CHAPTER 33: Fishes and Amphibians
Section 33-1: The Fish Body
Although they are very diverse, today‟s fishes share three key characteristics:
1. Gills: Fish obtain oxygen from the gas dissolved in water by using gills. Gills are made up
of rows of gill filaments, which are fingerlike projections through which gases enter and
leave the blood. In countercurrent flow, water passes over the gills in one direction as blood
flows in the opposite direction through capillaries in the gills. See Fig. 2, pg. 747.
2. Single-loop blood circulation: Blood is pumped from the heart to the capillaries in the gills.
From the gills, blood passes to the rest of the body and returns to the heart. The fish heart is
a chamber-pump with four chambers in a row (See Figure 3, page 748.
3. Vertebral column (backbone): All fish have an internal skeleton with a vertebral column that
surrounds the spinal cord. The brain is fully encased within a protective covering called the
skull or cranium.
Along with these characteristics, fish must also have:
; Specialized kidneys (with nephrons) in order to balance their salt and water intake.
Excess water and wastes leave the kidneys in a fluid called urine (marine fish have very
little urine and freshwater fish excrete large amounts of urine).
; Fish usually have separate sexes with external fertilization. In a process called spawning,
male and female gametes are released near one another in water (Figure 4, page 750).
Some species have internal fertilization (sharks, skates, and rays)
Section 33-2: Today’s Fishes
There are three groups of fishes:
1. Superclass Agnatha-Jawless Vertebrates (See Figure 5, page 751):
; The only extant groups are ? 60 species of lampreys and hagfishes. The most unusual
fishes found today.
; These creatures have been around for 330 million years. Today‟s agnathans are scaleless,
eel-like, have multiple gill slits, and skeletons made of cartilage.
; Lack paired appendages
; Sea lampreys feed by clamping onto the side of a live fish and using a rasping tongue to
penetrate in the skin. They are bloodsuckers. Larvae live in freshwater streams and the
mature forms migrate to the sea or lakes.
; Hagfishes are mainly scavengers, eating sick or dead fish and occasionally marine worms.
They are saltwater animals, and when threatened, they can produce huge amounts of slime
to slip away from predators.
2. Class Chondrichthyes: Cartilaginous fish like sharks, skates, and rays (See Figure 6, pg 762). ; Cartilaginous fishes because their endoskeletons are made of cartilage rather than bone.
Cartilage is strengthened by calcium carbonate.
; ? 750 living species.
; Sharks have light, streamlined bodies for quick movement in the hunt for prey ; Skates and rays are flattened bottom dwellers.
; The largest sharks and rays are suspension-feeders that feed on plankton ; Most sharks are carnivores that swallow prey whole or use teeth to tear flesh. ; The shark intestine has a spiral valve, a corkscrew shaped ridge that increases surface area
and prolongs the passage of food.
; Acute senses, like sharp vision and nostrils for smelling.
; Sharks have the beginnings of a lateral line system, a row of microscopic organs sensitive
to changes in the surrounding water pressure.
; Sharks have placoid scales. The shark‟s teeth are actually modified scales. They are
arranged in 6 to 10 rows, and when one tooth is worn down, it is replaced by the tooth
; Internal fertilization
; Have a cloaca, or common chamber that expels the digestive waste and contains the end of
the reproductive tract
3. Class Osteichthyes: Bony fishes
; The most numerous vertebrate class, ? 30,000 species.
; Both marine and freshwater
; Have an endoskeleton with a hard matrix of calcium phosphate
; Skin is covered by flattened scales
; Secrete a mucus for reduced swimming friction
; Breathe by drawing water over gills located in chambers. A flap called the operculum
covers these chambers (See Figure 8, page 756).
; Have a swim bladder, which is a specialized air sac that helps control the fish buoyancy.
Allows them to remain motionless.
; Have a fully developed lateral line system, which allows fish to perceive their position and
rate of movement in the water (See Figure 7, page 753).
; Streamlined bodies for swimming.
; Most are oviparous with external fertilization
; Two major classes of bony fish:
1. Ray-finned fishes (Subclass Actinopterygii): Bass, trout, perch, tuna, etc. Long rays
support their fins. Found in freshwater, marine ecosystems, or live in both. Teleosts
are the most advanced of the ray-finned bony fish (about 95% of all fish). See UP
CLOSE, page 754-755.
2. Lobe-finned fishes and lungfishes (Subclass Sarcopterygii): Mainly freshwater. Some
preexisting groups were large bottom dwellers with „walking fins‟. The only lobe-
finned fish extant today is the coelocanth. There are three genera of lungfishes alive
today. These live in ponds and can burrow into the mud when the ponds shrink.
Probably the ancestors to amphibians. See Figure 10, page 757.
Section 33-3: Amphibians
; Tetrapods are animals that have two pairs of limbs that support them on land and include the Amphibia, Reptilia, Aves, and Mammalia.
; Three of these groups, the Reptiles, birds, and mammals have an amniotic egg, which is a
shelled, water-retaining egg that enables these animals to complete their life cycle on land. These animals are called amniotes.
Class Amphibia: (Frogs, salamanders, and caecilians)
; The first vertebrates to walk on land, with the assistance of legs.
; They may have evolved from a lobe-finned fish
; Named “Two-lives” because of the distinct metamorphosis that occurs between the
larval (tadpole) and adult (frog) stage and because of their dualistic living in both
aquatic and terrestrial habitats. See Figure 34-16, page 771.
; Most adult forms breathe with lungs (internal, baglike respiratory organs that allow for
gas exchange between air and the bloodstream), while larval amphibians have gills. Fig.
12, pg. 759.
; Have double-loop circulation: Pulmonary veins return blood from the lungs to the
heart. The oxygen-rich blood is then pumped to the tissues. See Fig. 13, pg. 759 ; The atrium of the heart is partially divided into left and right sides by a septum. Fig. 14,
; Rely on moist skin to carry out gas exchange, which is called cutaneous respiration.
They rely on close connections to water for gas exchange.
; External fertilization, shell-less eggs are laid in moist habitats because they dry out
quickly. Mortality is high in some species. The fertilized eggs hatch into tadpoles (See
Fig. 16, page 761)
; Complex and diverse social behavior, especially around breeding season. ; Three extant orders
1. Urodela (“tailed ones”, salamanders): About 369 species. They have long bodies, long
tails, and smooth/moist skin. Some are entirely aquatic and others live on land as adults
or throughout their lives. The walk with a side-to-side bending (like Gordon). They do
not undergo a dramatic metamorphosis. See Fig. 17, page 764.
2. Anura (“tail-less ones”, frogs & toads): About 3500 species, more specialized for land.
Frogs can hop along the terrain, can be camouflage, have skin glands that are
poisonous, and can have bright patterns to warn predators. They live anywhere from
deserts to rainforests. They are mostly carnivorous-they eat insects (yummy). They
undergo a dramatic physical metamorphosis. See UP Close, page 762-763. 3. Apoda (“legless ones”, caecilians): About 150 species. Legless and nearly blind, they
resemble earthworms and are specialized for burrowing. Mainly live in tropical areas.
See Fig. 18, page 764.