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.

Response

Nictitating 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