THE FISHES: VERTEBRATES SUCCESS IN WATER
INTRODUCTION
=Modern fishes include an estimated 31,000 species, by far the most of all clades within the
Vertebrata.
=Fishes were the earliest vertebrates, with jawless species being the earliest forms and jawed
species evolving later.
=They are active feeders, rather than sessile, suspension feeders.
=The Agnatha (jawless fishes)—the hagfishes and lampreys—have a distinct cranium and
complex sense organs including eyes, that distinguish them from the invertebrate chordates, the
urochordates and cephalochordates.
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SUBPHYLUM VERTEBRATA
Superclass Agnatha/AGNATHANS: Jawless Fishes
=Jawless fishes (Agnatha) are craniates representing an ancient vertebrate lineage that arose over
550 million years ago.
=In the past, hagfishes and lampreys were sometimes recognized as separate clades within the
Agnatha, primarily because lampreys were regarded as true vertebrates, whereas hagfishes were
not. However, recent molecular data, both from rRNA and mtDNA, as well as embryological
data, provide strong support for the hypothesis that living agnathans—previously called
cyclostomes—are monophyletic, and thus share recent common ancestry.
=The defining features of the living jawless fishes are the lack of jaws and lack of paired lateral
appendages (fins). They also lack internal ossification and scales, although these are not defining
features of the clade.
=Some of the earliest jawless fishes were the armored ostracoderms (which translates to “shellskin”): vertebrate fishes encased in bony armor—unlike present-day jawless fishes, which lack
bone in their scales.
=Some ostracoderms, also unlike living jawless fishes, may have had paired fins. We should
note, however, that the “ostracoderms” represent an assemblage of heavily armored extinct
jawless fishes that may not form a natural evolutionary group.
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Membranous roof in the skull: The skull of Agnatha has a membranous roof, which sets
them apart from other vertebrates with bony skulls.
Single median nasal opening: Agnatha have a single nasal opening located in the midline
of the skull.
Notochord and fibrous neural tube: The vertebral column of Agnatha consists of a
persistent notochord, a flexible rod-like structure, surrounded by a fibrous neural tube.
Multiple gill slits: Agnatha typically possess a significant number of gill slits, ranging from
7 to 14 pairs.
Absence of a conus in the heart: Unlike other vertebrates, Agnatha lack a conus, a
specialized region in the heart.
Persistent hypophysial sac: Agnatha have a persistent hypophysial sac, a structure related
to the pituitary gland.
Semicircular ducts in the ear: Agnatha possess one or two semicircular ducts within their
ear, aiding in balance and orientation.
Elongated kidneys and archinephric ducts: Agnatha have long kidneys and archinephric
ducts, which are involved in excretory functions.
Absence of genital ducts: Agnatha lack specific genital ducts for reproductive purposes.
Well-developed pineal apparatus: Agnatha exhibit a fairly well-developed pineal apparatus,
which is involved in regulating biological rhythms and light detection.
Larval stage and endostyle: Agnatha typically undergo a larval stage that is microphagus,
meaning they consume small particles of food. They possess an endostyle similar to that
found in protochordates.
Additional general characteristics of Class Agnatha include:
Classification within the phylum Chordata and subphylum Vertebrata.
Lack of a recognizable stomach in the digestive system.
Complete absence of jaws, distinguishing them from jawed vertebrates.
Paired fins are often absent or reduced in size.
Thick bony scales and plates that were present in early species have been lost in modern Agnatha.
The skeleton is primarily cartilaginous rather than bony.
The notochord, which is a characteristic of embryonic development, persists into adulthood.
Presence of seven or more paired gill pouches for respiration and water filtration.
Class Myxini: Hagfishes
Lampetra fluviatilis from the German North Sea
GENERAL CHARACTERISTICS
1. Absence of jaws: Agnatha lack true jaws, making them distinct from other vertebrate
classes.
2. Lack of teeth, paired appendages, and exoskeleton: Agnatha do not possess teeth or paired
appendages like fins. Furthermore, they lack an exoskeleton.
=The class Myxini includes at least 70 species of hagfishes—eel-like scavengers that live on the
ocean floor and feed on living or dead invertebrates, fishes, and marine mammals.
=they are almost completely blind, sensory barbels around the mouth help them locate food by
smell and touch
=They feed using keratinized teeth on a movable cartilaginous plate in the mouth, which rasp
pieces of flesh from their prey. These feeding structures allow the gills to be used exclusively for
respiration, not for filter feeding as in the urochordates and cephalochordates.
