Fish locomotion

Fish, like these yellowfin tuna, use many different mechanisms to propel themselves through water
Like a plane or submarine, a fish has six degrees of freedom
Eels propagate a more or less constant-sized flexion wave along their slender bodies.
Tunas such as the bluefin swim fast with their large crescent-shaped tails.
Boxfish use median-paired fin swimming, as they are not well streamlined, and use primarily their pectoral fins to produce thrust.
Porcupine fish (here, Diodon nicthemerus) swim by undulating their pectoral fins.
Gymnotus maintains a straight back while swimming to avoid disturbing its electric sense.
Sharks are denser than water and must swim continually to maintain depth, using dynamic lift from their pectoral fins.
Flying fish gain sufficient lift to glide above the water thanks to their enlarged pectoral fins.
In the monoplane body plan of Exocoetus, only the pectoral fins are abnormally large, while the pelvic fins are small.
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Salmon larva emerging from its egg
Atlantic herring eggs, with a newly hatched larva
Freshly hatched herring larva in a drop of water compared to a match head.
Late stage lanternfish larva
A 9mm long late stage scaldfish larva
Larva of a conger eel, 7.6 cm
Bluefin tuna larva
Pacific cod larva
Walleye larva
Common sturgeon larva
Boxfish larva
Ocean sunfish larva, 2.7mm

In addition, some fish can variously "walk" (i.e., crawl over land using the pectoral and pelvic fins), burrow in mud, leap out of the water and even glide temporarily through the air.

- Fish locomotion

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Swim bladder

Internal gas-filled organ that contributes to the ability of many bony fish to control their buoyancy, and thus to stay at their current water depth without having to expend energy in swimming.

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How gas is pumped into the swim bladder using counter-current exchange.
The West African lungfish possesses a lung homologous to swim bladders
Most mesopelagic fishes are small filter feeders which ascend at night using their swimbladders to feed in the nutrient rich waters of the epipelagic zone. During the day, they return to the dark, cold, oxygen deficient waters of the mesopelagic where they are relatively safe from predators. Lanternfish account for as much as 65 percent of all deep sea fish biomass and are largely responsible for the deep scattering layer of the world's oceans.
Swim bladder display in a Malacca shopping mall
Fish maw soup
Swim bladder disease has resulted in this female ryukin goldfish floating upside down

Some of them can control their depth only by swimming (using dynamic lift); others store fats or oils with density less than that of seawater to produce a neutral or near neutral buoyancy, which does not change with depth.

Skate (fish)

Skates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays.

Early Eocene fossil stingray Heliobatis radians
A skate is caught in the Outer Banks using a Carolina rig
Full view of a skate's mermaid's purse. Roughly 125 mm in length.
The fibers of the skate electric organ are embedded in the muscles located lateral to the notochord in the tail.
Four developing embryos located in an opened big skate egg case (mermaid's purse)
Skates swim with their pectoral fins

The flattened body shape, ventral eyes and well developed spiracles of the skate allows them to live benthically, buried in the sediment or using a longitudinal undulation of the pectoral fins known as Rajiform locomotion to glide along the water floor.

Fish fin

Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish.

Ray fins on a teleost fish, Hector's lanternfish 
(1) pectoral fins (paired), (2) pelvic fins (paired), (3) dorsal fin,
(4) adipose fin, (5) anal fin, (6) caudal (tail) fin
Skeleton of a ray-finned fish
Lobe-finned fishes, like this coelacanth, have fins that are borne on a fleshy, lobelike, scaly stalk extending from the body. Due to the high number of fins it possesses, the coelacanth has high maneuverability and can orient their bodies in almost any direction in the water.
The haddock, a type of cod, is ray-finned. It has three dorsal and two anal fins
Cartilaginous fishes, like this shark, have fins that are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers
Caudal fin of a grey reef shark
Shark fin
Comparison between A) the swimming fin of a lobe-finned fish and B) the walking leg of a tetrapod. Bones considered to correspond with each other have the same color.
In a parallel but independent evolution, the ancient reptile Ichthyosaurus communis developed fins (or flippers) very similar to fish (or dolphins)
Similar adaptations for fully aquatic lifestyle are found both in dolphins and ichthyosaurs
In the 1990s, the CIA built a robotic catfish called Charlie, designed to collect underwater intelligence undetected

An example is the Robot Tuna built by the Institute of Field Robotics, to analyze and mathematically model thunniform motion.

