Fish have evolved to be expert navigators in both marine and freshwater environments. Today, there are dozens of thousands of extant fish species roaming our oceans, seas, lakes, and rivers. They may be classified as bony fish (superclass Osteichthyes), cartilaginous fish (superclass Chondrichthyes), and jawless fish (superclass Agnatha). Each of these groups boasts a number of massive species, many of which have become extinct.
The oldest fish species first appeared more than 500 million years ago, during the Ordovician period. Fish have evolved to take on all forms and sizes. Most of them are generally streamlined as their shape influences the speed with which they can travel and escape from predators. Some of them maintain a carnivorous diet, whereas others have developed a taste for the tiniest of crustaceans.
Many of the largest, extant species of fish are actually sharks. These cartilaginous species have cemented their status as some of the fiercest predators on earth. Others are known as gentle giants – mostly harmless to humans yet able to grow to several times our size. Whatever their preferences, these species (both extant and extinct) are a testament to the diversity and ecological complexity of our planet’s waters.
1) Leeds’ fish (Leedsichthys problematicus)
Likely the largest fish to have ever roamed our oceans, Leedsichthys problematicus was a bony, ray-finned member of the extinct Pachycormidae family. With remains that have been mistaken for that of a dinosaur, it is estimated to have measured about 54 feet (16.5 meters) long. This behemoth existed during the middle Jurassic to late Cretaceous period and may have disappeared due to climate change.
Often referred to as the “Leeds’ Fish” in honor of the paleontologist Alfred Nicholson Leeds, this species has baffled scientists for decades. Its fossil remains are largely fragmented, making it remarkably challenging to form conclusions on the true size and form of its skeleton. It appears that its skull and neural arch were mostly cartilaginous. Moreover, its jaw did not possess any teeth. This suggests that, like some of today’s extant filter feeders, it subsisted on zooplankton and other tiny marine animals.
2) Megalodon (Otodus megalodon)
An extinct member of the mackerel shark family (Lamnidae), the megalodon may have grown to a length of 34 – 67 feet (10.4 – 20.4 meters). This behemoth’s jaws and teeth are often modeled and displayed in museums to represent this species’ sheer size. Given its massive gape, it was most likely an apex predator that fed on large sea creatures, like small whales, other sharks, and unfortunate seals.
The megalodon existed during the Miocene and Pliocene epochs, which date back to more than 3.6 million years ago. Carnivorous, it significantly influenced the food chain wherever it was found in large numbers. Generally, it may have looked like a much larger version of the basking shark or the great white shark. As the monster in the wildly successful blockbuster “Jaws” is definitely bigger than any extant shark, some theories suggest that it may have been modeled after a megalodon.
3) Whale shark (Rhincodon typus)
Often misidentified as a whale, largely due to its common name, morphology, and size, the whale shark is actually a type of cartilaginous fish. This gilled giant is the largest living vertebrate outside of class Mammalia. Readily distinguished by its unique patterns of spots, which researchers use to differentiate one individual from another, it is found in the pelagic zones of moderately warm habitats.
The whale shark can grow to an average length of about 18 – 32 feet (5.5 – 9.8 meters). Despite its massive size, this fish has no interest in humans as food. Its flattened jaws are lined with tiny, densely packed teeth and filter pads meant for collecting krill, copepods, and fish larvae. Perceived as a gentle giant, the whale shark is known for being quite safe around swimmers and scuba divers in tropical reefs. In fact, in some parts of the Philippines, it is controversially considered the main tourist attraction.
4) Basking shark (Cetorhinus maximus)
Like the whale shark, the basking shark is a migratory filter feeder. It feeds by opening its enormous mouth close to the water’s surface, where it may appear to be basking under the warmth of the sun. As seawater moves into its mouth and exits through its gills, its gill rakers catch zooplankton, small fish, and other tiny invertebrates. An average-sized, actively feeding specimen can filter around 430 tons of water within a single hour!
Once commercially used as a source of hide, oil, meat, and fishmeal, the basking shark is now an endangered species. This large fish, which can grow to about 26 feet (8 meters) long and weigh as much as 4.65 tons, favors the pelagic zones of productive coasts. Though it may be found at depths of more than a thousand feet in winter, it may occasionally enter shallow, brackish waters to feed.
5) Beluga sturgeon (Huso huso)
One of the largest species of bony fish, the beluga sturgeon is an anadromous member of the Acipenseridae family. Individuals found in freshwater environments are considered the lengthiest fish of their habitats.