=Hagfishes are entirely marine and are found in oceans around the world, except for the polar
regions.
=Unique slime glands beneath the skin release a milky mucus (through surface pores) that upon
contact with water becomes incredibly slippery, making the animal almost impossible to hold.
This slippery mucus thus allows the hagfish to escape from the grip of predators.
=Hagfish can also twist their bodies into a knot, which provides additional leverage to feed.
=Sometimes hagfish enter the bodies of dead animals and eat carcasses from the inside out.
Interestingly, they do not have a stomach.
=There is no paired appendages; as do hagfishes, although they have one or two fleshy dorsal
fins
=Heart is two-chambered;
=Kidney is mesonephric type;
=Brain with 10 cranial nerves with small cerebellum;
Lamprey. These parasitic sea lampreys,
Petromyzon marinus, attach by suction to their
lake trout host, and use their rough tongues to rasp
away flesh in order to feed on the trout’s blood.
Note: some call them predators because they attack and
eventually fall off.
=Hagfishes have a cartilaginous skull, as well as a fibrous and cartilaginous skeleton, but the
major supportive structure is the notochord that runs the length of the body.
=In hagfishes, the notochord is not replaced by the vertebral column, as it is in true vertebrates,
and thus they may (morphologically) represent a sister group to the true vertebrates, making
them the most basal clade (group of organisms that share common ancestors) among the skullbearing chordates.
Class Cephalaspidomorphi (Lampreys)
The class Cephalaspidomorphi is named after the Greek word kephale = head + aspidos = blade
or shield + morphe = form.
This class of aquatic animals have no jaws and it was formerly known as Cyclostomata.
They are the oldest of the jawless vertebrates.
They contain most of the features of other vertebrates.
This group has placed apart from others due to the absence of the jaws.
Many of the living creatures in this class prefer to live parasitic lives. These are known as living
fossils. Because, from the time of origin, there has not been much structural change in their body.
They are known as Lamprey and it was named after Linnaeus.
In 2003, scientist Hickman, et al. first divided the living jawless animal or Agnatha into two
separate classes. One of them is called Myxini and the other is called Cephalaspidomorphi.
Previously these two classes belonged to class Cyclostormata and was identified as two
subclasses. Although there are evolutionary similarities, there are several fundamental
differences between the two classes. So far, 41 species of Cephalaspidomorphi have been
identified.
General Characteristics
=Notochord is persistent.
=There are no jaws.
=They have circular sucker like mouth with lots of teeth (adult lampreys are characterized by
rasping tongue within a toothed, funnel-like sucking mouth)
=The main axial skeletal system is cartilaginous or fibrous.
=Lampreys have two semi-circular ducts on each side of the head, while hagfish have one semicircular duct on each side of the head.
=There is no actual gill arch to protect the gills, but the gills, gill arteries, branchial baskets exist
outside the branchial nerve; the gills are located in the sac.
=The branchial basket is strongly attached to the skull.
=The paired fins (pectoral and pelvic) are missing.
=A middle nostril exists.
Lampreys live primarily in coastal and freshwater environments
Some species are marine, but all species spawn in fresh water
Eggs are fertilized externally, and the larvae (called ammocoetes) differ greatly from the adult
form, closely resembling the adult cephalocordate amphioxus. After spending three to 15 years
as suspension feeders in rivers and streams, they attain sexual maturity. After one to three years
of feeding on fish as ectoparasites during life in open water, the adults swim upstream, reproduce,
and die within days.
Gnathostomes: Jawed Fishes
=Gnathostomes, or “jaw-mouths,” are vertebrates that possess true jaws—a milestone in the
evolution of the vertebrates.
=one of the most significant developments in early vertebrate evolution was the development of
the jaw: a hinged structure attached to the cranium that allows an animal to grasp and tear its
food.
=Jaws were probably derived from the first pair of gill arches supporting the gills of jawless
fishes.
=Early gnathostomes also possessed two sets of paired fins, allowing the fishes to maneuver
accurately.
=Pectoral fins are typically located on the anterior body, and pelvic fins on the posterior. =
Evolution of the jaw and paired fins permitted gnathostomes to expand their food options from
the scavenging and suspension feeding of jawless fishes to active predation.
=The ability of gnathostomes to exploit new nutrient sources probably contributed to their
replacing most jawless fishes during the Devonian period. Two early groups of gnathostomes
were the acanthodians and placoderms (Figure 29.11), which arose in the late Silurian period and
are now extinct.
=Most modern fishes are gnathostomes that belong to the clades Chondrichthyes and
Osteichthyes (which include the class Actinoptertygii and class Sarcopterygii).