Tetraodontiformes

Order of highly derived ray-finned fish, also called the Plectognathi.

Long-spine porcupinefish, Diodon holocanthus: On the right is a blue-spotted grouper, Cephalopholis argus
Ocean sunfish
The honeycomb cowfish is part of the family Ostraciidae.
American whitespotted filefish Cantherhines macrocerus

Most members of this order – except for the family Balistidae – are ostraciiform swimmers, meaning the body is rigid and incapable of lateral flexure.

Fish anatomy

Study of the form or morphology of fish.

Anatomical directions and axes
Skeleton of a bony fish
Skeletal structure of a bass showing the vertebral column running from the head to the tail
Skeletal structure of an Atlantic cod
Skull bones as they appear in a seahorse
Positions of fish mouths: terminal (a), superior (b), and subterminal or inferior (c).
Illustration of barbels on the head of a fish
Moray eels have two sets of jaws: the oral jaws that capture prey and the pharyngeal jaws that advance into the mouth and move prey from the oral jaws to the esophagus for swallowing.
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Gill of a rainbow trout
Placoid scales from a shark. A: Epidermis; B: Dermis; C: Pulp core; D: Dentine; E: Basal plate; F: Enamel; G: Spine
Singular bowfin cycloid scale.
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Bowfin cycloid scales.
Fish scales. 1: Cycloid scale; 2: Ctenoid scale; 3: Placcoid scale; 4: Ganoid scale
Cycloid scale.
Fish scales. A: Ganoid; B: Cycloid; C: Ctenoid
The lateral line is clearly visible as a line of receptors running along the side of this Atlantic cod.
The haddock, a type of cod, is ray-finned. It has three dorsal and two anal fins.
Sharks possess a heterocercal caudal fin. The dorsal portion is usually larger than the ventral portion.
The high performance bigeye tuna is equipped with a homocercal caudal fin, finlets and keels.
Internal organs of a male yellow perch: A=gill, B=Heart atrium, C=Heart ventricle, D=Liver (cut), E=Stomach, F=Pyloric caeca, G=Swim bladder, H=Intestine, I=Testis, J=Urinary bladder
The black swallower is a species of deep sea fish with an extensible stomach which allows it to swallow fish larger than itself.
Internal organs of a female Atlantic cod: 1=Liver, 2=Gas bladder, 3=Ovary, 4=Pyloric caeca, 5=Stomach, 6=Intestine
Dorsal view of the brain of the rainbow trout
Cross-section of the brain of a porbeagle shark with the cerebellum highlighted
Skull of a northern pike
Skull of Tiktaalik, a genus of extinct sarcopterygian (lobe-finned "fish") from the late Devonian period

Pectoral fins: Found in pairs on each side, usually just behind the operculum. Pectoral fins are homologous to the forelimbs of tetrapods, and aid walking in several fish species such as some anglerfish and the mudskipper. A peculiar function of pectoral fins, highly developed in some fish, is the creation of the dynamic lifting force that assists some fish such as sharks in maintaining depth and also enables the "flight" for flying fish. Certain rays of the pectoral fins may be adapted into finger-like projections, such as in sea robins and flying gurnards.

Aquatic locomotion

Biologically propelled motion through a liquid medium.

