Section 12.2Fish

Fish• Are the most numerous and

widespread of the vertebrates

• Key feature of all fish are that they have gills for exchange of oxygen and carbon dioxide with the water

• There are nearly 22,000 species of fish– 59% live in slat water– 40% live in fresh water– 1% move regularly between

fresh and salt water

Fish

• Fish are divided into 3 classes:– Jawless Fish• Class Agnatha

– Cartilaginous Fish• Class Chondrichthyes

– Bony Fish• Class Osteichthyes

• Use to be more classes but some went extinct– 2 classes to be exact

Fish Groups

Jawless Fish

• Fall under the Class Agnatha• Have no jaw, and lack paired fins• They have:– Slimy skin– Eel-like bodies– Have notochord and skeleton made of cartilage

• The class Agnatha is made of about 70 species of lampreys and hagfish

Hagfish

• Are bottom dwellers• Scavenge by feeding on

dead/dying animals• Mouth is hard to see but

is surrounded by fleshy tentacles called “feelers”

• Even without a jaw has a row of tooth like plates allowing it to tear flesh from it food

Slime glands

Lamprey• Live in fresh and salt water• Spawn in streams and live

their as juvenilles• When mature undergo

metamorphosis to adults• As adults use their sucker like

mouth to attach to its prey, then use their teeth to tear a hole in the victim and suck out blood and other bodily fluids– They are parasitic and as a

result have negatively effected commercial fishing

– They are a big issue in the Great lakes

Lamprey

Fish Evolution• For the rest of the groups of fish the key evolution steps

taken by fish were:– Development of a jaw

• Allowed fish to utilize other food they could not before– Have 2 pairs of fins

• Allowed for better maneuvering in the water– Mineralization

• Allowed skeleton to be made of bone instead of cartilage • These new adaptations came during the late Silurian and

Devonian periods– Note not all classes made these steps and even though

successful for some it did not help others– Devonian is often referred to as the “age of fishes” due to their

growth and diversification

Jaw Evolution

• The jaws evolved via modification of skeletal rods that previously supported the anterior pharyngeal slits– The remaining gill slits remained the site of

respiration

Jawed Fish

• With the evolution of the jaw there were 4 new classes of fish that would come about

• The classes are as follows:– Chondrichthyes– Osteichthyes– Placoderms– Acanthodians

The Classes that Went Extinct

Placoderms• Were fishes with armoured

heads• Extinct by beginning of the

Carboniferous period, 360 million years ago

Acanthodians• Were spiny fishes• Extinct by beginning of the

Carboniferous period, 360 million years ago

(a) Coccosteus, a placoderm

(b) Climatius, an acanthodian

Class Chondrichthyes

• The cartilaginous fish• Class includes fish that

have jaws, paired fins and cartilaginous skeletons

• As a result this class includes:– Sharks– Rays– Skates

Sharks• Best known of the cartilaginous fish• The streamlined body and paired fins of sharks enhance

stability and maneuverability– Body is covered with placoid scales, help keep streamline and give

sand paper texture to skin – Powerful muscles power undulations of the body and caudal

fin to drive the fish forward.– The dorsal fins provide stabilization.– While some buoyancy is provided by low density

oils in large livers, the flow of water over the pectoral and pelvic fins also provides lift to keep the animal suspended in the water column. • This is why if sharks stop swimming they will sink

Sharks

• Majority are carnivorous– Funny thing is largest shark, the whale shark, is a filter feeder

• With this life style sharks have various adaptations that allow them to be successful– Their strong jaws and triangular teeth– Teeth are continually being replaced– Have sharp vision and excellent sense of smell– Sharks can detect electrical fields, including those generated by the

muscle contractions of nearby prey, through patches of specialized skin pores.

– The lateral line system, a row of microscopic organs sensitive to pressure changes, can detect low frequency vibrations • Allows shark to detect movement in the water

Shark Reproduction• Shark eggs are fertilized internally• Sharks then with their fertilized eggs do one of

the following depending on the species:– Oviparous sharks encase their eggs in protective

cases and lay them outside the mother’s body.• These hatch months later as juveniles.

– Ovoviviparous sharks retain fertilized eggs in the oviduct.• The embryo completes development in the uterus,

nourished by the egg yolk.– A few sharks are viviparous, providing nutrients

through a placenta to the developing offspring.

Rays and Skates• Rays and skates though closely related to sharks, have

adopted a very different lifestyle.– Most are flattened bottom dwellers that crush mollusks and

crustaceans in their jaws.– As their mouths are often on the ocean bottom they take in water

via modified gill slits called spiracles, as opposed to their mouth like most fish

– The enlarged pectoral fins are used like wings to propel the animal through the water.

– The tail of many rays is whiplike and may bear venomous barbs for defense against threats.

Class Osteichthyes• Key features of the bony fish: – Skeleton is made of bone – Have flexible fins that are better for steering and

propelling themselves• Some bony fish can swim around 80km/hr for short bursts

– Body covered in scales– Like sharks have lateral line system to allow detection

of movement in the water– Control buoyancy with an air sac known as a swim

bladder– Gills are covered by bony flap called operculum

• Breath by drawing water over their gills• They are the most numerous vertebrates

Typical Body and Structures

Swim Bladder• Is not found in primitive fish (like sharks) and for this

reason if they stop moving they sink• Where as bony fish with their swim bladder can stop

and will not sink• This works as the fish can change the amount of air

held in the swim bladder– This changes the fishes buoyancy allowing them to move

up/down in the water column– Change buoyancy by moving air between the swim

bladder and the blood • Air from blood into bladder, fish goes up• Air from bladder into blood, fish goes down

Reproduction

• Most are oviparious– This is external

fertilization– Female deposits eggs

and male release fluid called milt which contains sperm, to fertilize the eggs

Class Ostechthyes

• This class though is often divided into 3 subclasses based on the fish alive today:– Ray-finned fishes– Lobe-finned fishes– Lungfishes

Ray-Finned Fish• Most bony fish belong to

this subclass and are found all over the world– When you think of a fish

odds are it’s a ray-finned fish

• The fishes fins are supported mainly by long flexible rays

• Fins may be modified for maneuvering, defense, and other functions

Lobe-finned Fish• Most are extinct and those living are found in

only a few places and only found recently (late 1930s)

• Have muscular pectoral and pelvic fins– Fins supported by structures of bony skeleton

Lungfish

• Only found in the southern hemisphere

• Have gills and lung like air sacs allowing them remove oxygen from air and water

Homework

• Read pages 452-454

• Answer Questions– 1-7 page 454