Category: Fish

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Fish are the largest vertebrate group with more than 30,000 species. They show an amazing diversity, coming in many shapes and sizes, but are united by their aquatic lifestyle (although independently many have become air-breathing, and so too a number of species are capable of terrestrial excursions) that has led to the evolution of features such as gills for breathing and fins for locomotion. Many different groups are generally included in the fish, but two major subgroups can be identified. These are the cartilaginous fish (e.g. sharks and rays) and the bony fish (Actinopterygia), which include the teleosts, that account for the majority of fish species. Other important groups include the jawless fish (Agnatha) such as hagfish and lampreys, and lobe-finned fish (Sarcopterygii) such as coelocanths and lungfish (relatives of which evolved into tetrapods).

Despite the range of forms and behaviours, convergent evolution is rampant in this group. Perhaps the classic example is the striking similarity between tuna and the lamnid (and alopiid) sharks, which involve remarkably similar fusiform bodies, a special form of locomotion called thunniform swimming that employs a complex system of ligaments and endothermy ("warm-bloodedness") that has also evolved independently in various other fish (notably the marlin and swordfish). The tuna/lamnid convergence is only one of many examples of the strikingly parallel emergence of ecomorphs, i.e. fish with very similar shapes that have evolved in different parts of the world. Amongst the most notable instances are eel-like (angulliform) and pike morphologies, but many other examples are known, especially from freshwater assemblages. Cichlids in particular provide many fascinating examples.

In the marine realm, there is a whole series of fascinating convergences amongst the sharks and the related rays (collectively the elasmobranchs), in addition to the tuna/lamnid convergence already mentioned. In the former, one remarkable convergence involves reproduction, not only with the development of viviparity but also the evolution of a placenta. Nor is this convergence restricted to the sharks, but both viviparity and the placenta have evolved independently in many other groups of fish. In the jaws of rays, we find a surprising parallel to trabecular bones. Both sharks and rays are capable of electroreception, which has also independently evolved in bony fish (at least twice) and, more remarkably, monotreme mammals.

The repeated evolution of the swim bladder represents an interesting example of an anatomical convergence. This organ is not unique to fish but has also evolved in at least one species of cephalopod. In addition, the swim bladder has been repeatedly lost in fish, which has interesting implications relating to change in the developmental pathways responsible. The function of the swim bladder is dependent on the counter-current exchange system of the rete mirabile, which is itself rampantly convergent, and gas-proofing by guanine crystals that dramatically decrease the permeability of the swim bladder wall also find a very different role in the reflective surfaces of the eye/tapetum.

Many more fascinating convergences can be found among the fish, for example with respect to the lateral line system (analogues of which have evolved in e.g. cephalopods and crustaceans), the red/white muscle system (that is similar in squids) and feeding. Teeth have evolved at least twice and lungfish have evolved complex 'molariform' tooth plates analogous to those of certain reptiles and mammals. Some fish (e.g. damselfish) have independently evolved agriculture and maintain algal "gardens", reminiscent of those maintained by the marine mammalian herbivores, dugongs. Regarding locomotion, of course, fish principally swim, but some fish, such as the eel Moringua, have converged on burrowing forms and many groups of fish have actually learnt to walk. There is a striking example of a ray that has effectively evolved legs to engage in bipedal locomotion. In this context it is important to recall that the tetrapods evolved from the sarcopterygian fish in the Devonian, which is in itself a fascinating story of evolution, introducing further examples of convergence such as the independent evolution of fingers in a group known as the rhizodonts. In fish there is also evidence for laterality, social intelligence and personalities, so it is not surprising to find additional convergences in many aspects of behaviour, including socio-sexual systems (e.g. monogamy and parental care).

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Topic Title Teaser text Availablity
Light producing chemicals: how to make bioluminescence

The most remarkable luciferin in terms of its distribution is known as coelenterazine. This nitrogen-ring based molecule is found in nine separate groups, ranging from radiolarians to fish.


Flying through the air on a summer's evening or sparkling in the ocean you may see magical flashes of light that signal some of nature's most enchanting creatures, those that are bioluminescent.

Male fish building complex nests to entice females

Japanese pufferfish males expend gargantuan amounts of energy building complex sand nests to attract females, who lay their eggs there. Reminds remind one of bower-birds among others...

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Electrolocation and electrocommunication in weakly electric fish

Fish have eyes, but they live in a much more complex sensory world, where even electricity plays a surprising - and convergent - role.

Four-eyed fish n/a Not Available
Endothermy (“warm-bloodedness”) n/a Not Available
Bioluminescence in marine animals n/a Not Available
Pycnodontid fish dentition n/a Not Available
Fin collagen in tuna, dolphins and sharks n/a Not Available
Ultraviolet (UV) vision in insects and vertebrates n/a Not Available
Agriculture in damselfish

Don’t be tempted to think human agriculture is unique. On many coral rocks, there are very similar things going on…


Cichlids are one of the cause celebré of evolution, and rightly so because these freshwater fish show a remarkable diversity and exemplify many key aspects of adaptive radiation. But why so successful? The answer lies in the versatility of the jaws (and yes, they are convergent).

Foam nests in animals

Nests crop up everywhere, but one made out of foam? Might not sound like a great idea, but it is. And no surprise, it has evolved several times...

Lateral line system in fish and other animals

Some cavefish are completely blind, so how do they manage to navigate through their environment with astonishing ease?

