Fishes

Jaws and paired appendages

Pectoral fins o fishes are appendages that are usually located just behind the head, and pelvic fins are usually located ventrally and posteriorly.

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Both sets of paired fins increase the agility of fishes by giving them a more precise steering mechanism.

Class Chondrichthyes - Cartilaginous fishes -

Most are carnivores or scavengers,

Epidermal placoid scales and a cartilaginous endoskeleton.

700 species

Sharks are covered by tough skin with dermal, placoid scales.

These scales project posteriorly and give the skin a tough, sandpaper texture.

Posteriorly pointed scales also reduce friction with the water when a shark is swimming.

The teeth of sharks are actually modified placoid scales.

As the outer teeth wear and become useless, they are replaced by newer teeth moving into position from inside the jaw.

Crowns of teeth in different species may be adapted for shearing prey or for crushing the shells of mollusks.

Sharks are <1m to 10m in length.

Skates and rays are specialized for life on the ocean floor.

They usually inhabit shallow water, where they use their blunt teeth to feed on invertebrates.

Their most obvious modification for life on the ocean floor is a lateral expansion of the pectoral fins into wing-like appendages.

The sing ray has a tail modified into a defensive lash - the dorsal fin persists as a venomous spine.

Class Osteichthyes - bony fishes

Have some bone in their skeleton and or scales, an operculum covering the gill openings, and lungs or a swim bladder.

20000 species

Bony fishes probably evolved form ancient agnathans.

Subclass Dipneusti - lungfishes

Only 3 genera survive today.

When freshwater lakes and rivers begin to stagnate and dry, these fishes use lungs to breathe air

queensland, Australia, tropical Africa and tropical South America

When a lake or river has nearly dried, these lungfishes burrow into the mud.

They keep a narrow air pathway open by bubbling air to the surface.

After the substrate dries, the only evidence of a lungfish burrow is a small opening in the earth

Lungfishes may remain in a dormant state for 6 months or more.

When rain again fills the lake or river bed, lungfishes emerge from their burrows to feed and reproduce.

Subclass Crossopterygii - Ancient fishes that possess muscular fins.

1938 people fishing in deep water off the coast of South Africa brought up fishes that were identified as being from a group of supposedly extinct crossopterygians, called coelocanths

Since then, numerous other specimens have been caught in deep water around the Comoro Islands off Madagascar

Subclass Actinopterygii - Ray finned fishes

They usually possess swim bladders, which are gas-filled sacs located along the dorsal and the body cavity and used to regulate buoyancy.

Infraclass Chondrotei - sturgeons and paddlefishes

most sturgeons live in the sea and migrate into rivers to breed.

They are very large (up to 1000 kg) and have bony plates covering the anterior portion of the body.

Heavy scales cover the tail.

The mouth of a sturgeon is small, and its jaws are weak.

Sturgeons feed on invertebrates that they stir up from the sea or riverbed using their snout.

Because sturgeons are valued for their caviar, they have been severely overfished.

Infraclass Holostei - garp

Has thick scales and long jaws that it uses to catch fishes.

Infraclass Actinopterygii - modern bony fishes - Teleostei

20000 species

They have a symmetrical caudal fin and a swim bladder that has lost its connection to the digestive tract.

Teleosts adapted to nearly every available aquatic habitat.

Swimming is less energetically costly for a fish than running is for a terrestrial organism.

Friction between a fish and the water is reduced by the streamlined shape of a fish and the mucoid secretions that lubricate the body surface.

Fishes move through the water using their fins and body wall to push against the incompressible surrounding water.

Muscle bundles of most fishes are arranged in a () pattern.

Because these muscles extend posteriorly and anteriorly in a zig-zag fashion, contraction of each muscle bundle can affect a relatively large portion of the body wall.

The forked shape of the tail reduces surface area that could cause turbulence and interfere with forward movement.

Evolution of jaws transformed early fishes into efficient predators.

Most modern fishes are predators and spend much of their life searching for food.

Kinds of food that one fish eats at different times in its life varies.

For example, a fish may feed on plankton as a larva but switch to larger prey, such as annelids or smaller fish, as an adult.

Prey are usually swallowed while.

Teeth are often used to capture and hold prey, and some fishes have teeth that are modified for crushing the shells of mollusks.

Other fishes, such as carp, feed on a variety of plants and small animals.

A few are primarily herbivores, feeding on plants.

An enlargement, called the stomach, is primarily used for storing large, often infrequent, meals.

The small intestine, however, is the primary site for enzyme secretion and food digestion.

Sharks and other elasmobranches have a spiral valve in their intestine, and body fishes possess outpockets of the intestine, called pyloric ceca, which increase absorptive and secretory surfaces.

All vertebrates have a closed circulatory system in which blood, with red blood cells containing hemoglobin, is pumped by a heart through a series of arteries, capillaries, and veins.

The vertebrate heart develops from four embryological enlargements of a ventral aorta.

In fishes, blood flows from the venous system through the sinus venosus, the atrium, the ventricle, the conus arteriosus, and into the ventral aorta.

Five afferent vessels carry blood to the gills, where the vessels branch into capillaries.

Blood is collected by efferent vessels, delivered to the dorsal aorta, and distributed to the body.

Fishes live in an environment that contains less than 2.5% of the oxygen present in air.

To maintain adequate levels of oxygen in their bloodstream, fishes must pass large quantities of water across gill surfaces and extract the small amount of oxygen present in the water.

