Fish

BY SYDNEY WHITE MATT LEAGAN

General characteristics• Most fish have 7 fins.• Many fishes have well-developed eyes and a keen

sense of vision that they rely upon to find and capture food, avoid predation, locate shelter and find a mate. Many fishes see color and some are able to see in very low light levels.

• Comprising close to 80 percent of the body of many fishes, muscles provide power.

Phylogenetic Relations• Fish are in a group called Craniata which describes

a skull that surrounds the brain, olfactory organs, eyes, and inner ear.

• Zoologists do not know what animals were the first craniates were. Evidence suggests that the craniate lineage may go back about 750 million years.

• Recent cladistic analysis indicates that a group of fish called Hagfishes are the most primitive living Craniates.

Jawless fish (Agnatha)• They have no endoskeleton.• They breath through gills. They

have seven or more gill pouches which are supported by bronchial arches.

• They have a closed circulatory system with a two chambered heart.

• There are two sexes and reproduction involves internal fertilization. They do not have any signs of raising their young.

• Includes the Hagfish and the Lamprey.

Cartilaginous Fish (Chondrichthyes)• They do not endoskeleton, the only part that is considered bone

are the teeth and the jaw. Everything is made of placoid scales.• They breath through 5 to 7 gills. They lack a swim bladder so

they have to keep swimming in order to keep the water moving across their gills.

• They have a closed circulatory system with a 2 chambered heart.• They do not lay eggs and do not raise their young. They use

internal fertilization and their young go through direct development.

• This group includes the Sharks, Rays, Ratfish and Skates

Bony Fish (Osteichthyes)• They are the only group with actual bones.• They breath through 3 gill slits but some fish such

as the lungfish can breath through lungs.• The have a close circulatory system with a heart

that has 3 valves.• They are dioecious an fertilize both internally and

externally. Some bony fish do raise their young.

Locomotion• Fish move through the water using their fins and body

wall to push against the surrounding water.• Fish tend to have five fins. The dorsal fin, caudal fin,

anal fin, pelvic fin, and the pectoral fin.• Dorsal Fin-This fin is used for balance and staying upright• Caudal Fin-This is also known as the tail and used for forward

movement.• Anal Fin-This fin is also used for balance• Pelvic Fin and Pectoral Fin- These fins are use to help the fish

steer through the water.• The muscles are arranged in a sideways W pattern.

The muscle bundles extend posteriorly and anteriorly in a zigzag fashion, contraction

Nutrition• Most likely the earliest fist here filter feeders

and scavengers that sifted through the mud of the sea floor

• Modern fish are predators and spend the majority of their time searching for food.

• Prey can vary greatly from fish to fish. Some fish feed on invertebrate animals, but many fish feed on other vertebrates.

• Fish will usually swallow prey hold, some fish also have teeth that are modified for crushing shells. Some fish such as the herring, paddlefishes and the whale shark are filter feeders.

• Filter feeders have a long gill process called gill rakers that trap plankton while the fish swims through the water with their mouths open.

Digestion• The digestive tract of a fish is very similar

to that of other vertebrates. • The stomach is used to store large

infrequent meals.• The small intestine is the primary site for

enzyme excretion and food digestion.• Sharks and other Elasmobranchs have a

spiral valve in their intestine • Boney fish poses out pockets of the

intestine called the pyloric ceca which increase absorptive and secretory surfaces.

• Swim Bladder-a gas-filled sac, usually along the dorsal body wall of bony fishes. It is an out growth of the digestive tract and regulates the buoyancy of a fish

Circulation• All vertebrates have a closed circulatory system

where a heart pumps blood, with red blood cells containing hemoglobin through a series of arteries, capillaries and veins.

• In most fish the blood passes through the heart once with every circuit around the body.

• A few fish like the lungfish have lungs and the pattern of circulation is altered.

• In the lungfish the path to the gills is the same but a vessel to the lungs has developed as a branch of the aortic arch.

• Operculum-The cover of a gill chamber of a boney fish.

Gas exchange• Fish live in a place that contains less than 2.5%

present oxygen.• Most fish have a muscular pumping mechanism to

move the water from the mouth, into the pharynx, over the gills and out through the gill openings.

• This is achieved by muscles that surround the pharynx and the opercular cavity which is between the gills and the operculum.

• Some sharks and open ocean fish such as the tuna maintain water flow by keeping their mouths open. This method is known as Ram Ventilation.

• Gas exchange occurs at a spot called the Pharyngeal Lamellae

• The gill arches support two rows of gill filaments. Blood flows into these filaments through branchial arteries which break into capillary beds within the lamellae.

• Water and blood flow in opposite directions on either side. Water between the lamellae is saturated with oxygen. The water will always loose oxygen because it always encountering blood with a lower oxygen concentration.

• Gill arches-these support the gills• Gill filaments-extend from each gill arch and

include vascular folds called the pharyngeal lamellae

• Counter current exchange 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.

• Cloaca- A common opening for excretory, digestive, and reproductive systems

Swim bladders and lugs• Most fish have chambers called pneumatic sacs.

