Phylum Chordata: Vertebrates

Phylum Chordata: Vertebrates

• Fish represent the most diverse and successful group of living vertebrates.
• Almost half of all vertebrate species are fish, totaling around 28,000 living species.
• Approximately 200 new fish species are described each year.
• Despite their abundance and diversity, fish remain the least known group of vertebrates.
• Estimates suggest only slightly over 1/3 of fish species in the Amazon river basin have been collected and described.
• Fish are also the oldest known vertebrates.
• The first true vertebrates appeared in the fossil record 530 MY ago (early Cambrian).
• A fossil shark species from 310 MY ago was the first vertebrate known to migrate.
• This species lived in rivers but swam to the sea to breed.
• The first fish appeared over 500 MY ago and were small, bottom-dwelling animals without jaws, teeth, or paired fins.
• These early fish likely sucked up small particles of food from the bottom sediments.
• For about 50 million years, fish were the only vertebrates on Earth.
• They eventually developed primitive fins that improved their swimming ability.
• Fish also developed heavy bony armor and roamed the earth in this form for over 20 million years.
• Eventually, most fish lost the heavy armor and developed more streamlined bodies, jaws, and paired fins.
• This "modern" design produced almost all the varieties of fish that exist today.
• All fish are aquatic and highly adapted for aquatic life.
• Fish occupy virtually every kind of freshwater and saltwater habitat.
• There are no terrestrial fish, although some, like the "walking catfish", can survive considerable time outside of water and crawl on land.
• The closest to terrestrial is the "mangrove killifish," which normally lives in muddy mangrove swamps but can be found stuffed into insect tracks in rotting logs when they dry up.

Size of Fish

• The smallest fish is the stout infantfish, Schindleria brevipinguis, found in Australia; males are 7 mm long, females are 8.4 mm, and they weigh 1 mg, other small fish include Pygmy Goby less than 1/2 inch.
• The largest fish is the whale shark, reaching about 50-70 feet and 40 tonnes.
• Most fish continue to grow throughout life, unlike birds and mammals which stop growing at adulthood.
• Growth is generally temperature-dependent.
• Annual rings are produced in scales, otoliths, and other bony parts, allowing for accurate age determination.

Skin

• Fish skin consists of two layers: the epidermis and dermis.
• The epidermis usually secretes slimy mucous, reducing friction by up to 66% and improving swimming efficiency.
• The dermis can be tough and leathery or relatively thin.
• The dermis produces scales in most fish.
• Scales are under the slimy epidermis.
• Scales are made of enamel and sometimes dentin, the same basic components as teeth.

Skeleton

• The fish skeleton begins as a cartilage framework.
• The cartilage is replaced by bone in most fish by adulthood.
• Bone is a tissue unique to vertebrates, originating as an external protective covering.
• It later evolved into an internal supporting framework.
• A highly flexible "backbone" of cartilage or bone is the main support and framework for swimming muscles.
• The number of vertebrae on a fish varies from 16 to over 400.
• Most fish have ribs that support swimming muscles, but they are not used for "breathing".
• Bone or cartilage supports the dorsal, ventral, and caudal fins (unpaired fins).
• Pectoral and pelvic fins are paired appendages, known as the "appendicular skeleton" and are homologous to arms and legs.
• Fins act as rudders, for balance, feelers, weapons, sucking discs, or lures to attract prey.

Movement

• Most of a fish's body mass is bundles of segmented muscle tissue called myomeres (myotomes).