A placoderm. Dunkleosteus was an
enormous placoderm from the Devonian
period, 380 to 360 million years ago. It
measured up to 10 meters in length and
weighed up to 3.6 tons. Its head and neck
were armored with heavy bony plates.
Although Dunkleosteus had no true teeth,
the edge of the jaw was armed with sharp
bony blades.
Class Chondrichthyes: Cartilaginous Fishes
=The class Chondrichthyes (about 1,000 species) is a morphologically diverse clade, consisting
of subclass Elasmobranchii (sharks, rays, and skates, together with the obscure and critically
endangered sawfishes), and a few dozen species of fishes called chimaeras, or “ghost sharks” in
the subclass Holocephali.
=Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage.
=This clade arose approximately 370 million years ago in the early or middle Devonian.
=They are thought to be descended from the placoderms, which had endoskeletons made of bone;
thus, the lighter cartilaginous skeleton of Chondrichthyes is a secondarily derived evolutionary
development.
=Parts of shark skeleton are strengthened by granules of calcium carbonate, but this is not the
same as bone.
=Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for a
part or all of their lives.
=Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws
and teeth to tear it into smaller pieces.
=Sharks have abrasive skin covered with tooth-like scales called placoid scales.
=Shark teeth probably evolved from rows of these scales lining the mouth. A few species of
sharks and rays, like the enormous whale shark, are suspension feeders that feed on plankton.
=The sawfishes have an extended rostrum that looks like a double-edged saw.
=The rostrum is covered with electrosensitive pores that allow the sawfish to detect slight
movements of prey hiding in the muddy sea floor.
=The teeth in the rostrum are actually modified tooth-like structures called denticles, similar to
scales.
Shark. Hammerhead sharks tend to school during
the day and hunt prey at night.
=Sharks have well-developed sense organs that aid them in locating prey, including a keen sense
of smell and the ability to detect electromagnetic fields.
=Electroreceptors called ampullae of Lorenzini allow sharks to detect the electromagnetic fields
that are produced by all living things, including their prey. (Electroreception has only been
observed in aquatic or amphibious animals and sharks have perhaps the most sensitive
electroreceptors of any animal.)
=Sharks, together with most fishes and aquatic and larval amphibians, also have a row of sensory
structures called the lateral line, which is used to detect movement and vibration in the
surrounding water, and is often considered to be functionally similar to the sense of “hearing” in
terrestrial vertebrates.
=The lateral line is visible as a darker stripe that runs along the length of a fish’s body.
= Sharks have no mechanism for maintaining neutral buoyancy and must swim continuously to
stay suspended in the water.
=Some must also swim in order to ventilate their gills but others have muscular pumps in their
mouths to keep water flowing over the gills.
Whale shark in the Georgia
Aquarium. Whale sharks are filterfeeders and can grow to be over 10
meters long. Whale sharks, like
most
other
sharks,
are
ovoviviparous.
=Sharks reproduce sexually, and eggs are fertilized internally.
=Most species are ovoviviparous: The fertilized egg is retained in the oviduct of the mother’s
body and the embryo is nourished by the egg yolk. The eggs hatch in the uterus, and young are
born alive and fully functional.
=Some species of sharks are oviparous: They lay eggs that hatch outside of the mother’s body.
Embryos are protected by a shark egg case or “mermaid’s purse” (Figure 29.14) that has the
consistency of leather.
=The shark egg case has tentacles that snag in seaweed and give the newborn shark cover.
=A few species of sharks, e.g., tiger sharks and hammerheads, are viviparous: the yolk sac that
initially contains the egg yolk and transfers its nutrients to the growing embryo becomes attached
to the oviduct of the female, and nutrients are transferred directly from the mother to the growing
embryo.
=In both viviparous and ovoviviparous sharks, gas exchange uses this yolk sac transport.
Figure 29.14 Shark egg cases. Shark embryos
are clearly visible through these transparent
egg cases. The round structure is the yolk that
nourishes the growing embryo.
In general, the Chondrichthyes have a fusiform or dorsoventrally flattened body, a heterocercal
caudal fin or tail (unequally sized fin lobes, with the tail vertebrae extending into the larger upper
lobe) paired pectoral and pelvic fins (in males these may be modified as claspers), exposed gill
slits (elasmobranch), and an intestine with a spiral valve that condenses the length of the
intestine.
They also have three pairs of semicircular canals, and excellent senses of smell, vibration,
vision, and electroreception.
