The Fishes

Very successful!

The oldest vertebrate group, but there is no evidence that they are declining from a period of earlier glory as did their amphibian and reptile successors. There are more bony fishes alive today than ever before, and no other group threatens their domination of the seas. They are more successful in terms of number of species (>21,000 species) and in numbers of individuals (countless billions) than all the other vertebrate classes combined.

Some of the reasons for fish success: Can move swiftly with fins and streamlined body and

can hang motionless at any depth by varying their buoyancy by adding or removing gas from their swim bladder.

Excellent sensory equipment. Acute olfactory and visual senses and unique lateral line system which is sensitive to water currents and vibrations (provides "distance touch").

Gills are the most effective respiratory devices in the animal kingdom for extracting oxygen from water.

Some of the reasons for fish success: Osmotic regulation. With highly developed organs of

salt and water exchange, bony fishes are capable of fine-tuning their body fluid composition in either freshwater or marine environments.

Complex behavioral mechanisms. Can deal with emergencies more cleverly than can any invertebrate (except for some cephalopods) and many have evolved elaborate reproductive behavior concerned with courtship, nest building and care of the young.

Class Agnatha (jawless fishes) Ostracoderms (all extinct)

Bizarre beasts! Water-living, with fishlike bodies and tails, but armored with thick plates of bone or bonelike material. They had scales over the trunk and tail, where motility for swimming was essential, and a solid layer of armor plates over the head region. (Forerunners of our skull?)

No jaws (mouth a small hole or crosswise slit). Probably were filter-feeders as were their predecessors.

No limbs or paired fins. At most a pair of flaps behind the head region or small spines at the sides of the body.

Ostracoderms Dorso-ventrally flattened bottom dwellers

grubbing in the mud. Moved from marine to fresh water

environment early in evolution. Why the armor? Protection. From what? Ostracoderms were not predators (why not?) Ancient cephalopods and crustaceans weren't a threat. (why not?) Eurypterids (water scorpions) were.

Eurypterid

Ostracoderms Eurypterids lived in same habitats and were much

larger (6 inches for fish vs. dozen feet for eurypterids. Eurs. had biting mouth parts and were obviously carnivorous. Basic food source for them must have been the Ostracoderms and only enemy of the fishes must have been the eurypterids.

Later vertebrates increased in size, became faster-swimming, and many migrated to the sea where they were comparatively free from attack. Correlated with these developments water scorpions dwindled in importance and disappeared. Higher vertebrates tended to loose armor, but portions survive even in the highest forms. In us may be superficial position of bones of head.

Ostracoderms Armor may also have provided protection

from flooding tissues in fresh water. Bone was important reservoir for Ca and phosphate ions.

Ostracoderms The fossil record and the kidney support the

conclusion that they involved in freshwater. In most fishes and the amphibians the kidneys

produce a dilute watery urine. The primitive kidney drains water from the interior of

the body and is an ideal osmoregulator for a freshwater organism.

It is a problem for a marine organism. In time blood became more dilute than sea water

(approx 1/3 as salty). This fixed the body fluid concentration and ionic composition for all vents. to evolve later, whether aquatic, terrestrial, or aerial (or marine).

Ostracoderms

What happens when returned to sea? Majority of fishes now live there. Two solutions.

1.The advanced bony fishes evolved special glands which excrete superfluous salts into the water passing through the gills.

2. The sharks use urea to increase salt content and become isosmotic or hypertonic to sea water. Protects him even though his kidneys continue to pump water out as they did in fresh water ancestors.

Living Agnatha (lampreys and hagfishes) Hagfishes are entirely marine scavengers. 32

species. Not very popular with fishermen. Lampreys (stone sucking) refers to habit of grasping stone with mouth to maintain its position in a current. 19 species in N.A., ½ of which are parasitic.

Nonparasitic species probably descended from the parasitic ones (don't feed as adults. No teeth, alimentary canal). All breed in f.w. streams. Long larval life (3-7yrs). Looks like archetypic chordate.

Major problem in great Lakes. Came in when Welland Ship Canal built in 1829. Took over 100 yrs., but decimated fishing industry in 50s and 60s. Under control now. Killed larvae and restocked lakes with resistant species.

 Placodermi (extinct jawed fishes). Placoderms are a highly varied group of

armored fish that appeared at the end of the Silurian period during the peak of ostracoderm radiation and became extinct about 150,000,000 yrs later. Descended from the Ostracoderms. Successful?

 Placodermi - Importance Hinged upper and lower jaws. Made possible a

truly predatory vertebrate. Could eat food that was not microscopic. First pair of gill arches was modified into the primitive upper and lower jaws. No fossil record of steps in process. Infer from embryological evidence.

Paired fins. Could swim a great deal better than the Ostracoderms.

Early evolution in fresh water, but radiated into marine environment.

  Placodermi - Importance Like Ostracoderms the head and thoracic

region was covered with large heavy armor plates; a few were encased in small bony scales.

At least some had true lungs. First vertebrates to make that statement.

From them probably came the Chondrichthyes (cartilaginous fishes) and the Osteichthyes (bony fishes).

 Class Chondrichthyes (Sharks, Rays, Skates and Chimera). A major evolutionary side branch. Appeared approx. 375,000,000 yrs. ago.

Greatest radiation approx 280,000,000 m.y.a.. Most are, and have been marine predators and scavengers. Used for food (1% of fish market) and leather.

