Adaptive Radiation in Fishes | Vertebrates | Chordata |Zoology Notes

Introduction :- Adaptive Radiation In Fishes

The term "fish" denotes organisms residing in water. The origin and evolution of fishes form an intriguing subject, extensively studied by numerous scientists who have conducted comprehensive research on the topic. By examining living specimens and fish fossils, researchers have estimated the timeframe of fish origin and traced their evolutionary path. The earliest fossil superclass of fishes, Ostracoderm, provides valuable insights into their ancient history.

Within the realm of vertebrates, there exist 9 classes, including two extinct species – Ostracoderm and Placoderm. The initial two classes, Ostracodami and Cyclastomata, are jawless and fall under the Agnatha group. The remaining 7 classes possess jaws, categorized under the Gwerthostomata group. This classification encompasses two superclasses: Tetropoda, which comprises creatures with limbs, and Pisces, which includes aquatic beings. The specialized study of fishes is known as Ichthyology.

Origin Of Fishes

The origin of Fishes first occurred with the ancient life form Ostracoderm. This implies that they were found in fresh segments called calovals during the Ordovician Period, using scales. These segments were formed in a fragmentary manner, and no such fact was found in nature that was preserved for fossil studies. They were carefully preserved for fossil studies, which hints at their existence through scales during the Ordovician Period. Jamoytius and Theodus, discovered in Silurian England, are among the earliest chordates. V-shaped myotomes and continuous median and lateral fin folds were present on its body. Additionally, Craniates fossils are likely ancestors of Jamoytius. Some fragmentary organisms, categorized and named in the Ostracoderms group, are referred to as fishes without jaws, known as Agnatha. Jawed fishes, which we call Gnathostomes, originated from Agnatha; hence, they are considered the ancestors of Agnatha.

Evolutionary Drivers in Fishes

In history, the presence of a jaw has been considered the most significant development. The importance of this evolution has been extensively explained by scientists. Organisms lacking jaws, like jawless vertebrates, lacked the ability to withstand various levels of life. This capability emerged through Ostracoderm during the Silurian or Devonian period, revealing virtually endless possibilities for evolution. These creatures occupied ecological niches that remained open. The jawless vertebrates have been divided into two categories.

(1) Ostracoderm [Extinct]

(2) Cyclostomata.


Ostracoderm (Extinct):- Jawless armored ostracoderm, late Silurian. Or were found in the Devonian period. All of them were related to cyclestomates, because the structure of their brain, ears and blood vessels were similar.

(2) Cyclostomata

Cyclostomata includes living examples of jawless fishes such as lampreys and hagfishes, demonstrating ongoing importance and diversity within the evolutionary path of fishes. These are the most primitive of living craniates and have a low level of organization as demonstrated by lampreys and hagfish. ,

⫸Adaptive Radiation In Bony Fishes

Bony fishes exhibit adaptive radiation or divergent evolution, a phenomenon observed across various habitats worldwide. With over 20,000 species, their diversity surpasses that of all other vertebrates. Modern bony fishes trace their origins back 200 million years, with significant groups emerging during the Triassic period, laying the foundation for their evolutionary success.

Adaptive Radiation In Fishes

Diversity OF Bony Fishes:- The ray-finned fish are extraordinarily diverse in terms of body form color, habit, and habitat, Behaviour and so forth.

Bony Fishes In Devonian Period

Diversity in Species:- Bony fishes include more then 20,000 species all over the world inhabiting in lakes, ponds, rivers, ditches, Springs and Ocean constituting 90% of the living fish. The teleosts comprise at least (27) different orders and (390) families, the majority of which are marine. Their ancients ancestry extending in to the past for some (500) million years has allowed a vast span of time for evolutionary divergence.

Diversity In Shape and Size :-  Bony fishes have experienced various evolutionary adaptations in morphology. Commonly, they have torpedo-shaped(fusiform). In free swimming species the body approximates the theoretically perfect streamlines form to reduce water pressure during swimming, although they have a vast variety in shape and size. These range from globe-shaped (Globiform- puffers, Tetradontidae), Serpentine (Anguiliforms, eels, anguilla) thread-like of snipe eel, strongly flattened (Pleuronectes), irregular shape (Hippocampus) etc. Their size ranges from smallest pendula 1-2 cm to large lung fishes. Ocean sun fish weighing over 2 tones of body water. They may be of butterfly like (sea moth, pegasus umitengu). A rare giant oarfish (Regalecus glens), 7 meter(23ft ) long, captured in 1996. 

