Chapter 41 - Fishes 41-1 Introduction to Vertebrates Phylum: Chordata – Subphylum Vertebrata Like all chordates have the 4 required characteristics – Notochord, dorsal nerve cord, pharyngeal pouches, postanal tail 3 distinguishing characteristics – Vertebral column – Cranium – endoskeleton composed of bone or cartilage

Classification More than 45,000 species that occupy many different habitats Major groups of vertebrates – Class Agnatha: Lamphreys and Hagfishes – Class Chondrichthyes: Sharks, Rays, Skates – Class Osteichthyes: Bony Fishes (ex: salmon, catfish) – Class Amphibia: Amphibians (ex: frogs, salamanders) – Class Reptilia: Reptiles (ex: turtles, lizards, snakes) – Class Aves: Birds – Class Mammalia: Mammals

Evolution Most biologists think vertebrates originated about 550 mya – Oldest know fossils are of jawless fish Most did not have paired fins Bodies covered with heavy, bony scales Skeleton made of cartilage Diversified into many evolutionary lines Origin of Jaws – First fish w/ jaws and paired fins appeared 440 mya Seize and manipulate prey, increased stability and maneuverability – Jaws probably evolved from the first pair of gill arches

41-2 Jawless Fishes, Sharks, and Rays The term fish refers to 3 distinct classes Life in the water – Body plan of fish is well adapted Streamlined shape, muscular tail Paired fins: allow maneuverability Unpaired fins: increase stability Most tissues are denser than water Can control amount of gas (regulate vertical position) Use gills for gas exchange – Scales limit diffusion Lateral Line System: row of sensory structures that run the length of the fish’s body

Homeostasis Gain/lose water through osmosis – Based on salt content of environment Kidneys and gills play a large part in ridding body of waste products

Class Agnatha Jawless fish: Hagfishes and Lampreys Eel-like body, cartilaginous skeleton, unpaired fins Notochord remains throughout life

Hagfishes Small eyes that are beneath the skin Bottom dwellers in cold marine waters No jaws, so they feed by sawing the fish with its toothed tongue – Often burrow into body of dead fish and eat from the inside out Evade capture by tying in knots or secreting bad tasting slime When not feeding they remain hidden in burrows

Lampreys Have large eyes Live in both freshwater and marine environment – Always breed in freshwater – External fertilization Free living or parasitic – Feeding: attach by suction, tear a hole with toothy tongue, secrete chemical to prevent clotting

Class Chondrichthyes Sharks, Skates, and Rays Skeletons composed of cartilage Have moveable jaws, paired fins, and skeletons Almost all species live in salt water Skin covered with placoid scales: – Tooth-like spines that feel like sandpaper – Reduce turbulence of water flow

Sharks Many different food sources – Largest sharks (Whale and Basking) eat plankton Swim in a side-to-side motion b/c of their asymmetric tail fins. Behind their heads are paired pectoral fins that jut out of their bodies like the wings of a plane

Sharks Cont’d The shark’s mouth has 6 to 20 rows of backward- pointing teeth – When a tooth breaks from front row, another moves forward to take its place Amazing senses – Nostrils have specialized nerve cells that connect with olfactory bulbs in the brain – Sensitive to electrical fields (think muscle contractions) Gas exchange requires a continuous passage of water over a shark’s gills.

Rays and Skates Flattened bodies w/ paired wing-like pectoral fins – Some have whiplike tails Rays: diamond or disk-shaped bodies Skates: Triangular bodies Primarily bottom dwellers

Adaptations Gas exchange requires a continuous flow of water across the gills Do not release ammonia – Convert to urea, raises conc. of solutes to match sea water Maintain position in water – Generates lift as it swims to counteract sinking Uses a lot of energy – Store large amounts of lipids in the liver (increase buoyancy)

Reproduction Internal fertilization – Male transfers sperm into female body with modified pelvic fins called claspers Some lay yolky eggs right after fertilization Many allow eggs to develop inside the body – Young are born live No cartilaginous fish provide parental care for young after birth

41-3 Bony Fishes Class Osteichthyes – 95% of all known species of fishes Characterized by 3 key features – Bone Harder and heavier than cartilage – Swim bladder (or lungs) Gas filled sac used to control buoyancy – Scales Protection and help reduce water resistance

Characteristics Lobe-Finned Fishes – Have fleshy fins supported by a series of bones – Only seven species exist today – Ancestors of amphibians and all other terrestrial vertebrates Ray-Finned Fishes – Have fins that are supported by long, segmented, flexible bony elements called rays Probably evolved from scales Very diverse

External Anatomy (Yellow Perch) On each side of the head is the operculum – Hard plate that protects the gills Fins – Caudal fin: extends from the tail – 2 Dorsal fins (anterior and posterior) & Anal fin (ventral): help keep fish upright and in a straight line – Pelvic & pectoral fins: navigate, move up and down Skin (scales) – Covered in thin, round, overlapping disks – Grow throughout life of fish

External Anatomy

Digestive System Generalized carnivores Jaws armed with many sharp teeth that point inward to keep smaller fish and other prey from escaping Food passes from mouth  pharynx  esophagus  stomach  intestine  anus – Digestion takes place in out pockets of stomach called pyloric ceca – Liver and pancreas secrete digestive enzymes (bile and insulin) that help break down food

Circulatory System Delivers oxygen and nutrients & transports waste Consists of: – Two-chambered heart – Blood vessels – Blood containing red and white blood cells Heart pumps blood through arteries to capillaries – Small, thin-walled vessels in the gills – Blood picks up O2 and releases CO2 – Travels to body tissues for exchange Blood returns to heart through veins

The Heart 4 chambers in a row Deoxygenated blood empties into a collecting chamber called the sinus venosus Moves into larger atrium: causes contraction Sped up blood moves into the muscular ventricle – Main pumping chamber, driving force of circ. system Moves into final chamber, conus arteriosus – Has an elastic wall, contains valves to prevent blood from flowing back into the ventricle

Yellow Perch Heart

Respiratory System Large surface area of gills allows rapid gas exchange Gills supported by four sets of curved pieces of bone on each side of head – Each has double row of thin projections called gill filaments – Water taken in mouth, pumped across gill filaments, exits behind operculum Water flows away from head, blood toward – Known as countercurrent flow

Excretory Systems Kidneys filter chemical wastes from blood – Results in urine (contains ammonia, water, and ions) – Carried from kidney through system of ducts to the urinary bladder Stored then later expelled

Swim Bladder Gas bladder or swim bladder – Thin-walled sac in abdominal cavity – Contains mixture of oxygen, carbon dioxide, and nitrogen obtained from bloodstream – By regulating amount of gas in sac, fish adjust overall density and thus move up or down in water or hover at given depth

Nervous System Nervous system consists of: – Brain in three sections: forebrain, midbrain, hindbrain Olfactory bulb: process information on smells Cerebrum: integrate sensory info from other areas of the brain Optic tecum: receives and processes visual, auditory, and lateral line impulses; turns body toward/away from stimulus Cerebellum: coordinates motor function Medulla oblongata: regulates organs; relay station for stimuli – Spinal Cord Extends length of body, carries nerve impulses to and from the brain – Nerves that lead to and from all parts of the body Spinal nerves: connect spinal cord to internal organs, muscles, sense organs

Reproduction Spawning behavior is very diverse Separate sexes – ovaries in female = eggs; testes in male = sperm; both released through opening just rear of anus External Fertilization (usually) High mortality rate, often lay large #s of eggs A few bear live young – Female receives sperm during mating, and fertilization is internal – Carries eggs in body until young are born

Reproduction Cont’d