Nonvertebrate Chordates, Fishes, and Amphibians
Biology I: Chapter 30
CHORDATES
Chordates• Phylum Chordata
• Fishes, amphibians, reptiles, birds and mammals
• Four key characteristics:
1. Dorsal, hollow nerve cord
2. A notochord
3. Pharyngeal pouches
4. Tail that extends beyond the anus
Dorsal, Hollow Nerve Cord
• Nerves branch from this cord at regular intervals
• Nerves connect to internal organs, muscles and sense organs
Notochord
• Long support rod
• Runs through the body just below the nerve cord
• Most chordates only have in the embryonic stage
Pharyngeal Pouches• Paired structures in the throat
(pharynx) region
• Fishes and amphibians: slits develop that connect the pharyngeal pouches to the outside of the body
• The slits may then develop into gills that are used for gas exchange
Tail
• All chordates have a tail that extends beyond the anus at some point in their lives
• The tail can contain bone and muscles and is used in swimming by many aquatic species
Most Chordates are Vertebrates
• More than 99% of all chordates are in the subphylum Vertebrata
• Vertebrae: individual segments that make up the backbone; encloses and protects the spinal cord
• Backbone is part of an endoskeleton, or internal skeleton
Nonvertebrate Chordates• The two groups
• Soft-bodied marine organisms
• Have the four key characteristics at some point in their lives
• Tunicates– Filter feeders
• Lancelets– Small, fish-like creatures
Fishes• Aquatic vertebrates that are characterized by:
• Paired fins– Used for movement
• Scales– Used for protection
• Gills– Used for exchanging gases
Evolution of Fishes
• Fishes were the first vertebrates to evolve
• The evolution of jaws and the evolution of paired fins were important developments during the rise of fishes
The First Fishes• Jawless creatures whose
bodies where armored with bony plates
• Lived in the oceans during the late Cambrian Period, about 510 mya
• Fishes kept this armored, jawless body plan for 100 million years
The Age of Fishes
• Ordovician and Silurian Periods: 505-410 mya, fishes underwent a major adaptive radiation
• Devonian Period: “Age of Fishes”
• Some were jawless with little armor
• Ancestors of modern hagfishes and lampreys
• Others were armored and ultimately became extinct about 360 mya
The Arrival of Jaws• Other ancient fishes kept their bony armor and
possessed a feeding adaptation that would revolutionize vertebrate evolution: JAWS
• Jawless fishes – Limited to eating small particles of food that they filter out
of the water or suck up like a vacuum cleaner
• Jaws can hold teeth and muscles– Much wider variety of food– Defend themselves by biting
The Arrival of Paired Fins
• More control of body movement
• Fin tails and powerful muscles gave greater thrust when swimming
• Enabled fishes to move in new and varied patterns
• This enabled fishes to use their jaws in complex ways
The Rise of Modern Fishes
• Although the early jawed fishes soon disappeared, they left behind two major groups that continued to evolve and still survive today
– Ancestors of modern sharks and rays: skeletons made of resilient cartilage
– Group that evolved skeletons made of true bone
Form and Function in Fishes
• Adaptations to aquatic life include:
– Various modes of feeding
– Specialized structures for gas exchange
– Paired fins for locomotion
Feeding
• Herbivores, carnivores, parasites, filter feeders, detritus feeders
• A single fish may exhibit several modes of feeding (carp: eat what is available) while others are very specialized (barracuda: carnivore)
• Pyloric ceca: finger-like pouches found in many species of fish that secrete digestive enzymes to help digest food
Respiration• Most fishes exchange gases using gills located on either side of
the pharynx
• The gills are made up of feathery filaments
• Lampreys and sharks have several gill openings on the side of the body
• A number of fishes, such as the lungfish, have an adaptation that allows them to survive in oxygen-poor water or in areas where bodies of water often dry up
Respiration
Circulation• Closed circulatory systems with a heart that pumps blood
around the body in a single loop
• Heart consists of 4 parts:
– Sinus venosus: thin-walled sac that collects blood from the fish’s veins
– Atrium: a large muscular chamber that serves as a one-way compartment for blood
– Ventricle: thick-walled muscular chamber; pumping portion of the heart
– Bulbus arteriosus: connects to a large blood vessel called the aorta, which moves blood to the fish’s gills
Circulation
Excretion• Most fishes rid themselves of nitrogenous wastes in
the form of ammonia
• Gills, kidneys
– Saltwater fishes• Lose water by osmosis: kidneys return as much water to
the body as possible
– Freshwater fishes• Gain water by osmosis: kidneys pump out plenty of
dilute urine
Response• Well-developed nervous systems organized around a
brain
• Cerebrum: area of the brain responsible for all voluntary activities of the body
• Cerebellum: region of the brain that coordinates body movements
• Medulla oblongata: area of the brain that controls the functioning of many internal organs
Response
• Lateral line system: sensitive receptor system that enables fish to detect gentle currents and vibrations in the water (the 6th sense)
Movement• Most move by alternately contracting paired sets of
muscles on either side of the backbone
• Because their body tissues are more dense than the water they swim in, sinking is an issue for fishes
• Swim bladder: gas-filled organ found in many bony fishes that adjusts their buoyancy
Reproduction• Oviparous: term used to refer to animals whose eggs
hatch outside the mother’s body
• Ovoviviparous: term used to refer to animals whose young are born alive after developing in eggs inside the mother’s body
• Viviparous: term used to refer to animals that bear live young that are nourished directly by the mother’s body as they develop
Groups of Fishes
• Over 24,000 living species that are extremely diverse
• Jawless fishes
• Cartilaginous fishes
• Bony fishes
Jawless Fishes
• Have no true teeth or jaws
• Skeletons are made of fibers and cartilage
• Lack vertebrae; keep notochords as adults
• Two main classes:– Lamprey– Hagfishes
Sharks and Their Relatives
• Class Chondrichthyes
• Sharks, rays, skates,
• Also: sawfishes and chimaeras
• Cartilage, not bone
350 Living Species• Curved tails
• Torpedo-shaped bodies
• Pointed snouts
• Mouth underneath
• Enormous number of teeth
• Always exceptions!