A very large lobed liver produces squalene oil (a lightweight biochemical precursor to steroids)
that serves to aid in buoyancy (because with a specific gravity of 0.855, it is lighter than that of
water).
Rays and skates comprise more than 500 species.
They are closely related to sharks but can be distinguished from sharks by their flattened bodies,
pectoral fins that are enlarged and fused to the head, and gill slits on their ventral surface (Figure
29.15).
Like sharks, rays and skates have a cartilaginous skeleton.
Most species are marine and live on the sea floor, with nearly a worldwide distribution.
Unlike the stereotypical sharks and rays, the Holocephali (chimaeras or ratfish) have a
diphycercal tail (equally sized fin lobes, with the tail vertebrae located between them), lack scales
(lost secondarily in evolution), and have teeth modified as grinding plates that are used to feed
on mollusks and other invertebrates (Figure 29.15b).
Unlike sharks with elasmobranch or naked gills, chimaeras have four pairs of gills covered by
an operculum. Many species have a pearly iridescence and are extremely pretty.
Osteichthyes: Bony Fishes
=Members of the clade Osteichthyes, also called bony fishes, are characterized by a bony
skeleton.
=The vast majority of present-day fishes belong to this group, which consists of approximately
30,000 species, making it the largest class of vertebrates in existence today.
=Nearly all bony fishes have an ossified skeleton with specialized bone cells (osteocytes) that
produce and maintain a calcium phosphate matrix.
=This characteristic has been reversed only in a few groups of Osteichthyes, such as sturgeons
and paddlefish, which have primarily cartilaginous skeletons.
=The skin of bony fishes is often covered by overlapping scales, and glands in the skin secrete
mucus that reduces drag when swimming and aids the fish in osmoregulation.
=Like sharks, bony fishes have a lateral line system that detects vibrations in water.
=All bony fishes use gills to breathe.
= Water is drawn over gills that are located in chambers covered and ventilated by a protective,
muscular flap called the operculum.
=Many bony fishes also have a swim bladder, a gas-filled organ derived as a pouch from the gut.
The swim bladder helps to control the buoyancy of the fish.
= In most bony fish, the gases of the swim bladder are exchanged directly with the blood.
=The swim bladder is believed to be homologous to the lungs of lungfish and the lungs of land
vertebrates.
Bony fishes are further divided into two extant clades: Class Actinopterygii (ray-finned fishes)
and Class Sarcopterygii (lobe-finned fishes).
Actinopterygii (Figure 29.16a), the ray-finned fishes, include many familiar fishes—tuna, bass,
trout, and salmon among others—and represent about half of all vertebrate species. Ray-finned
fishes are named for the fan of slender bones that supports their fins.
In contrast, the fins of Sarcopterygii (Figure 29.16b) are fleshy and lobed, supported by bones
that are similar in type and arrangement to the bones in the limbs of early tetrapods. The few
extant members of this clade include several species of lungfishes and the less familiar
coelacanths, which were thought to be extinct until living specimens were discovered between
Africa and Madagascar. Currently, two species of coelacanths have been described.
Figure 29.16 Osteichthyes. The (a) sockeye salmon and (b) coelacanth are both bony fishes of the Osteichthyes
clade. The coelacanth, sometimes called a lobe-finned fish, was thought to have gone extinct in the Late
Cretaceous period, 100 million years ago, until one was discovered in 1938 near the Comoros Islands between
Africa and Madagascar
Figure 29.15 Cartilaginous fish. (a) Stingray. This stingray blends into the sandy bottom of the ocean floor. A
spotted ratfish (b) Hydrolagus colliei
AQUATIC ADAPTATIONS IN VERTEBRATES
Introduction
Aquatic adaptations occur in those animals which live in water habitat, viz., fresh, brackish or
sea water. They are called aquatic animals or hydrocoles. Based upon the phylogenetic history
of the aquatic animals, following two types of hydrocoles have been recognized:
1. Primary aquatic animals
The primarily aquatic animals are those in which the phylogenetic history is restricted to water
as habitat. Therefore, all their adaptations are originally designed to meet the necessities of
aquatic life. Generally, by Adaptations Pit Viper has heat sensitive pits between each eye and
nostril. This enables them to pick up changes in temperature around them. (Aquatic Adaptations,
Volant Adaptations and Desert Adaptations) primarily aquatic forms are meant the fishes, which
have never had a terrestrial ancestry, but have evolved from more primitive aquatic progenitors.
As a consequence, their adaptation to a dense watery medium is perfect and they do not suffer
as those secondarily adapted do through their inability to breathe water. They are, therefore, the
primitive gill breathing vertebrates.
