Introduction to Fishes

Characteristics of Fishes• All fish share four

common characteristics– 1. Fish have an

endoskeleton– 2. Fish have gills– 3. Fish have closed-

loop circulation– 4. Fish have Kidneys

• These adaptations allow fishes to live in oceans and freshwater habitats around the world

Movement and Response:• A fishes’ endoskeleton, fins, swim bladders,

and lateral line allow for swimming and sensing their underwater environment– Endoskeleton (Internal skeletons): • Most fish have a complete endoskeleton made of

either cartilage or bone.• Muscles that attach to the endoskeleton make strong

movements possible

Movement and Response• Fins and swim bladders: –Fish use fins, which are located on the back

and bellies, to allow them to increase their stability–Paired fins allow fish to turn, dive, climb

rapidly, move forwards, backwards, and up and down in the water–Fish use a swim bladder to regulate their

vertical position in the water by controlling the amount of gas that is in the swim bladder.

Movement and Response• Sensory Organs: – Sight: Fish use their eyes to sense light– Smell: Fish use one or two nostrils that leads to the

olfactory sacs to sense odor– Taste: Fish use taste buds in their mouths, lips, fins, and

skin – Sound: Fish use their inner ears – The lateral Line: this sense organ is a system of small

canals in the skin that are lined with cells sensitive to water vibrations cause by currents and pressure waves. The information obtained from the lateral line is used to direct a fish’s movement as they swim

Respiration and Circulation• Fish obtain oxygen from the water• The major respiratory organ of a fish is the gills.– Gills are made up of rows of filaments through which

gas enters and blood leaves.• Countercurrent Flow:– Occurs when water passes over the gills in one

direction as the blood flows in the opposite direction through the gills’ capillaries.

– Countercurrent flow allows the gills to be a very efficient respiratory organ allowing fish to extract 85% of the dissolved oxygen in the water around them

1. Oxygen-rich water enters the mouth and passes over the gills2. The water passes over the gill filaments from front to back. Blood moves through the filaments from back to front. When blood enters the filaments it is oxygen poor (Blue). When it exits the filaments it is oxygen rich (red)3. The oxygen-rich blood flows through the body. By the time it returns to the heart, the blood is oxygen poor again

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Respiration and CirculationSingle-Loop blood Circulation•After blood leaves the gill capillaries it moves through the vessels in the fish’s body•The blood then enters the heart•Fish have a simple chamber-pump heart•Blood collects in the heart’s atrium•Next the heart’s ventricle pumps the blood back into the gills•All fish have single-looped circulation except lungfish

Excretion • The body of most vertebrates is about 2/3

water and if they lose too much water they will die

• Minimizing water loss has become a key evolutionary challenge facing all vertebrates, even fish!

• Fish lose water due to osmosis.– Osmosis causes a net movement of water through

the membranes, such as gills and skin, towards regions of higher ion concentrations

Excretion• Salt and Water Balance: – Saltwater Fish

• The ion salt concentration of sea water is 3x that of most marine fishes’ tissues

• Therefore, fish lose water to their environment through osmosis• Salt water fish drink sea water and pump out the excess salt

through their gills to make up for lost water– Freshwater fish

• Their bodies have more salt than the surrounding water so they take in water due to osmosis

• The additional water they take in dilutes the salt• They regain salt by actively taking them in from their environment

– Because of the opposite challenges faced by the freshwater and saltwater fish very few fish can move between salt water and fresh water habitats

Excretion• Kidneys play an important role in maintaining

a fish’s salt and water balance– A kidney is an organ that regulates the body’s salt

and water balance and removes metabolic wastes from the blood

– Excess water and body wastes leave the kidneys in the form of fluid called urine• Saltwater fish only excrete a small amount of urine

because it is high in salt concentration, and excrete the majority of the body’s waste through the gills• Freshwater fish excrete a small amount of body waste

through the gills and the majority of their body waste is excreted through large amounts of diluted urine

Reproduction• Most fish reproduce sexually through external fertilization– External fertilization occurs through a process called spawning

where male and female gametes are released near one another in the water

– Spawning may occur between two individuals or in large groups. • When spawning occurs in large groups it produces millions of fertilized

eggs and increases the number of individual that will survive

• In sharks, skates, and rays eggs are fertilized inside the female’s body– In most species the egg will develop inside the female’s body and

the young are born live– The newborns are much larger than fish that are hatched from

eggs– The newborns are also less likely to be eaten by a predator

because they are born larger

Assessment Questions• Discuss the key characteristics found in all

fishes• Describe how fish use their fins• Summarize how the countercurrent flow helps

a fish obtain oxygen• Explain why fresh water fishes excrete more

urine than marine fishes do• Contrast reproduction in sharks with that of

most other fishes