rome.
Pietsch, T. W. (2005). "Dimorphism, parasitism, and sex revisited: modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes)." Ichthyological Research, 52(3), 207-236.
Frequently Asked Questions
How much voltage can an electric eel produce, and can it kill a human?
Electric eels (Electrophorus electricus) can generate up to 860 volts in short bursts, as measured by researcher Kenneth Catania at Vanderbilt University. While this voltage is substantial -- roughly seven times the output of a standard North American wall outlet -- the amperage is relatively low, typically around 1 ampere for only a few milliseconds. This means a single discharge is unlikely to kill a healthy adult human directly, though it can cause involuntary muscle contractions severe enough to cause drowning in shallow water, or cardiac complications in individuals with pre-existing heart conditions. Catania's research also demonstrated that electric eels can leap from the water to press their chins against perceived threats, delivering a more concentrated shock by completing the circuit through the target's body rather than through surrounding water.
How far can flying fish actually glide, and how do they stay airborne?
Flying fish (family Exocoetidae) do not truly fly but rather glide using greatly enlarged, wing-like pectoral fins. After building speed underwater at approximately 60 to 70 kilometers per hour and breaking the surface at a steep angle, they spread their rigid pectoral fins and become airborne. Documented glides have covered distances exceeding 200 meters (over 650 feet) and lasted up to 45 seconds, with the fish occasionally dipping its elongated lower tail fin into the water to generate additional thrust and extend the glide without fully re-entering. Some species, known as four-winged flying fish, also have enlarged pelvic fins that provide additional lift surface, allowing for longer and more controlled flights.
Is it true that all clownfish can change sex, and how does this work?
Yes, all clownfish (genus Amphiprion) are sequential hermaphrodites, meaning they have the biological capacity to change sex during their lifetime. Specifically, they are protandrous hermaphrodites -- they are all born male and can transition to female. In each anemone group, the largest and most dominant individual is the breeding female, the second largest is the breeding male, and all remaining individuals are non-reproductive males. When the dominant female dies, the breeding male undergoes a permanent sex change to become the new female, and the next-largest non-breeding male matures into the new breeding male. This transition involves hormonal changes that alter gonad structure and is irreversible once complete. This biological reality means that in an accurate retelling of Finding Nemo, Marlin would have become female after the loss of his mate.