Regarding the body form of planktonic organisms (which float passively at or about the surface),
some are globular such as Noctiluca, some have umbrella shape (medusa of Obelia, Aurelia) and
some have barrel-like shape (tunicates such as Doliolum and Salpa).
• Swimming organs
In vertebrates, the primary aquatic animals are the fishes. The fish move (swim) by the help of
fins and also by lateral undulations of the flexible body. The fins of fish are of two types, the
median fins and paired fins.
The median fins include dorsal fins, caudal fins and ventral fins. The paired fins are the pectorals
and pelvic. The caudal fin is the chief propeller, the dorsal and ventral fins help to keep body
vertical and the pectoral and pelvic fins help in propulsion and in making changes in direction.
Primary aquatic animals include protozoans, sponges, coelenterates, some annelids, molluscs
and arthropods; echinoderms; and chordates such as cephalochordates (Amphioxus),
urochordates (Herdmania, Doliolum, Salpa, etc.), cyclostomates (Petromyzon, Myxine), fishes,
etc.
2. Secondary aquatic animals
These are those hydrocoles which have a record of terrestrial life in their phylogeny (i.e., they
are descended from ancestors which led a life on land). Secondary aquatic animals are lungbreathers, mainly amphibious vertebrates, which through stress of circumstances such as
inhospitable lands, where food was scarce or severe competitions were forced to return once
more to the water.
Consequently, these animals show in their body structure, the evidences which speak of their
ancestry from land-living animals, e.g., Pila, frog and other amphibians, swamp river turtles
(Notosaurs, Phytosaurs), crocodiles, birds such as Ichthyornis, penguins, albatrosses, petrels,
ducks, geese, etc., and mammals such as Hippopotamus, Otter, whales, porpoises, etc.
Primary Aquatic Adaptations
•
Body contour
The form of body depends upon the habits of life. The majority of fixed and partly sedentary
forms have radially symmetrical body forms, e.g., sponges (Sycon, Euplectella, Hyalonema,
etc.,), Hydra, Obelia, Aurelia, Metridium (sea anemone), echinoderms such as Holothuria,
Echinus, Astropecten or star fish. The active locomotor type has fusiform, spindle-shaped
elongated and wormlike bodies. The spindle form is the characteristic of fishes and wavy, wormlike form is found in the annelids (Nereis).
The piscine body is designed for fast locomotion in water. There occurs a side-to-side
compression of head, body and tail into a beautifully curved streamlined fish form. Head is subconical. There is no protuberance over the body, which would retard the swift passage of the
animal through water. Further, since the relative weight is more on the upper half of the fish body
(on account of the myotomes) a dead fish will float ventral side turned upwards.
• Respiration
The primary aquatic animals are able to respire inside the water, without the need to come up to
the surface. The exchange of respiratory gases takes place between the blood of these animals
and the water outside.
There are two methods of aquatic breathing:
1. through diffusion through general body surface, e.g., protozoans, coelenterates and
planktonic larvae, and
2. with the help of special organs called branchia or gills, e.g., prawns, and other
crustaceans; Unio, Pila and other mollusks; and many vertebrates such as fishes,
tadpole of frog and salamanders. Indeed, gills of fishes are most remarkable aquatic
breathing organs utilizing dissolved oxygen of water.
• Air bladder
Advance bony fishes contain air bladder (or swim bladder) which serve as an accessory
respiratory organ and hydrostatic organ. Air bladder is a hollow outgrowth of the alimentary
canal and is filled with gas or air.
• Lateral line sense organs
Fishes have lateral line systems extending all over the body. It contains neuromast organs which
act as rheoreceptors (i.e., detect pressure changes in surrounding water).
• Skin
Skin of fishes is rich in mucous glands and/or is protected with scales.
Secondary Aquatic Adaptations
There occur fleshy and fin like expansions of the body wall in whales and ichthyosaurs which
help in propulsion. These fins may be dorsal or caudal. Dorsal fin is present in killer whale, while
absent in Delphinopterus and Balaena. Caudal fin (also called caudal or tail fluke) of marine
mammals in horizontal (vertical in reptiles) and the bone divides the tail into two equal parts
rather than running into one lobe. In turtles oar propulsion occurs by fin-like limbs; but in whales,
sirenians, etc., tail propulsion takes place as their hindlimbs become disappeared.
•
Enlargement of size
Aquatic vertebrates tend to be larger in size because in these creatures’ energy, which in
terrestrial form is exhausted in gravitational forces, is turned into growth. For example, largest
sulphur-bottom whale (Balaenoptera musculus) is several times bigger than the largest elephant.