Air enters these sacs and gas exchange occurs across the vascular surface. This is use by fish such as the Indian Climbing Perch, Lungfish. In this case they act as lungs. In other fish they are used as swim bladders.

Nervous and Sensory Functions• Like most vertebrates the nervous

system includes a brain and a spinal chord.

• Sensory structures are widely distributed over the body in addition to structures for touch, water temperature, vision, hearing, equilibrium and balance and detecting water movements.

• Fish have what are call external nares, these are openings that lead to olfactory receptors. They eyes of a fish are similar to other vertebrates but have no eyelid.

• Just like other vertebrates the receptors for equilibrium, balance and hearing are in the inner ear of a fish.

• Semicircular canals detect the rotational movement while other sensory patches help with equilibrium and balance by detecting the gravitational pull.

• Fish can hear, vibrations pass from the water through the bones of the skull to the middle ear, and some fish have chains of bony ossicles that connect the back of the skull to the swim bladder in which the vibrations strike the fish, amplified by the swim bladder and sent to the skull through the ossicles

• All fish have what is called a Lateral-Line System this consists of sensory pits in the epidermis that connect to canals just beneath the epidermis. These are used to detect water currents, or a predator or prey.

Electric Fish• Some fish are capable of Electroreception. This is

the ability to detect electrical fields that fish and other organisms generate.

• This ability has been demonstrated in over 500 species in seven families of Chondrichthyes and Osteichthyes. This ability is also highly evolved in sharks and rays

• The best known fish for producing and electric current is the electric Eel

• The organs that produce the currents are in the trunk of the eel and can deliver shocks that exceed 500 volts.

Excretion and Osmoregulation• Osmoregulation is a major function of

the kidneys and gills of fish.• Kidneys are located close to the midline

of the body, dorsal to the peritoneal membrane that lines the body cavity.

• The excretory structures are called nephrons and can be found in all vertebrates.

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

• Filtrate passes into a tubule system where the nutrients is absorbed then finally excreted

• Cloaca-A common opening for excretory, digestive, and reproductive systems.

Freshwater fish excretion and Osmoregulation

• Freshwater fish live in an environment containing few substances.

• To control excess water buildup and ion loss by never drinking and only taking in water when they are feeding.

• Freshwater fish have very short tubular systems so little water is reabsorbed. This makes fish produce large quantities of dilute urine.

• Active transport of ions into the blood at the gills compensates for this loss of ions.

Saltwater fish Excretion and Osmoregulation• Marine fish have the opposite problem. The

environment they live in contains 3.5% ions and their tissues contain 0.65%

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

• The nephrons of marine fish possess small glomeruli and long tubule systems.

Osmoregulation Cont.

Reproduction and development• Fish will often produce large number

of eggs with the hopes that they will be fertilized by the sperm of a male.

• Most of the fish are oviparous which means that the eggs develop outside of the female from the stored yolk.

• Some fish have special structures for sperm transfer. Fish like sharks have modified pelvic fins called claspers

• In many cases care of the embryos is limited or nonexistent. Some fish will build and tend nests.

• Some of the best brooders are the seahorses

Classification of Living Fish• Craniata

• Skull surrounds the brain, olfactory organs, eyes and inner ear. Unique embryonic tissue, neural crest, contributes to a variety of adult structures, including sensory nerve cells and some skeletal and other connective tissue structures.

• Subphylum Hyperotreti• Fishlike; skull consisting of cartilaginous bars; jawless; no paired appendages; mouth

with four pairs of tentacles; olfactory sacs open to mouth cavity; 5 to 15 pairs of pharyngeal slits; ventrolateral slime glands. Hagfish

• Subphylum Vertebrata• Vertebrae surround nerve chord and serve as primary axial support.• Hyperoartia

• A large, sucker-like mouth, reinforced by cartilage. Gill arches with spine-shaped processes.

• Class Cephalaspidomorphi• Sucking mouth with teeth and rasping tongue; seven pairs of pharyngeal slits; blind olfactory

sacs. Lampreys• Gnathostomata

• Hinged jaws and paired appendages; vertebral column may have replaced notochord; three semicircular canals

• Class Chondrichthyes• Tail fin with large upper lobe; cartilaginous skeleton; lack opercula and a swim

bladder or lungs. Sharks, skates, rays, ratfishes.• Subclass Elasmobranchii

• Cartilaginous skeleton may be partially ossified; placoid scales or no scales or no scales. Sharks, Skates Rays

• Subclass Holocephali• Operculum covers pharyngeal slits; lack scales; teeth modified into crushing

plates; lateral-line receptors in an open groove. Ratfishes.• Class Osteichthyes

• Subclass Sarcopterygii• Paired fins with muscular lobes; pneumatic sacs function as lungs. Lungfishes

and coelacanths• Subclass Actinopterygii

• Paired fins supported by dermal rays; basal portions of paired fins not especially muscular; tail fin with approximately equal upper and lower lobes; blind olfactory sacs.