A female that was collected from Russia’s Volga Estuary measured around 24 feet (7.3 meters) long and weighed more than 1,500 kg (3,300 pounds). Specimens that measure more than 10 feet (3 meters) long are now extremely rare due to their slow rate of growth and their overexploitation as a source of caviar.
The beluga sturgeon is now a critically endangered species and is largely extirpated from most regions of its natural range. This long-lived fish reaches sexual maturity at 12 – 22 years of age, with females requiring a longer developmental period, so adult specimens with viable eggs are exceedingly rare. To meet North American demand for its extremely expensive roe, Sturgeon Aquafarms (the only beluga sturgeon farm in the US) was provided with a license to grow this species.
6) Southern sunfish (Mola alexandrini)
The southern sunfish is also known as the giant or bumphead sunfish. One of the more peculiar types of large, oceanic fish, this species has an unusual morphology. Instead of having a well-balanced arrangement of fins, it has no caudal or pelvic fins at all. Instead, its heavy body is propelled by a pair of bony, median fins (one dorsal and one anal fin) close to where a tail should be. Though these fins may look incredibly awkward and misplaced, they actually help this species take vertical movements through the water column.
Capable of weighing as much as 6,000 pounds (2,700 kg), the southern sunfish is truly one of the heaviest fish to have ever existed! More spherical than long when viewed from its side, it can look much like a suspended moon from a distance. Living specimens are often misidentified as a more popular congener, the ocean sunfish.
7) Ocean sunfish (Mola mola)
A first encounter with an ocean sunfish, be it through media or in-person, is an eye-opening experience for just about anyone, regardless of their age or background. This morphological oddity has had much to teach researchers in terms of swimming efficiency and functional adaptations in marine environments. With a height that rivals its body’s width, it lacks an actual tail fin. Instead, it has a pseudotail, which looks much like a reduced caudal fin and may function as a rudder.
Able to leap 10 feet (3 meters) into the air, dive to depths of 600+ feet (183 meters), and bask horizontally on the water’s surface, the ocean sunfish’s laterally flattened body can move actively through the water. Its largest recorded specimens have measured around 10.8 feet (3.3 meters) long and weighed as much as 5,000 pounds (2,270 kg). White to grey in color, it may have a slightly mottled appearance.
8) Great white shark (Carcharodon carcharias)
One of the most feared animals in the world, the great white shark is a ferocious apex predator. Found in both coastal and offshore locations, this massive fish is set apart by its torpedo-like shape, its bulky build, and its remarkably sharp set of upper and lower teeth. Due to its reputation as a dangerous fish, one which can severely harm humans, it has been the subject of much research. Despite its fearsome nature, its populations are vulnerable to anthropogenic disturbances.
The only living member of its genus, Carcharodon, the great white shark is “great” indeed. With a lifespan of up to 70 years or more, it can grow to a maximum length of about 20 feet (6 meters) and weigh up to 5,000 pounds (2,270 kg). Given its sheer size, it can readily feed on considerably large prey. Many marine dolphins and seals may struggle to escape this fish’s tough jaws. Its only predator, which reaches greater average lengths in comparison, is the orca (Orcinus orca).
9) Giant oceanic manta ray (Mobula birostris)
Arguably one of the most visually majestic swimmers in open water, the giant oceanic manta ray is the largest member of the superorder Batoidea. This endangered cartilaginous fish is distinguished by its flattened appearance and its triangular extensions, which function as forceful, pectoral fins. These can flap up and down in water, much like wings. Incredibly graceful, these give the fish a wingspan that may measure as much as 29 feet (8.8 meters) long!
Despite its generally massive size, this manta ray has evolved to subsist entirely on plankton and other tiny animals found in the water column. It feeds by swimming with its mouth wide open and by using specialized lobes to control the flow of water. A steady swimmer, it may migrate on its own or in groups with dozens of individuals.
10) Greenland shark (Somniosus microcephalus)
The elusive Greenland shark is an anatomically large member of the Somniosidae family of sleeper sharks. Its females, which tend to grow larger than males, can measure up to 23 feet (7 meters) long and weigh as much as 3,000 pounds (1,360 kg). Characterized by tiny eyes, a rounded snout, and relatively small fins and gills for its size, it is known for being a slow swimmer. Despite its reduced speed, however, it can use its large buccal cavity to suck in its prey and consume them whole.