Muscles

• Fish have a relatively small body cavity for other organs.
• Muscles are segmented with zig-zag "W"-shaped bands along the sides.
• Muscles are mainly for swimming, although some fish can walk, crawl, burrow, and "fly."
• Less energy is required for swimming than other forms of vertebrate locomotion because they don't need to fight gravity.
• Water is 800 times denser than air, reducing the need to fight gravity.
• Each myomere consists of short muscle fibers connected to tough connective tissue that is also attached to the next myomere.
• Myomeres extend across several vertebrae, allowing for more power and fine control.
• Myomeres produce an "S" shaped swimming motion.
• Fish "push" on the water to move.
• Water is relatively dense and non-compressible.
• The fastest fish exchange the snake-like motion for a more rigid position where most of the flexing is toward the tail only.
• Overall, swimming speeds are not particularly fast compared to running or flight due to the high density of water.
• Larger fish can usually swim faster.
• Cruising speed is usually much slower than top speed.
• Speeds reported for fish are often speeds when they jump out of the water, giving the impression it is faster.
• Flying fish can glide above the water for 20-40 seconds.
• Dorsal and ventral fins improve swimming efficiency.

Feeding and Digestion

• After the evolution of jaws, fish were freed from deposit feeding and filter feeding.
• Most modern fish are carnivores.
• Small, numerous, sharp teeth are used to seize prey.
• Most fish have very flexible jaws to engulf large prey.
• Some can eat prey as large as they are.
• Most fish produce new teeth continuously throughout life.
• A few fish that scrape algae from hard surfaces have teeth that can bend (rather than break) as they scrape.
• Most fish lack moveable tongues.
• If tongues are present, they are not moveable.
• Fish swallow their food whole; they don't "chew", as chewing would produce pieces that might clog their gills.
• The intestine became longer to provide a greater surface area for absorbing a variety of nutrients.

Respiration

• Most fish have gills for getting O2 from water.
• Gills consist of thin feathery sheets with many blood vessels.
• Some fish can also breathe through their skin.
• A few fish can breathe air.
• The bearded goby flourishes in a dead zone off the coast of Namibia.
• These fish can survive in waters where oxygen is <10% and toxic hydrogen sulfide levels are high.
• They accomplish this by slowing their metabolism and returning to surface waters at night.

Circulation

• Blood circulation is closely tied to gas exchange through gills.
• Blood is pumped through arteries and veins by a simple heart.
• Fish have a 2-chambered heart (1 auricle-1 ventricle).
• Blood flows through a single circuit: heart, arteries, capillaries, veins, back to the heart.
• Blood is first pumped through gills and then out to the rest of the body.

Body Temperature/Thermoregulation

• Fish are cold-blooded, or poikilotherms, having a body temperature the same as their environment.
• Some fish maintain a higher temperature in their swimming muscles, as much as 20°C warmer than the surrounding water, as demonstrated by tunas, mako sharks, and swordfish.
• Other fish elevate the temperature of their brains and retinas such as marlins.
• These higher temperatures promote swimming and nervous activities.
• It promotes more precise vision and faster reflexes in predators.

Nervous System

• Fish brains are relatively small and simple compared to other vertebrates, but considerably more developed than in invertebrates.
• The fish brain is made up of several distinct functional areas: cerebrum (higher centers), cerebellum (coordination of movement), and brain stem (automatic activities).
• Fish sleep, staying motionless for several hours.
• Some marine species may bury themselves in sand or spin mucus "sleeping bags" each night to sleep.

Sense Organs

• Light doesn't travel as far in water, which also causes turbidity, while sound and pressure waves travel faster and farther.
• The lateral line system is "distance touch" and the most important sense in fish for sensory information, used to detect food or danger.
• A set of interconnected tubes and pores along the sides of the head and body detects water movements and unusually low frequency sounds.
• Electroreceptors detect body electricity of other fish and prey.
• Chemoreceptors are equivalent to the sense of smell.
• Fish have paired immovable eyes and most lack eyelids.
• In vertebrate eyes, a circle of muscle (ciliary muscle) focuses the lens for near and far vision.
• In most fish, the muscles of the lens are relaxed for near vision.
• In sharks (as well as amphibians, snakes, and humans) eyes are relaxed for far vision.
• Muscles in fish eyes move the lens forward or backwards to focus, unlike eyes that stretch the lens to change focus.

Variations in Fish Eyes

• One eye of flounders migrates to the same side as the other during embryonic development.
• A few freshwater fish have "bifocal" lenses to see above and below the water simultaneously,enhancing prey perception.
• Fish can see in color.
• They can also detect polarized light for navigation.
• Fish eyes get proportionately larger in dimmer waters.
• Fish eyes can be small or absent altogether in complete darkness, for example cavefish.