Jellyfish in motion
A eurasian coot swimming
The flagellum of a Gram-negative bacteria is rotated by a molecular motor at its base
Salmon spermatozoa for artificial propagation
Shrimp paddle with special swimming legs (pleopods)
Daphnia swims by beating its antennae
Octopuses swim headfirst, with arms trailing behind
Jellyfish pulsate their bell for a type of jet locomotion
Scallops swim by clapping their two shells open and closed
Open water fish, like this Atlantic bluefin tuna, are usually streamlined for straightline speed, with a deeply forked tail and a smooth body shaped like a spindle tapered at both ends.
Many reef fish, like this queen angelfish, have a body flattened like a pancake, with pectoral and pelvic fins that act with the flattened body to maximize manoeuvrability.
The leopard shark angles its pectoral fins so they behave as hydrofoils to control the animal's pitch
The slowest-moving fishes are the sea horses, often found in reefs
Common toad (Bufo bufo) swimming
Nile crocodile (Crocodylus niloticus) swimming
Immature Hawaiian green sea turtle in shallow waters
Macroplata
Comparative skeletal anatomy of a typical otariid seal and a typical phocid seal
Animated representation of lobstering
Chinstrap penguin leaping over water
Swimming dog

From the point of view of aquatic propulsion, the descent of modern members of the class Reptilia from archaic tailed Amphibia is most obvious in the case of the order Crocodilia (crocodiles and alligators), which use their deep, laterally compressed tails in an essentially carangiform mode of propulsion (see Fish locomotion).

Megalodon

Extinct species of mackerel shark that lived approximately 23 to 3.6 million years ago (Mya), from the Early Miocene to the Pliocene epochs.

The depiction of a shark's head by Nicolas Steno in his work The Head of a Shark Dissected
Diagram of the chronospecies evolution of megalodon
Megalodon tooth with two great white shark teeth
The great white shark (Carcharodon carcharias) and megalodon were previously thought to be close relatives.
Restoration assuming a similarity in appearance to the great white shark
Sculpture in the Museum of Evolution in Puebla, Mexico
Reconstruction showing the position of the replacement teeth
Reconstructed jaws on display at the National Aquarium in Baltimore
Reconstructed megalodon skeleton on display at the Calvert Marine Museum
Coprolite attributed to megalodon
Vertebra of a whale bitten in half by a megalodon with visible gashes from teeth
Megalodon may have faced competition from macroraptorial sperm whales, such as Livyatan (above).
Artistic impression of a megalodon pursuing two Eobalaenoptera whales
Collection of teeth of juvenile megalodon and C. chubutensis from a probable nursery area in the Gatún Formation of Panama
Megalodon may have become coextinct with smaller baleen whale species, such as Piscobalaena nana.
discovered megalodon teeth which were erroneously dated to be around 11,000 to 24,000 years old.

This is unlikely since the sand tiger shark is a carangiform swimmer which requires faster movement of the tail for propulsion through the water than the great white shark, a thunniform swimmer.

Ghost knifefish

The ghost knifefishes are a family, Apteronotidae, of ray-finned fishes in the order Gymnotiformes.

Apteronotus albifrons
Orthosternarchus tamandua
Sternarchorhynchus oxyrhynchus

They are distinguished from other gymnotiform fishes by the presence of a caudal fin (all other families lack a caudal fin) as well as a fleshy dorsal organ represented by a longitudinal strip along the dorsal midline.

Ichthyosaur

Ichthyosaurs (Ancient Greek for "fish lizard" – ἰχθύς or ichthys meaning "fish" and σαῦρος or sauros meaning "lizard") are large extinct marine reptiles.