Greenland sharks are able to feed on squid, eels, smaller sharks, and protein-rich fish found throughout their native range. These generalist feeders may also actively hunt pinnipeds. Given their preference for cool temperatures, usually just above freezing, they have a markedly slow metabolic rate. Another remarkable fact about this large fish is its longevity. Able to live for more than 200 years, it is the most long-lived of all vertebrates.
11) Tiger shark (Galeocerdo cuvier)
The marine counterpart of a jungle tiger, this voracious hunter is known for preying on the widest range of prey items. No fish, turtle, sea snake, cephalopod, or dolphin is exempt from its non-selective appetite. Virtually anything that moves and can fit into its mouth may be targeted. This macropredator may occasionally attempt to consume non-living, manmade items as well. Unfortunately, as it continues to be fished in some areas, its populations are near-threatened.
Female tiger sharks can grow to a maximum length of around 18 feet (5.5 meters) and weigh as much as 3,000 pounds (1,360 kg). Males, which are comparably smaller, rarely grow to more than 12 feet (3.7 meters) long. Unsurprisingly, their growth rate is largely dependent on the availability of food. Their success as predators is due, in part, to their well-evolved dentition. Their teeth, which point to the side, can slice through thick exoskeletons and bone.
12) River beluga (Huso dauricus)
Also known as the “Kaluga”, the river beluga is one of the largest anadromous sturgeons. In terms of size, it is slightly smaller than the beluga sturgeon. Likewise once extensively hunted for its commercially valuable roe, its wild populations are now critically endangered. Though it is protected all throughout its native range, it continues to be illegally poached by determined fishers.
The river beluga has a maximum length of around 18.6 feet (5.7 meters). Distinguished by its rows of dorsal scutes and its triangular head, it has a flexible mouth with a gape that is quite large relative to its body size. Its young feed on insect larvae, small crustaceans, and zooplankton in freshwater habitats. Its adults, which spawn in estuarine locations, can feed on fish and shellfish.
13) Giant armored fish (Dunkleosteus terrelli)
The giant armored fish was a massive placoderm that existed more than 350 million years ago (during the Late Devonian period). Believed to have been one of the first oceanic species to cement its role as an apex predator, it is estimated to have reached a maximum size ranging anywhere from 13 – 33 feet (4 – 10 meters) long. This hefty fish likely weighed more than a ton at sexual maturity. Its largest specimens may have even maxed out at 4 tons!
The class (Placodermi) to which this species belongs is now wholly extinct. It was notable for including plate-skinned or armored prehistoric fish. These species had tough skull plates and jaws that could open and close at high speed. Their strong bite force, which may have ranged between 4400 N to 5300 N, is one of the greatest among all animals. One might surmise that this easily made up for their lack of teeth.
14) Chinese paddlefish (Psephurus gladius)
Recently declared to be extinct in the wild, the Chinese paddlefish was a gigantic freshwater vertebrate. This fish may have grown to lengths of up to 23 feet (7 meters) in the Yangtze and Yellow rivers. Its impressive length is due, in part, to its narrow, paddle-shaped rostrum. This usually measured as much as a third of its entire body length. Its skull was also elongated and is more narrow than that of its only living relative, the American paddlefish (Polyodon spathula).
The Chinese paddlefish maintained a mostly solitary lifestyle. It relied on its strong swimming skills and on passive electroreception to deftly capture prey. Cyprinids, catfish, anchovies, and gobies were some of its favorite treats.
15) Marlin (Istiompax and Makaira spp.)
Marlins are some of the largest ray-finned fishes. Three notable species – the Indo-Pacific blue marlin (M. mazara), black marlin (I. indica), and Atlantic blue marlin (M. nigricans) – are able to grow to about 16 feet (5 meters) long and may weigh more than 2,000 pounds (907 kg). These members of the Istiophoridae family are set apart by their pointed bills, slender shape, lengthy dorsal fin, and crescent-shaped tail.
Exceptional swimmers, marlins are popular as sporting fish and are believed to be some of the fastest fish on the planet. Some species may boast sustained swimming speeds of more than 30 km (18.6 miles) per hour; their burst speeds may go up to 128 km (80 miles) per hour. They are often compared to and mistaken for sailfish and spearfish, which are their close relatives. Wherever they are found, these fish make for valuable catches as they pack a hefty mass of high-protein flesh.
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