Other example includes giant sharks and squids.
Pectoral paddles of whales and sirenians exhibit the following adaptations:
=The restriction of movements corresponding to the elbow and wrist joints.
=The fusion between digits.
=Increase in the number of phalanges, called hyperphalangy
=Increase in the number of digits for increase of expanse of paddling surface, called
hyperdactyly.
•
Stream-lined body
The body shape is stream-lined like primarily adapted forms: neck constriction disappears and
tail enlarges, e.g., Ichthyosauria (extinct fish-lizards), Cetacea (whales, dolphins, and porpoises),
Sirenia (manatees and dugongs), Pinnipedia (walrushes and seals). Frog also contains streamlined body.
•
Submergence
All secondary aquatic animals need to develop capacity of submergence since swimming below
water surface demands such an adaptation. For example, in whales the ribs are strongly arched,
the lungs are massive, the external nostrils communicate with the median “blow hole” which is
closable. Certain adult aquatic insects too are able to increase their period of submergence by
storing air inside the subelytral space, e.g., Nepa.
•
Shortening of neck
There occurs reduction of length and mobility of neck. In whales’ cervical vertebrae (which are
seven in number like other mammals) are fused to form a solid and compressed mass of bone.
• Disappearance of hairs, skin glands, etc
In whales and sirenians, the skin becomes naked due to loss of hairs. The hair loss is compensated
by the formation of a fatty layer below the skin (blubber) for the retention of the bodily heat. The
blubber also has a hydrostatic advantage (e.g., it helps in floatation or to keep positions in the
water and act in combination with the buoyancy of the aquatic medium). Sweat or oil glands
disappear as they have nothing to do with the aquatic mode of life. Muscles and nerves also
atropy from the integument due to its thickening and immobility.
• Mouth arrangement
Since jaws are not used for mastication in whales they lost the power of movement. Teeth become
simplified (homodont in dolphins) and greater in number. In sperm whale, teeth are present only
on one jaw or entirely absent from both the jaws (e.g., baleen whale).
• Skull modification
In certain aquatic mammals (e.g., Dolphins, porpoises) the cranium is shortened and front part
of the skull becomes elongated to acquire the shape of a rostrum. In the skull of the cetacea, the
zygomatic arch is reduced to a vestige.
•
Simplification of vertebrae
In secondarily aquatic forms (vertebrates) the vertebrae tend to be simple. In Ichthyosaurs,
vertebrae are simple with biconcave centra like the fishes. Various secondary articulations or
zygapophysis become reduced, as body weight is supported by water. The chest too becomes
cylindrical. The rib articulations are modified and are central, i.e., they are articulated to the
centrum and are not articulated to the transverse processes. Sacrum in cetaceans and sirenians is
more or less reduced, since it does not withstand and transmit the supporting impact of the
hindlimbs, as does in terrestrial forms.
•
Disappearance of excrescences
The external ears (pinnae) which hinder water locomotion tend to disappear, since they collect
sound waves in air medium and are useless in aquatic forms. Thus, ears are reduced in
amphibious mammals and are lost in whales, true seals and walruses. The nostrils (nares) move
towards the apex of head as in whales, ichthyosaurs, phytosaurs, etc. Nares are often capable of
being closed (e.g., otter). Likewise, eyes become water-adapted by shifting higher on the face as
in hippopotamus.
•
Occurrence of locomotory paddles (fins)
•
Lightness of bones
The bones in aquatic forms are light and spongy. In whales, their interstices are filled with oil.
Fish – Definition, Examples, Characteristics
Sea snake: A reptile that spends part of its time in the water.
Jellyfish: An invertebrate with a gelatinous body and tentacles.
Starfish: An echinoderm with radial symmetry, distinct from the bilateral symmetry of fish.
Adaptations for Living in an Aquatic Environment
Fish have a range of adaptations that allow them to thrive in aquatic environments. These
adaptations help them to move efficiently, breathe, find food, avoid predators, and reproduce.
Here are some key adaptations:
Fish are aquatic vertebrates that typically have gills, fins, and a streamlined body. They inhabit
diverse environments, from freshwater rivers and lakes to salty oceans and deep seas.
Shared Characteristics with Other Vertebrates
Fish, like all vertebrates, possess a backbone or spinal column. This characteristic places them
within the subphylum Vertebrata. Other shared features include a brain protected by a skull, a
complex nervous system, and bilateral symmetry.