Trabeculae (skeletons) n/a Not Available
Pufferfish (and inflation)

Pufferfish are some of the most extraordinary fish to have evolved, especially because of their capacity to swallow water and inflate themselves to something like a football. Not only that but some representatives can be deadly to the unwary diner...

“Broken jaw” – mandibular and maxillary jaw joints

At first sight having a jaw with a joint seems a contradiction in terms, but such exist and not only are obviously functional, but needless to say convergent.

Eel-like (“anguilliform”) fish

Within the African catfish, eel-like forms have evolved four times independently, and other expmales include the Neotropical swamp eel, the true eels (which include the morays) and the lamprey.

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Moray eels

Eels masquerading as snakes sounds interesting, and that is before they go hunting with their friends the groupers...


Suction feeding in fish, amphibians, reptiles and aquatic mammals

Probably everyone is familiar with the walrus, but did you know that it generates a vacuum in its mouth to suck clams out of their shells? And this is just one example of suction feeding, the feeding mode typically used by bony fish…


Not many foods served in a restaurant can kill you, but pufferfish is the exception. Tetrodotoxin, the toxin responsible for such culinary fatalities, reveals a fascinating story of convergent evolution...

Teiid lizard dentition: convergence with other reptiles, mammals and fish

Teiids are skink-like lizards whose members show a stunning diversity of tooth types, providing rich evidence of convergence within the teiids themselves, in distantly related reptile groups and even in certain mammals and fish.

Mimicry in fish and other marine animals n/a Not Available
Independent eye movement in fish, chameleons and frogmouths

One of the most surprising convergences amongst animals is that seen between a small fish that lives in coral sands, known as the sandlance, and the lizards known as chameleons.

Pharyngeal jaws in teleost fish

One of the great evolutionary breakthroughs in the teleost fish was the conversion of some of the elements that supported the gill bars into a second set of pharyngeal teeth that complemented the oral teeth. See how a fish becomes a snake!

Durophagy (hard prey-eating) in fish

Plenty of animals have an extraordinary capacity to crush hard prey and this has evolved independently many times in the vertebrates. If you suspect it is a durophage, watch your fingers!

Gliding reptiles

In the reptiles, different forms of skin membrane (called ‘patagia’) and in some extinct species, primitive feathers, have evolved convergently as adaptations for gliding.

Mammal-like placentation in skinks (and fish)

“Only two types of vertebrates [have] evolved a reproductive pattern in which the chorioallantoic placenta provides the nutrients for fetal development. One is [...] the eutherian mammals […], and the other, a few lineages of the family Scincidae.” A.F. Flemming (2003) J Exp Zool 299A 33-47

Defensive spines in animals

Sea-urchins, porcupines (and porcupine fish), lizards and many other animals bristle with defensive spines.

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Telephoto eyes in animals

Pursued by the paparazzi? Watch out for those animals equipped with telephoto lenses...

Silk production and use in arthropods

Remarkably, fossil silk is known, especially from amber of Cretaceous age. Material includes both silk with trapped insects, possibly from an orb-web, and strands with the characteristic viscid droplets that are the key in trapping prey.

Electric fish: insights into convergence

Ever seen an electric eel in an aquarium? Don’t dare putting your hand in the tank...

Agriculture: from ants to dugongs

Human farmers tending their fields are a familiar sight. But don't forget about those fungus-farming termites or the fish with a garden of algae…

Sharks and rays (elasmobranchs): insights into convergence

In terms of sensory evolution the elasmobranchs are of particular interest, because independently of other fish and even some mammals (e.g. duck-billed platypus) they have evolved electrosensory systems.

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Swimming and thermoregulation in sharks and tuna

Thunniform swimming depends on a large, lunate tail that is joined to the rest of the body via a narrow peduncle. Whilst the tail flicks backwards and forwards, so propelling the animal, the rest of the body hardly moves sideways.

Developmental genetic pathways to convergence

At first sight there is a fairly simple dichotomy between convergent features that have effectively the same genetic basis, and those where the same feature emerges but the underlying genetics are different. The former, however, is somewhat more complicated...

Parental care in vertebrates, echinoids, molluscs and brachiopods

The independent evolution of parental care is far more widespread than birds and mammals, extending as far as molluscs and echinoderms!

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Transparent tissues: eyes, bodies and reflective surfaces

Read on if you want to know about the numerous animal equivalents to the invisible man...

Polarized light detection in arthropods, fish and cephalopods

In bees detection of polarized light from different quadrants of the sky is an important component in their navigation.

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Personality in vertebrates and cephalopods

Personality in the vertebrates might, therefore, be deeply embedded in their phylogeny, although this does not rule out the convergent appearance of more complex personality traits in more advanced vertebrates, notably birds and mammals.

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Viviparity (live birth) in animals

Viviparity is rampantly convergent, with famous examples in the reptiles, notably the lizards and snakes.

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Lekking in birds, fish, mammals and cephalopods

Complex interactions between males and females prior to mating have evolved independently many times. Amongst the most familiar examples are leks.

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Electroreception in fish, amphibians and monotremes

From an evolutionary point of view, electroreception is particularly intriguing as a sense modality that has been repeatedly lost and reinvented again.

Swim bladders of fish and the octopus Ocythoe

Swim bladders have evolved independently in fish and in Ocythoe octopus females.

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Ultraviolet (UV) absorption in vertebrates and cephalopods

In some vertebrates (fish, mammals) and cephalopods we find an interesting convergence whereby some of the incoming ultraviolet is screened out.

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