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The skull found by Joseph Anning in 1811
The torso found by Mary Anning in 1812
Diagram of the skeletal anatomy of Ichthyosaurus communis from an 1824 paper by Conybeare
"Professor Ichthyosaurus" shows his pupils the skull of extinct man, caricature of Charles Lyell by Henry De la Beche (1830)
Hawkins' specimens are still the showpieces of the Natural History Museum
Ichthyosaurus figure at Crystal Palace Park (1854)
Typical Holzmaden fossils: adult and juvenile Stenopterygius quadriscissus
Precious opal replacing ichthyosaur backbone, display specimen, South Australian Museum
Grippia longirostris from the early Triassic of Spitsbergen was already well-adapted to an aquatic lifestyle.
Hupehsuchus
Mixosaurus
Shonisaurus popularis
Stenopterygius resembled a modern dolphin.
Many ichthyosaur lineages continued into the Cretaceous.
Restoration of Platypterygius kiprijanovi – Albian-Cenomanian of Kursk region (Russia)
CGI restoration of Ichthyosaurus communis
Examples of distinct features shared both by dolphins and derived ichthyopterygians
The skull of Temnodontosaurus platyodon has the typical ichthyosaurian shape with an elongated snout and large eye sockets.
Ichthyosaur vertebra from the Sundance Formation (Jurassic) of Natrona County, Wyoming: Note the characteristic hourglass cross-section. (Scale in mm.)
In this specimen seen from below, what looks like a breastbone is in fact the fused coracoids
Ichthyosaur 'paddle' (Charmouth Heritage Coast Centre)
In this arm of Ophthalmosaurus icenius, an additional upper row of elements has developed, ending above in an extra lower arm bone.
A Holzmaden ichthyosaur in which the preparer found organic remains in the position of the dorsal fin, but failed to locate any for the flippers.
An ichthyosaur coprolith
Temnodontosaurus acutirostris with ammonoids
Caypullisaurus is attacked by the crocodylomorph Dakosaurus
Despite their considerable size, the flippers of the Amazon river dolphin are mainly used as rudders.
Temnodontosaurus had the largest eyes of any known vertebrate, indicating a good diving capacity
Chaohusaurus with three juveniles
Detail of a female Stenopterygius with a great number of fetuses in her belly, one of which has been expelled
Life restoration of a female Maiaspondylus giving birth

Their bodies were elongated and they probably used an anguilliform locomotion, swimming by undulations of the entire trunk.

Mosasaurus

Type genus (defining example) of the mosasaurs, an extinct group of aquatic squamate reptiles.

Faujas' 1799 interpretation of the second skull's excavation
MNHN AC 9648, the second skull and holotype of M. hoffmannii, was nicknamed "the great animal of Maastricht".
M. missouriensis holotype, with the Harlan snout and Goldfuss skull; drawn in 1834 and 1845 respectively
1892 drawing of IRSNB 3119, of one of many M. lemonnieri skeletons described by Louis Dollo.
An 1854 depiction of Mosasaurus in Crystal Palace Park
Life restoration of M. hoffmannii, one of the largest known mosasaurs
Size range of Mosasaurus compared with a human
The Penza specimen, one of the largest known fossils of Mosasaurus
Annotated schematic of a M. hoffmannii skull
Closeup of M. hoffmannii teeth, with a replacement tooth developing inside the root of the lower right tooth
Closeup of a M. beaugei palate, showing the smaller pterygoid teeth on their namesake bones
Life restoration of M. missouriensis
Fossil skull of the proposed new species M. glycys
Skull of M. conodon
Skeleton of M. beaugei
Well-preserved fossil of M. missouriensis
Skull of M. lemonnieri
The skull of M. hoffmannii was adapted to withstand powerful bites.
Reconstruction of an M. hoffmannii forelimb
Sclerotic ring of Mosasaurus
Restoration of M. hoffmannii preying on a sea turtle
Like modern crocodiles, Mosasaurus likely grappled their rivals' heads.
M. hoffmannii specimen IRSNB R25, with an infected fracture to the left dentary (seen between the two middle tooth crowns in the back)
Fragmentary skull of a juvenile Mosasaurus (NHMM 200793) from Geulhem, Netherlands
Mosasaurus inhabited the Western Interior Seaway of North America and Mediterranean Tethys of Europe and Africa.
Restoration of M. beaugei, which is known from Morocco and Brazil
Mosasaurus coexisted with bony fish such as Xiphactinus, sea turtles like Protostega and plioplatecarpine mosasaurs in North America.
Mosasaurus fossils were found in the Seymour Island of Antarctica, which once provided cool temperate waters.
Mosasaurus inhabited offshore ocean habitats of various depths.
Mosasaurus went extinct as a result of the K-Pg extinction event; its last fossils were found at or close to the boundary, which is represented by the thick dark band separating the lighter and darker layers of this cliff.

The swimming style was likely sub-carangiform, which is exemplified today by mackerels.