Chemoreceptors

• Chemicals travel well in water.
• There is no clear distinction between smell and taste.
• Chemoreceptors are located in the mouth, around the head, and sometimes all over the body.
• Catfish and loaches have barbels around their mouth (barbels also for touch).
• Olfactory sacs are their nose.
• Fish don't "breath" through their nose.
• Nostrils are just sensory pits.
• Most fish have a good sense of 'smell', for prey detection and communication.
• Fish lack taste buds.
• Fish can apparently discriminate between bits of food and trash they take in, and expel unwanted items that they eat.

Hearing and Touch

• Sound is an important means of communication in fish, especially deepwater fish.
• Fish have an inner ear with distinctive bones that vibrate when sound waves pass through, sending a signal to the brain.
• Barbels detect touch.

Orientation and Balance

• The otolith organ and semicircular canals are use for detecting orientation and changes in motion.

Excretion

• Like other vertebrates, have kidneys to remove most metabolic (Nitrogen wastes).
• Kidneys also play a role in salt/water balance.
• Gills also play a role in excretion,secreting NH3 and other N wastes.

Osmoregulation

• The body fluids of all animals must be in balance with their environment or they must have mechanisms that allow them to regulate their own salt and water balance.
• Marine invertebrates have body fluids with the same salt concentrations as seawater (3%), isotonic to their environment.
• Freshwater fish have salt content ~100 times greater than the freshwater they live in (Hypertonic).
• There is a tendency for them to gain water and lose salts.

Marine Bony Fish

• Most alive today evolved from freshwater fish.
• So, they have salt content about one third that of seawater, it is hypotonic to their environment.
• Marine fishes tend to lose water and gain salt.
• Scales and mucus make a fish's body almost completely impermeable to water.
• Most gain or loss of water and salt is through gills or the lining of the mouth.

Maintaining Salt and Water Balance

• Freshwater fishes must avoid drinking,urinate, and absorb salt ions through gill and from food.
• Marine (bony) fishes drink large amounts of seawater,produce little urine,while pumping out excess salts from gills.
• Because of these osmotic limitations, 90% of all fish are restricted to either freshwaters or saltwater.
• Only 10% of fish can easily move between freshwater and saltwater = euryhaline fish and can actively regulate to maintain salt balances.

Reproduction and Development

• Only a few are hermaphrodites, with male and female gonads but only one functional at a time.
• Also, only a few reproduce by parthenogenesis.
• Most fish are dioecious.
• Genders may not be easily distinguished externally.
• Most with external fertilization (oviparous).
• But a few have internal fertilization.
• Some, eg. guppies, can store sperm for months.
• In addition, a few with internal fertilization bear live young.
• Most produce large numbers of eggs, of which less than 1/million will survive to maturity.
• Most spawn at certain times of the year,when spawning is temperature dependent.
• Temperature is critical for survival of eggs and young.

Reproduction in Marine fish

• In most marine fish, males and females come together in great schools and release millions of egg and sperm.
• Eggs become part of the plankton through embryonic and larval development.

Reproduction in Freshwater fish

• Freshwater fish often have elaborate mating behaviors, many make nests, bear live young, and/or show parental care.

Kinds of Fish

• There are 28,078 species of fish.
• There are 3 different classes of vertebrates categorized as "fish":
  • Jawless fish (Agnatha) with 108 species, like Hagfish and Lampreys.
  • Cartilaginous fish (Chondrichthyes) with 970 species, including sharks, stingrays, manta rays, and sawfish.
  • Bony fish (Osteichthyes) with 27,000 species (96% of all fish).
    • Bony fish are the most abundant living group.
    • The two main types of bony fish are ray-finned fish and lobe-finned fish.