Fish Characteristics
Fish possess several distinct features that distinguish them from other vertebrates:
Gills: For extracting oxygen from water.
Fins: For propulsion, steering, and balance.
Scales: Covering the body for protection and reducing water resistance.
Lateral Line System: A sensory organ that detects water movements and vibrations.
Swim Bladder: An internal gas-filled organ that helps maintain buoyancy (in most bony fish).
Examples of Fish and Misidentified Non-Fish
Invertebrates, mammals, birds, reptiles, and amphibians are not fish. Also, just because an animal
has the word “fish” in its name, it doesn’t mean it’s a fish:
Examples of Fish:
Salmon (Salmo salar): A well-known migratory species that lives in both fresh and saltwater.
Goldfish (Carassius auratus): A popular freshwater aquarium fish.
Great White Shark (Carcharodon carcharias): A large predatory marine fish.
Manta Rays (genus Mobula): Large triangular-shaped fish with the largest brain to body ratio of
all fish.
Non-Fish Examples Often Mistaken for Fish:
Dolphin: A marine mammal often confused with fish due to its aquatic lifestyle.
1. Body Shape and Streamlining
Streamlined Body: Most fish have a streamlined body shape that reduces water
resistance and helps them to swim efficiently.
Lateral Compression: Many fish have bodies that are laterally compressed. This helps
them maneuver easily through water.
2. Locomotion and Fins
Fins: Fish have various types of fins (dorsal, pectoral, pelvic, anal, and caudal fins)
that aid in propulsion, steering, and stability. The caudal (tail) fin is particularly
important for propulsion.
Fin Placement: The placement and size of fins varies, adapting different species to
specific environments, such as swift currents or stagnant waters.
3. Respiration
Gills: Fish extract oxygen from water using gills, which are highly efficient at
extracting dissolved oxygen. Gills have a large surface area and thin membranes to
facilitate gas exchange. (The lungfish also has lungs and it can breathe air.)
Counter-Current Exchange: The blood flow in gills runs opposite to the flow of water,
maximizing oxygen uptake.
4. Buoyancy Control
Swim Bladder: Many bony fish have a swim bladder. A swim bladder is an internal
gas-filled organ that helps maintain buoyancy. By adjusting the gas volume in the
swim bladder, fish control their buoyancy and remain at different water depths
without expending much energy.
Oil-Rich Livers: In cartilaginous fish like sharks, the liver is rich in oil. The oil helps
with buoyancy.
5. Sensory Systems
Lateral Line System: A series of sensory organs along the sides of the fish that detect
vibrations and water movements, aiding in navigation and predator avoidance.
Electroreception: Some fish detect electrical fields generated by other organisms,
which is useful for hunting and navigation in murky waters.
Well-Developed Eyes: Many fish have well-developed eyes adapted to their specific
environment, whether it is the bright, clear waters near the surface or the dark, murky
depths.
6. Feeding Adaptations
Mouth Position and Shape: The shape and position of a fish’s mouth depend on its
feeding habits. For example, bottom feeders have downward-facing mouths, while
predators have forward-facing mouths.
Teeth and Jaw Structure: Different fish have various types of teeth and jaw structures
suited to their diet, from the sharp teeth of predators to the grinding plates of
herbivores.
7. Camouflage and Defense
Coloration: Many fish have coloration that helps them blend into their environment,
whether through counter-shading (darker on top, lighter on the bottom) or mimicry.
Scales and Armor: Scales provide protection from predators and parasites. Some fish
have bony plates or spines for added defense.
8. Reproductive Adaptations
Diverse Reproductive Strategies: Fish exhibit a wide range of reproductive strategies,
from laying thousands of eggs to live-bearing young. These strategies help ensure the
survival of their offspring in various environments.
Nesting and Brooding: Some fish build nests or exhibit parental care to protect their
eggs and young from predators.
9. Temperature Regulation
Ectothermy: Most fish are ectothermic, meaning their body temperature is the same
as the surrounding water temperature. This adaptation helps them survive in various
thermal environments.
Regional Endothermy: Some fish, like certain species of tuna and sharks, maintain
higher temperatures in specific body regions (muscles, brain). This enhances their
performance in cold waters.
10. Hydrodynamics
Mucus Secretion: Many fish secrete mucus over their scales, reducing friction and
helping them move more smoothly through water. It also offers protection against
pathogens.
Evolutionary History of Fish
Fish are among the earliest vertebrates, with origins tracing back to over 500 million years ago
during the Cambrian period. Early jawless fish, such as ostracoderms, gave rise to more complex
forms. The development of jaws in fish like placoderms marked a significant evolutionary
milestone, leading to the diversification of the group into the vast array of species seen today.