Most fishes use a muscular pumping mechanism to move the water into the mouth and pharynx, over the gills, and out of the fish through the gill openings.

This pump is powered by muscles surrounding the pharynx and the opercular cavity, which is between the gills and the operculum.

Holding their mouth open while swimming is called ram ventilation.

Gas exchange across gill surfaces is very efficient.

Gills are supported by gill arches.

Gill filaments extend from each gill arch and include vascular folds of epithelium, called gill lamellae.

Blood is carried to the bills and into gill filaments in branchial arteries.

The arteries break into capillary beds in gill lamellae

Gas exchange occurs as blood and water move in opposite directions on either side of the lamellar epithelium.

This countercurrent mechanism provides very efficient gas exchange by maintaining a concentration gradient between the blood and the water over the entire length of the capillary bed.

Climbing perch, lungfishes, and ancient crossopterygians, pneumatic sacs function as lungs.

In other body fishes, pneumatic sacs act as swim bladders.

Fishes maintain their vertical position in a column of water in four ways.

One way is to incorporate low-density compounds into their tissues.

Fishes (especially their livers) are saturated with buoyant oils.

A second way fishes maintain vertical position is to use fins to provide lift.

A third adaptation is the reduction of heavy tissues in fishes.

The bones of fishes are generally less dense than those of terrestrial vertebrates.

The fourth adaptation is the swim bladder.

Using a swim bladder, buoyancy can be regulated to meet the day to day need of a fish by precisely regulating the volume of gas in it.

garp and sturgeons gulp air at the surface to force air into their swim bladders.

Most teleosts have swim bladders that have lost a functional connection to the digestive tract.

Gases are secreted into the swim bladder from the blood using a countercurrent exchange mechanism in a vascular network called the rete mirabile.

Gases may be reabsorbed into the blood at the posterior end of the bladder.

The e central nervous system of fishes, as in other vertebrates, consists of a brain and a spinal cord.

Fishes possess specialized receptors for olfaction, vision, hearing, equilibrium=, and balance, and for detecting water movements.

Opening in the snout of fishes, called external nares, lead to olfactory receptors.

In most fishes, receptors are located in blind-ending olfactory sacs.

The eyes of fishes are similar in most aspects of structure to those found in other vertebrates.

they are lidless, however, and the lenses are round. Focusing is accomplished by moving the lens forward or backward in the eye.

Receptors for equilibrium, balance, and hearing are located in the inner ears of fishes and their functions are similar to those of other vertebrates.

Vibrations may be passed from the water through the bones of the skull to the middle ear, and a few fishes have chains of bony ossicle.

Vibrations strike the fish, are amplified by the swim bladder, and sent through the ossicle to the skull.

Running along each side and branching over the head of most fishes is a lateral-line system.

The lateral-line system consists of sensory pits in the epidermis of the skin that connect to canals that run just below the epidermis.

Lateral lines are used either to detect water currents or for detecting a predator or a prey that may be causing water movements in the vicinity of the fish.

Low-frequency sound may also be detected.

Osmoregulation is a major function of the kidneys and gills of fishes.

As with all vertebrates, the excretory structures in the kidneys are called nephrons.

Nephrons filter blood-borne nitrogenous wastes, ions, water, and small organic compounds across a network of capillaries called a glomerulus.

The filtrate then passes into a tubule system, where essential components may be reabsorbed into the blood.

The filtrate remaining in the tubule system is then excreted.

To control the excess buildup of water and loss of ions, freshwater fishes never drink, and only take in water when feeding.

Also, the nephrons of freshwater fishes are numerous and frequently possess large glomeruli and relatively short tubule systems.

Thus, large quantities of very dilute urine are produced.

Marine fishes fact the opposite problem.

Their environment contains 3.5% ions, and their tissues contain approximately 0.65% ions.

Marine fishes, therefore must combat water loss and accumulation of excess ions.

They drink water and eliminate excess ions by excretion, defecation, and active transport across gill surfaces.

The nephrons of marine fishes frequently possess small glomeruli and long tubule systems.

Up to 90% of nitrogenous wastes are eliminated as ammonia by diffusion across hill surfaces.

the remaining 10% of nitrogenous waste are excreted as urea, creatine, or creatinine.

These wastes are produced in the liver and are excreted via the kidneys.

not unusual for a fish to produce millions of eggs in a single season.

The vast majority of these millions of potential adults will never survive to reproduce.

many eggs will never be fertilized, may fertilized eggs may wash ashore and dry, may eggs and embryos may be smashed by currents and tides, and others will fall victim to predation.

Some fishes show mating behavior that helps ensure fertilization, or nesting behavior that protects eggs from predation, sedimentation, and fouling.

Large schools and the release of eggs or sperm by one individual often releases spawning pheromones that induce may other adults to spawn.

The vast majority of fishes are oviparous, meaning that eggs develop outside the female from stored yolk.

ovoviviparous

viviparous.

Some fishes have specialized structures that aid in sperm transfer.

male elasobranchs, for example, have modified pelvic fins called claspers.

During copulation, a clasper is inserted into the cloaca of a female.

Sperm travel along grooves of the clasper.

in may fishes, care of the embryos is limited of nonexistent.

Some fishes however construct and tend nests, and some carry embryos during development.

Some of the best-known brooder include the seahorses and pipefishes.

Males of these closely related fishes carry embryos throughout development in ventral pouches.

The male brazilian catfish broods embryos in an enlarged lower lip.

Most fishes do little if any caring for young after they have hatched.