Agnatha (Jawless Fish)

• There are 108 living species of jawless fish.
• Jawless fish are remnants of the earliest fish in the fossil record and are the oldest known vertebrates.
• They are not technically "vertebrates" since they have no vertebrae, just a cartilage rod for support, but they are clearly relatives of the other fish groups.
• Jawless fish are the most ancient and primitive vertebrate group.
• Fossils discovered that are over 500 MY of jawless fish.
• Jawless fish are the only living vertebrate group with no jaws and also lack paired fins.
• Main groups of Agnatha:
  • Ostracoderms - all extinct
  • Hagfish
  • Lampreys

Ostracoderms

• Circular or slit -like mouth opening without jaws.
• Some of the earliest vertebrate fossils are in this group which lived from the late Cambrian to Ordovician Periods.
• The fossils recently pushed the date back to 530 MY.
• Ostracoderms flourished for 150 MY and became extinct at end of Devonian Period.
• Fish were armored with heavy dermal bony plates for protection against eurypterids and cephalopods.
• Ostracoderms had no paired fins.
• They probably stayed close to the bottom and were likely deposit feeders or filter feeders.
• Ostracoderms may have filtered water for food,using pumping action of their muscular pharynx, but not with cilia & mucus.
• Some ostracoderms may have even been predators.
• Later groups of ostracoderms had paired pectoral fins,whcih improved swimming ability
• They also exhibited a more sophisticated nervous system and sense organs that are in common with todays lampreys.

General Characteristics of living Agnatha:

• All modern surviving relatives of ancient Agnatha have lost their bony armor.
• They can be freshwater or marine dwellers.
• Live with a long eel-like body form.
• Without jaws or paired fins, generally poor swimmers.
• Have smooth, slimy skin, no scales.
• Skeleton is a simple rod of cartilage = notochord, no bone.
• Gills located inside 5-16 pairs of pore-like gill openings. Cartilage gill supports hold the gill-slits open.
• A spiracle at top of head can draw water in and over gills.
• Poor sense organs.
• Parasitic or scavengers
• The two main groups of fish today are Hagfish and Lampreys.

Hagfish

• There are approximately 65 species.
• All are marine.
• They are about 18" long and the largest known is almost 4' long.
• Live in deep waters.
• Almost completely blind and eyes have degenerated.
• They are mainly scavengers that eat dead or dying fish, molluscs, annelids, etc.
• Hagfish have small eyes,enter dead or dying animal through an orifice or by digging into the animal, enter dead or dying animal through an orifice, and can quickly find food by touch and smell.
• They have 2 keratinized toothed plates on its tongue.
• Hagfish rasps bits of flesh from carcass
• Hagfish can absorb food directly into their body through their skin.

Hagfish Circulation

• Hagfish have an unusually low pressure circulatory system
• with 1 main heart and 3 accessory hearts to boost blood pressure through gills.

Hagfish Excretion

• Hagfish are the only vertebrate with body fluids the same concentration as salt water (invertebrate trait).

Hagfish Protection

• Secrete copious amounts of slime at 500 ml/min.
• Milky fluid from slime sacs is along sides of the body.
• Fluid on contact with seawater forms a very slippery material making them impossible to hold and can secrete enough slime to turn a bucket of water into a gel in a few minutes.
• This formation can clog the gills of predators and may be able to extricate themselves from jaws of predator by "knot tying” behavior.
• Hagfish breeding habits are still relatively unknown.
• Some hagfish are hermaphroditic as the animal has both male and female gonads but only one or the other is functional.
• Lampreys have no larval stage.

Conodonts

• Numerous fossils of small tootlike structures have been known for years.
• They are commonly used as index fossils to date sediments.
• Until 1980 fossils of complete animal were found so scientists were unsure what animal produced them.
• Conodonts were an agnathan, apparently most closely related to hagfish, and had toothlike plates in throat area.
• Conodonts possess features of notochord, swimming muscles as myomeres, paired eyes, and similar inner ear structure.