Diversity In Habitat :-  Live in almost all types of habitats fresh water as well as marine Bony fishes have experienced various evolutionary adaptation in physiology to live in a wide range of ecological habitats with diverse temperature levels. Antarctic bullhead notothen(Nototheria coriiceps), an Actinopterygii species that lives in freezing waters of the Southern Ocean, represents a powerful adaptation to the extremal low  temperature.

Diversity In Locomotion:- Fishes are able to move through water causing almost no turbulence bony fishes may swim, walk, fly, or be immobile. The have various type of fins supported by rays and flaps (lobed fin, latimeria), fatty adipose fins(salmons and trouts),fin reduces to a few spines(stickle back), walk on land with limb like fins(lung fishes). Mucous coated streamlined bady of fishes reduces viscous drag in water. Some are passive travellars like Echeneis (Remora) Exocoetus(Fly fish) glides over water.

Diversity In Food and Feeding :- Bony Fishes they feed on nearly all types of organic matter. Herrings(Clupia herringus) in north sea follow plankton concentration. Mackerals (Scombers) follow herrings for their food bony fishes may be herbivore or carnivore or may be omnivore on the basic of their food. Phytoplanton, Zooplankton, Small insects, Crustacean larvae and small fishes are the main food of them. Some are voracious feeder like Gambusia affinis feeds on mosquito larvae. 
Diversity In Body Colour:- Bony Fishes they exhibit a huge variety of beautiful colours. They are of grey( Labeo, Catla), mattled green( Oriental sweet lip fish, Plectorhinchus vittatus), Red (Betta) yellow (Tang fish), Green carp or grass carp (Ctenoparyngodon idella), pink(Gorami, Sichild), Blue (Betta), Sky blue(Blue tang fish), Orange(Gold fish) violet(purple betta), and so many beautiful coloure Stripped colors, Haemulon Sciurus) are there.

Diversity In Fish:- Bony fishes have both unpaired(Dorsal, Caudal, anal and Ventral fins)  and Paired Fins (Pectoral and Pelvics).They differ from lobe-finned fishes in that then fins consist of webs of skin supported by spines(rays) made of bone or horn.

         First dorsal fin is modified in sucker in Echeneis.

         Dorsal fin modified in to spines in stickle back.

         Ventral fin modified in to ovipositor in Bitterling (Rhodeus)

         Pectorals modified in to wigs like structures for gliding. 

Diverstiy In Sense Organs:- Bony fishes and they have different type of sensory system, including vision, hearing, chemoreception, electroreception, lateral line sensation and so forth.

        Light levels decrease as water depth increases. In fish vision could be changed by functional amino acid changes in the visual opsin pigment and rhodopsin

         A recent study explored rises and falls of opsin genes in 59 Actinopterygii and explained the significance of these opsin gene modification for environmental adaptation 

Diversity In Having Special Organs:- The electric eel(Electrophorus electricus) Electric Ray (Torpedo) and various other fish can produce electric organ discharges(EODs), which may be low voltage for electrolocation and high voltage to stun prey or offer protection.

       Some coastal water fish like Anomalopskeptontron and Photoblepharon palpebratus and Porichthyes possesses light producing organs.

              Amphipnous cuchia produce sound in stress conditions with its air bladder.

              Ichthyotorism or Poisonous fish - At least 1000 species of fish are thought to be venomous. Eg toxin within their skin-scombroids, Gempylid etc. toxin in glandular secretion-Box fish or trunk fish. They may produce toxins in their skin

Diversity In Air Bladder:- Air bladder or a gas filled sac, arising as a dorsal outgrowth from the gut in most of the bony fishes. There may be oxygen. carbon di-oxide or nitrogen gas in the bladder. Therefore it also called as gas bladder. There is a variety the air bladders in fish eg. physostomous bladder with dorsal opening in Acipensor, bifurcated in polypterus(Bichir), double type bladder is found in perch, cod, hoddoc, Toad fish


Adaptive radiation in fishes stands as a testament to the remarkable diversity and resilience of life. Through evolutionary innovation and adaptation fishes have conquered diverse habitats, from the depths of the ocean to freshwater lakes and streams. By understanding the mechanisms and significance of adaptive, we gain insights into the intricate tapestry of life on earth and the importance of preserving it for future generation. Lets us continue to marvel at the wonders of nature and strive to protect the delicate balance of ecosystems that sustain us all.

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