Bony Fishes• Class Osteichthyes
• Skeletons made of hard, calcified tissue: bone
• Ray-finned fishes
– Rays or spines that support the fins
– Only 7 living species of bony fish are not ray-finned
• Lobe-finned fishes
Ecology of Fishes
• Anadromous: fishes that spend most of their lives in the ocean but migrate to fresh water to breed
– Examples: lampreys, sturgeons, and salmon
• Catadromous: fishes that spend most of their lives in fresh water but migrate to the ocean to breed
– Example: European eel, American eel
AMPHIBIANS
Amphibian• Have survived for hundreds of millions of years
• The only modern descendants • of an ancient group that
gave rise to all other land vertebrates
• Amphibian means “double life”…live in both water and on land
Amphibian• Vertebrate
• Lives in the water as a larva and on land as an adult (with some exceptions)
• Breathes with lungs as an adult
• Has moist skin that contains mucus glands
• Lacks scales and claws
Evolution of Amphibians
• The first amphibians to climb onto land probably resembled lobe-finned fishes similar to the modern coelacanth
• The amphibian had legs, appearing about 360 mya
Evolution of Amphibians• Early amphibians evolved several adaptations that
helped them live at least part of their lives out of water:
– Bones in the limbs and limb girdles became stronger for more efficient movement
– Lungs and breathing tubes enabled them to breathe air
– The sternum formed a bony shield to support and protect internal organs, esp. the lungs
Evolution of Amphibians• Soon after they appeared, amphibians underwent a major
adaptive radiation
• Some were huge: Eogyrinus was about 5 meters long!
• Amphibians became the dominant form of animal life in the warm, swampy fern forests about 360-286 mya
• Climate changes caused many of the swamps to disappear
• Most amphibians became extinct
Evolution of Amphibians
• Only three orders of small amphibians survive today:
• Frogs and toads
• Salamanders
• Caecilians
Form and Function in Amphibians
• Although the class Amphibia is relatively small, it is diverse enough to make it difficult to identify a typical species
• We will focus on the structures found in frogs…
Feeding• Tadpoles
– Filter feeders or herbivores that graze on algae
– Eat almost constantly
• Adult amphibians – Almost entirely carnivorous
• Mouth esophagus stomach small intestines large intestines cloaca
Respiration
• Larval amphibians – Gas is exchanged
through the skin and gills
• Adult amphibians– Lungs and skin
Circulation• Double loop
• 3 chambered heart
First Loop
• Carries oxygen-poor blood: heart lungs and skin
• Takes oxygen-rich blood: lungs and skin heart
Circulation
Second Loop
• Transports oxygen-rich blood: heart rest of the body
• Transports oxygen-poor blood: body heart
Circulation
Excretion• Amphibians have kidneys that filter wastes from the
blood
• Urine: The excretory product of the kidneys
• Urine travels through tubes called ureters into the cloaca
• Urine is passes outside or temporarily stored in a bladder above the cloaca
Reproduction
• Eggs do not have shells and tend to dry out if they are not kept moist
• Most species lay eggs in the water
• The male fertilizes them externally
• In a few species (most salamander), eggs are fertilized internally
Reproduction• The male climbs onto the
female’s back
• The female releases the eggs that are then fertilized
• The eggs are in a transparent jelly, useful for attaching the eggs to underwater plants and that provides nourishment to the growing cells
Reproduction
• Some amphibians abandon their eggs after they are laid
• Others incubate the young in their mouth, on their back, or even in their stomach!
Movement
• Amphibian larvae
– Often move very much like fishes, wiggling their bodies and using a flattened tail for propulsion
Movement
• Adult amphibians
– Most use their front and back legs to move in a variety of ways
– Jump, climb, run, etc.
ResponseNictitating Membrane:
• Moveable transparent membrane in amphibians located inside the regular eyelid
• Protects the surface of the eye from damage under water and keeps it moist on land
Response
Tympanic Membrane
• Eardrum of amphibians
• Inside the skull
• Vibrates in response to sound, allowing hearing
Groups of Amphibians• Salamanders: Order Urodela
– Long bodies and tails– Four legs– Carnivores
• Frogs and Toads: Order Anura– Frogs: Long legs and lengthy jumpers– Toads: short legs and short hops– Adults lack tails
• Caecilians: Order Apoda– Legless animals that live in
water or burrow in moist soil or sediment
Ecology of Amphibians
• Have no feathers, fur or scales
• Many are ideal for predators such as birds and mammals
Ecology of Amphibians• Amphibians have adaptations
to protect themselves
• Many have skin colors and markers that allow them to blend in with the environment
• Some release toxins
• Some are brightly colored as a warning to predators