Hagfish Human Impacts

• Hagfish are a bane to some commercial fishermen using gill or set nets so they have often devoured internal contents of fish often devoured internal contents of fish by the time they pull catch in hagfish.
• Hagfish are not often a pest with trawl fishing.
• Hagfish are collected for "leather" to make golf bags and boots today.
• Their slime has unusual properties since it is reinforced with strong, stretchy, spidersilk-like protein fibers.
• The substance is being researched for potential uses for stopping bloodflow in accidents and surgeries.
• Scientist are also investigating its use for parachutes, packaging and clothing to make synthetic fabrics similar to nylon and polyester which are petroleum based.
• Some species are in serious decline due to over harvesting.

Lampreys

• There are 38 species. The US has 20 species in North America.
• They reach 15 - 60 cm and some species are nearly 1 M long.

Lampreys Feeding and Digestion

• Most are parasitic. Others don't feed as adults.
• Parasitic adults attach to prey by sucker like mouth,rasp away flesh with teeth to suck out blood, and inject anticoagulant.
• When finished, the lamprey releases its hold,leaving the host to sometimes dies from wound

Lampreys Reproduction

• All species spawn in winter or spring.
• They spawn in shallow freshwater streams.
• The male builds nest,and uses oral disc to move stones and make a shallow depression.
• Next, the female joins him.
• The female attaches to rock to hold position in current.
• The male attaches to female and fertilizes eggs as they are released.
• Adults die after spawning.
• Eggs are sticky, covered with sand and in 2 weeks eggs hatch into ammocetes larvae.
• Close resemblance to amphioxus (cephalochordate).
• Larvae drift downstream and burrow into sand.
• Larvae are suspension feeders for 3-7 years,feeding on microscopic organisms
• Then rapidly metamorphose into adults: larger eyes,oral disc with rasping teeth,nostrils shift to top of head,and body shorter and rounder.
• All lampreys migrate up freshwater streams to breed.
• Eggs and larvae develop in freshwater.
• Young of marine species then migrate to ocean until sexually mature.

Human impact on Lampreys

• Petromyzon first invaded the great lakes in 1913-1918 (bioinvasion) then destroyed great lakes fisheries in 1950's
• Rainbow trout, whitefish, lake herring, and other species populations were destroyed.
• Then, their numbers began to decline in early 1960's due to depleted food
• Expensive control measures have been introduced, using expensive larvicides placed in selected spawning streams.
• Today, some native species have been restocked and are now thriving again.

Jawed Fish: Origin of Jaws & Paired Fins

• The oldest known jawed vertebrates appeared 420 MY (Silurian), about 100 MY after the jawless fish first appeared.
• Notable species from this time include placoderms and acanthodians.

Evolution of Jaws

• Jaws were one of the major events in the history of vertebrates that freed from bottom feeding an allowed access to a much greater variety of food sources
• Jaws evolved from gill supports (gill arches).
• In certain primitive fish the jaws resemble gill arches in shark embryology, jaws develop from gill arches, and cranial nerve branching and placement in jaws and gills of cartilaginous fish resembles each other.
• Initially, jaws just "closed the mouth' that allowed feeding on larger foods like seaweeds, plant and animals
• Later, jaws became armed with dermal scales that evolved into teeth.
• Jaws allowed predatation on larger active prey.
• And around the same time that jaws allowed eating larger pieces of food, the stomach was formed from an expanded pouch of the gut.

Origin of Paired Fins

• Acanthodians and placoderms were also 1st fish with paired fins.
• All living jawed fish (gnathostomes) have paired pectoral and pelvic fins.
• Pectoral fins appeared before pelvic fins.
• Probably originated as stabilizers for swimming and might have begun as folds in skin along sides of animal, with later muscle attachments, becoming moveable.
• In addition to moveable jaws and paired fins,all jawed fish also have paired nostrils and 3 semicircular canals for equilibrium.