Fish Taxonomy
Fish are classified into three major groups:
Agnatha (Jawless Fish): Includes hagfish and lampreys.
Chondrichthyes (Cartilaginous Fish): Includes sharks, rays, and skates.
Osteichthyes (Bony Fish): The largest group, including both ray-finned (Actinopterygii) and
lobe-finned fish (Sarcopterygii).
Detailed Taxonomy of Fish
Agnatha:
Class Myxini: Hagfish
Class Petromyzontida: Lampreys
Chondrichthyes:
Subclass Elasmobranchii: Sharks and rays
Subclass Holocephali: Chimaeras (ratfish)
Osteichthyes:
Subclass Actinopterygii: Ray-finned fish
Order Perciformes: Perches and allies
Order Cypriniformes: Carps and minnows
Order Siluriformes: Catfish
Subclass Sarcopterygii: Lobe-finned fish
Order Coelacanthiformes: Coelacanths
Order Ceratodontiformes: Lungfish
Approximately 40% of fish species live in freshwater, while the remaining 60% inhabit marine
environments.
Typical Anatomy of a Fish
Fish External Anatomy
Most fish have streamlined bodies and scales. Their gills, fins, and swim bladder are distinctive
external features.
Body Structure: Streamlined for efficient swimming.
Gills: Located on the sides of the head for breathing.
Fins: Dorsal, pectoral, pelvic, anal, and caudal fins for movement and stability.
Scales: Ganoid, cycloid, or ctenoid scales providing protection.
Swim Bladder: Present in many bony fish for buoyancy control.
Key Physiological Aspects
Like humans, fish have a heart, digestive system, kidneys, and several of the same senses.
However, they have gills instead of lungs, often lay eggs, and sometimes sense electrical fields.
Circulation: Fish have a closed circulatory system with a heart that pumps blood in a single
circuit.
Respiration: Gills extract oxygen from water and expel carbon dioxide.
Digestion: Begins in the mouth, continues through the esophagus, stomach, intestines, and ends
at the anus.
Excretion: Kidneys filter waste from the blood, excreting ammonia or urea.
Reproduction: Can be oviparous (egg-laying), ovoviviparous (egg-hatching inside the mother),
or viviparous (live-bearing).
Senses: Includes vision, smell, taste, touch, hearing, and the lateral line system for detecting
water movements.
Fish Cognition
Fish exhibit a range of cognitive abilities, including problem-solving, memory, and learning.
Studies indicate that fish navigate complex environments, recognize individuals, and exhibit
social behaviors. Manta rays are among the fish that pass the mirror test and may be self-aware.
Electrogenesis in Fish
Some fish, like electric eels and electric rays, possess specialized cells called electrocytes that
generate electric fields. These electric fields are used for navigation, communication, and
defense. Also, some fish sense electric fields.
Thermoregulation in Fish
Most fish are ectothermic (cold-blooded), relying on external sources to regulate their body
temperature. However, some species, like the opah (Lampris guttatus), are endothermic (warmbloodedness). Other endothermic or partially endothermic fish include billfish, tuna, the great
white shark, and thresher sharks. An endothermic fish maintains elevated body temperatures
through metabolic heat production.
Conservation Status
Many fish species are threatened or endangered due to overfishing, habitat destruction, pollution,
and climate change. Conservation efforts are crucial to protect these species and their
ecosystems.
Economic and Ecological Importance
Fish play vital roles in ecosystems as predators, prey, and contributors to nutrient cycles.
Economically, they are essential for commercial and recreational fisheries, providing food and
livelihoods for millions of people worldwide.
Behavior and Social Structure
Fish exhibit a wide range of behaviors, from solitary hunters like the great white shark to
complex social structures seen in schooling fish like herring. Some fish, like cichlids, display
parental care and protect their young from predators.
Practical Applications and Studies
Fish are important in various scientific research fields, including genetics, physiology, and
environmental science. Zebrafish (Danio rerio) are a model organism in developmental biology
and genetics research due to their transparent embryos and rapid development.
Interesting Fish Facts
Some fish display unusual “unfishlike” behaviors:
Lungfish: Breathes air and survives out of water for extended periods.
Clownfish: Exhibits sequential hermaphroditism, changing sex during its lifetime.
Mudskippers: Amphibious fish that walk on land using their pectoral fins.
Goblin Shark: Possesses a protrusible jaw that extends to catch prey.
Parrotfish: Produce a mucus cocoon at night to protect themselves from predators.