Class Chondrichthyes - Characteristics Skeleton is only of cartilage. Notochord persistent. Heterocercal tail in which the dorsal lobe is larger

than the ventral lobe and includes the tip of the upturned vertebral column. Primitive; similar to the Placoderms. Your text suggests that it evolved to offset the tendency for fishes shaped as these are (and were) to rise up when swimming. The asymmetrical tail pushes the animal down so it remains stable.

Class Chondrichthyes - Characteristics Placoid scales = tooth-like projections

(dentacles) present in the skin and spines that form the leading edge of the dorsal fins of some species. They are believed to be remnants of bony armor that encased their predecessors. Teeth in mouth develop similarly and are continuously being formed and migrating forward to replace older or broken teeth throughout life.

Class Chondrichthyes - Characteristics Digestive system. Ventral mouth (with jaws) --> long

pharynx --> esophagus --> stomach (J-shaped) --> intestine (with spiral valve) --> cloaca.

Spiral valve is unique. Consists of a spiraling flap of absorptive tissue which greatly increases the surface of the intestine. Sharks swallow large chunks of food and spiral valve slows movement of food so digestive enzymes can work on it. Also increases the surface area for absorption.

Have liver, pancreas and gall bladder.

Class Chondrichthyes - Characteristics Kidneys osmoregulate by increasing salt

content of blood with urea and by excreting excess salt from kidney and rectal gland.

Brain. Two olfactory lobes, two optic lobes, a cerebellum, and a medulla oblongata. Not too good for deep thoughts.

Osteichthyes (bony fishes) The bony fishes experienced one of the

greatest adaptive radiations of any group. Their adaptations have fitted them for every

aquatic habitat except the most completely inhospitable.

Body forms vary from fusiform speeders to sedentary flatfish and everything in between. Read text for some idea of diversity of form and physiology which have led to such a radiation.

Osteichthyes First appeared in fresh-water deposits of late

Silurian. Didn't invade marine environment until Mesozoic (135,000,000 years ago).

Fish experts are as bad as Arthropod experts. Can't agree on even the broadest lines of evolution of the Osteichthyes.

Could have come from some ostracoderm group or from some placoderm group (I prefer the latter, but am certainly no expert).

Osteichthyes The important evolutionary trend is the

division of the Osteichthyes into the Actinopterygii (ray-finned fishes) and the Sarcopterygii (lobe-finned fishes) in the Devonian (345-405 m.y.a).

There is general agreement that Sarcopterygii came from placoderms so if they had a common ancestor the ancestor of both was a placoderm.

Osteichthyes - Characteristics Bony skeleton, notochord persists in

some, usually homocercal tail. Scales of various types are generally

present. Some are similar to the toothlike scales of sharks, but have surfaces of true enamel so there are no placoid scales.

Terminal mouth usually with many teeth, jaws present and more highly developed than in Chondrichthyes.

Evolution of jaws in fishes

Osteichthyes - Characteristics Respiration by four pairs of gills in common

gill chamber, covered by bony operculum (= movable external flap). Each gill has toothed raker that keeps food out of the gills.

In most, muscles in gills aid in pumping water from mouth through gills.

Nostrils in most fish not connected to respiratory passages; used only for smell.

Osteichthyes - Characteristics Primitive forms had lungs. Valuable in fresh water

where considerable seasonal drought occurred. In Actinopterygii lungs became swim bladder.

Selective advantage of a swim bladder is in the marine environment where the problem of buoyancy is greater than the problem of obtaining oxygen from the air.

Basic fish circulatory system. Two-chambered heart, closed system of vessels, 4 pairs of aortic arches, nucleated red blood cells.

Evolutionary significance Sarcopterygians are important because they

were the ancestors of the first land vertebrates.

Have internal nostrils, fleshy lobed, paired fins, and heavy scales that consist of a bony base covered by a thick layer of cosmine (a dentine-like substance) and spongy bone. Divided into lungfish and Coelocanths (living) and Rhipidistians (extinct).

Crossopterygians

Represented by only one living species, the marine coelocanth, Latimeria chalumnae. All coelocanths were believed to be extinct, but in 1939 the first specimen of Latimeria was dredged up from the depths of the sea off the east coast of South Africa.

Formerly thought to be ancestral to terrestrial vertebrates, now believed to be ancestors of specialized marine fishes.

http://www.museum.hu-berlin.de/home.asp?page=http://www.museum.hu-berlin.de/auss/rundgang/evolution_latimeria.asp?lang=1

Crossopterygians Preadapted for terrestrial life. Had lungs

since the time of placoderms and appendages that were well suited to a semi- walking gait and posture.

The limbs of crossopterygians were very effective for moving over the debris of a swamp environment, certainly much more so than the ray fins of actinopterygians, which are relatively easily torn.

Crossopterygians Latest research seems to put a freshwater

family, the rhipisistians, as the ancestor of the terrestrial vertebrates.

Skull bone pattern closest to earliest known amphibians.

Lobe fins highly developed. Each lobe had small bones and muscles which enabled fish to move along river bottoms like “walking catfish” of today. Better than lungfishes.

Lungs for air breathing, unlike swin bladder “lungs” of coelocanths.

Evolutionary significance The end of the Devonian was a period of great

seasonal droughts. Assume that lungs of crossopterygians allowed them to move from one pool to another as each dried up. Fleshy lobed, paired fins aided them in their movement on land.

Earliest amphibians. were little more than fishes (encountered dry land only as went between pools). Why become terrestrial? Insects as food supply and not much competition from other large animals? More diverse habitats?