Main Groups of Jawed Fish

• Acanthodians (extinct)
• Placoderms (extinct)
• Chondrychthyes
• Osteichthyes

Acanthodians

• First group of jawed fish to appear in the fossil record (430-260 MY).
• Most are relatively small, 4-6 inches.
• Acanthodians had scales rather than bony plates.
• Most were covered with small, diamond shaped dermal bony scales.
• Teeth are small and sharp on the jaw.
• Probably active swimmers.
• Gave rise to the bony fish.
• Devonian = "age of fishes" due to the explosion in fish diversity once jaws the fins evolved.

Placoderms

• Appeared ~20 MY after the first acanthodians.
• Most were medium-sized; 1-2 feet long but some specimens reached up to 30 feet long.
• Heavily armored fish. Dermal bony plates over front 1/3rd to 1/2 of body, but the rest of the body had small bony scales or was without armor.
• Were relatively poor swimmers.
• First vertebrates to have necks separating their heads from forelimbs. It allowed them to move their heads independently.
• It was a very diverse group.
• Some were predators,others were bottom dwellers,some had crushing mouthparts and ate shellfish.
• One species, was the earliest known example of a vertebrate giving live birth that was one species recently found, Materpiscis attenboroughi, 380 MY,.
• Both placoderms and acanthodians went extinct at end of Devonian

Class Chondrichthyes (Sharks and Rays)

• There are 970 species.
• Last of the 4 fish groups to appear in the fossil record.
• Retain many primitive fish characteristics that have changed little from earliest fossils.
• All but a few are marine.
• Most are 6-15 feet long.
• Includes the largest fish and second largest of all living vertebrates.
• Whale shark filter feeder up to 60 feet long.
• Great white gets up to 30 feet long.
• Body form is either fusiform (spindle shaped) like Sharks.
• Or flattened like rays which spend most time on or gliding near shallow bottoms.

Skin and Scales of Sharks and Rays

• Skin is very tough & leathery
• Muscles of shark pull on skin rather than pulling on the skeleton
• Have scales reduced to small, hard, knife-like (placoid) dermal scales embedded in skin that stick out from skin
• Scales are made of bony tissue that have same structure as tooth including enamel, dentin & pulp cavity
• Reduced scales enhance swimming efficiency
• Scales are continuously shed and replaced throughout life

Shark Support

• All members of the group have a skeleton made mostly of cartilage.
• Although their ancestors had bony skeletons, sharks retained mineralization in teeth, scales & spine

Shark Muscles & Movement

• Sharks are the most graceful and streamlined of all fish.
• Sharks have excellent swimming ability.
• With powerful dorsal and caudal fins for propulsion and paired appendages: pectoral and pelvic fins.
• Sharks exhibit rigid, non flexible pectoral fins in with hammerhead sharks using its head for steering since pectoral fins are not moveable.
• Sharks are also among some of the fastest fish.

Shark Buoyancy

• All sharks are slightly heavier than water.
• To keep from sinking, sharks must keep moving as the shape of sharks tail provides lift.
• A large liver is rich in fats and oils giving sharks near neutral buoyancy.
• The configuration doesn't need to use much energy to maintain position in the water column.
• In migrating sharks, the liver can enlarge to 25% of the animals weight to provide energy needed for long migrations.

Feeding & Digestion

• Most are predators yet, by nature, most tend to be timid & cautious.
• Have powerful jaws,suspended from chondrocranium by ligaments.
• Capable of free movement (somewhat similar to the flexibility in the jaws of some snakes).
• Teeth only grasp and tear apart prey and don't chew
• Teeth and (dermal) scales of sharks are essentially identical except for size.
• Are enamel covered dentin that has same structure and composition and both shed regularly.
• Teeth are enlarged scales that form replaceable rows of teeth.
• Usually, it's the only part of a shark preserved as fossils: fossil shark teeth because they are easily lost, constantly replaced.

Examples of Sharks

• Sawfish :flails its toothed snout in schools of fish then sucks up the injured ones. Some sharks are plankton feeders,scavengers
• Skates and rays: have broad, blunt, cobblestone-like teeth for crushing clams, oyster, etc.

