1. Introduction to Fishes

2. 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

3. 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

4. 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.

5. 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

6. 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

7. 1. 2. 3.
1. Oxygen-rich water enters the mouth and passes over the gills 2. 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

8. Respiration and Circulation
Single-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

9. 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

10. Excretion Salt and Water Balance: Saltwater Fish Freshwater 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

12. 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

13. 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

14. 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