Digestive System

• Food passes from the mouth, down the esophagus to the large stomach, where it has large esophagus and stomach to allow for the passage large, unchewed pieces of food.
• Most digestion occurs in the stomach and the first part of the intestine
• Contains two major accessory digestive organs:a very large multi-lobed liver, and secretes bile which is stored in the gall bladder until needed to aid in the digestion of fats and lipids.

Liver and Pancreas

• Liver also processes and stores nutrients absorbed from the intestine and helps to remove toxins that might have been taken in.
• Sharks also have livers with huge stores of oil to make the animal more buoyant in the water.
• A glandular pancreas also produces dozens of digestive enzymes to help break down proteins, carbs and lipids as well as help control sugar levels in the body.
• Most of the remainder of the digestive tract functions in absorption of the food once it is digested.in sharks, part of the small intestine is modified into a spiral valve (=valvular intestine) which slows the movement of the food through the system and increases the area available for absorption of these nutrients
• Undigested materials pass through a short large intestine (colon) into the cloaca and out of the body.

Sharks Respiration

• Gills are inside 5 pairs of gill slits like to agnatha
• A pair of spiracles behind the eyes can take in water when mouth is occupied with food.
• Sharks must be moving or there must be some current to move water over the gills to extract oxygen from the water.
• All fish have a closed circulatory system with a simple,2 chambered heart and a single circuit of blood flow.
• Blood is pumped from the heart to the gills to pick up oxygen, then to the rest of the body to deliver nutrients and oxygen and transfer wastes before returning to the heart.
• The Spleen the main organ that produces blood cells, filters the blood and removes "worn out" cells.

Shark Nerous System and Senses

• Includes a lateral line system
• A series of pores running over the surface of the head and down the sides of the body, containing mechcanoreceptors and electroreceptors
• Used to detect vibrations and current, and tiny electrical currents produced by the nearby animals.
• Especially useful in murky waters.
• Species also utilize chemoreceptors especially found in olfactory pits on head open through nostrils.
• Widely spaced nostrils allow shark to more easily locate prey and can detect prey half mile or more distant.
• Most species contain functional eyes but not particularly good and are used at close.
• Some speices like Hammerhead sharks have eyes with wide spacing to improve depth perception.
• Lastly, species also utilizes ampullary organs (of Lorenzini) electroreceptors in head that sense bioelectric fields around animals. Used especially attack.
• Sharks have relatively large kidneys that retain urea to help maintain internal fluids isosmotic to sea water.
• There’s also a rectal gland assists kidney. It secretes excess salts.
• Most sharks, but only a few kinds of other fish posses a cloaca: a single chamber that receives products from the intestine, kidneys and reproductive system and opens to the outside through a single pore

Chondrichthyes Reproduction and Development

• Most fish and almost all vertebrates are dioecious while most fish have external fertilization.
• all chondrichthyes have internal fertilization along with male sharks & rays with claspers on pelvic fins which are used to transfer sperm and not clasping.
• Usually produce only a few eggs at a time and no member of the group produces >12 at a time.
• Many retain eggs in body till hatch and bear live young! (viviparous) where development lasts 6 months to 2 years.
• Some sharks have primitive uterus and placenta and provide "uterine milk" for developing young.

Special Traits for some sharks

• Others get extra nutrition by eating eggs and siblings in uterus and some sharks and skates have internal fertilization but deposit eggs in horny (keratin) capsule,a "mermaid's purse”.
• Each "purse" may contain several eggs and often has "tendrils” to attach to objects
• There is no parental care after eggs are laid or young are born.

Cartilaginous Fish

• Skates and Rays are made up of ~ half of all chondrichthyes.
• They mainly prefer bottom dwelling.
• Species are dorsoventrally flattened with an enlarged pectoral fins.
• Species move in wavelike fashion
• Contain gills that open on underside with a large spiracle.
• Electric rays are generally slow, sluggish fish that live in shallow waters.

Muscles

• Have some muscles modified into electric organs to shock prey or stun predators with cells connected in parallel.
• High amperage with low voltage.
• Contain electric rays that each cell generates .02-.05 volts and can generate combined shock of ≤35 Volts.
• The amount of electricity varies greatly.
• Can only give a few shocks before it has to rest and eat.
• Others include Sawfish. They are were once common predators in coastal bays and lagoons.

Species Traits

• Like murky water and can tolerate low salinities.
• Aztecs referred it as an “earth monster"
• It grows to about 20 feet long
• Uses its “saw” to slash and flail about to kill or stun prey,then as a ladel to orient and swallow their prey
• Bear live young, 6-12 at a time, each ~1 foot long already with “saw"
• Considered a nuisance to fishermen where they were caught unintentionally as bycatch in tangled and torn nets for centuries
• They virtually disappeared by the 70's
• Now a protected species with teeth of snouts still sold for artificial spurs for fighting cocks.
• Also has fins sold for up to $3000 in asian markets for an alternative kind of “shark fin soup"

Shark ecology

• Sharks are the top predators in many ocean food chains.
• Sharks contain symbiotic relationships such as commensalism, some species may be more mutualistic by removing parasites and pathogens from their host's skin: Shark Suckers (remoras).
• Remoras are bony fish with one of the dorsal fins modified into a suction disc.Up to 3' long.
• They use large “suction cup" like disc on the top of their heads to attach to sharks

Remoras

• Are attached to sharks during most of their life.
• Sharks sucker feeds on debris produced from the shark's feeding activities and parasites on skin of shark, or mainly on the hosts feces.
• Some species are host specific.
• Pilot Fish about 2' long with dark blue stripes and at as cleaner fish help to keep the sharks healthy.
• Sharks will eat anything; including parasites and bodily wastes off the shark that are also territorial about the shark that are swim along side them for sometimes long distances as in not allowing others to 'horn in'.

Symbioses: Parasitism

• Parasitic flatworms, annelids, roundworms ,different kinds known to parasitize Sharks and rays.
• Only one species of shark itself is actually a parasite : the Cookiecutter Shark and they parasitizes fish and sea mammals.
• Hosts include marlins, whales, larger sharks after finding a host they fasten their highly specialized mouth equipped with a pair of suction-cup like lips onto their flanks.

Cookiecutter Sharks

• Closes its spiracles to form a tight seal.
• Uses its teeth to bite down and wheels in a circle to dislodge a plug of flesh from its host.the scars left by the sharks in sea animals is common.
• There are some reports of human victims being "hosts”
• Underwater photographers ship wreck victims usually night time attacks.

Cartilaginous Fish

• Nektonic fish Sharks swordfish
• Benthic Fish skates & rays guitar fish
• Hammerhead Shark is made up of loners,slower than most sharks use head for maneuvering,with eyes far apart improve depth perception, often basks near surface of water.
• Guitarfish are all tropical and warm temperate waters,are sluggish bottom dwellers and eat shellfish, worms and small fish.
• Skates & Rays are benthic fish known as well.

Human Impact

• Shark attacks have about 60-70 per year with 1-12 fatalities with major species including great white mako tiger bull hammerhead.
• casualities reported in from the Australian region:
  • In 2008 in US 4 people were killed in shark attacks,while 108 were killed by cows
• Shark fishing has had a major impacted as ~40 Million/yr are harvested worldwide.
• Recent estimates (2012) are that shark reef worldwide have declined by 90%
• They are harvested for their skeletons that is dried and powdered and used as an “herbal remedy" to 'cure cancer' with the primary cause is China's growing appetite for shark fin soup.

Shark Fin soup

• Sells for up to $100/bowl.
• For example Dubai alone exports 500 tonnes of shark fins and other which exports 500 tonnes (~ half the world shark fin production)
• its generally a legal harvest but increasingly being banned: and for example, "finning” has been outlawed in US.

Medicinal/Pharmaceuticals

• Used by ancient Egyptians as “electrotherapy” treatment for arthritis and gout
• There’s medicinal chondroitin for joint treatment and extracts are being tested are weight loss.

Class Osteichthyes Bony Fish

• are the most successful vertebrate class.
• Contain 27,000 sp; (96% of all fish =more species