Chapter 18
The Fishes: Vertebrate
Success in Water
Evolutionary Perspective
• Phylogenetic Relationships
– Subphylum Craniata
• Skull surrounds brain, olfactory organs,
eyes, and inner ear.
• Infraphylum Hyperotreti
• Infraphylum Vertebrata
– Fossils record
• Craniates and bone date earlier than 500
mya.
Survey of Fishes
• Infraphylum Hyperotreti—Class
Myxini
Hagfishes
Head supported by cartilaginous bars.
Lack vertebrae and retain notochord
4 pairs of sensory tentacles around
mouth
– Ventrolateral slime glands
– Marine
– Scavenge dead and dying fish
–
–
–
–
Figure 18.4 Class Myxini.
Survey of Fishes
• Infraphylum
Vertebrata
– Vertebrae
surround nerve
cord.
– Ostracoderms
• Extinct agnathans
• Bony armor
• Bottom dwellers
Figure 18.5 An ancient
Silurian seafloor with two
ostracoderms (Pteraspis and
Anglaspis).
Survey of Fishes
• Class Petromyzontida
– Marine and freshwater
– Most are predators as adults, filterfeeders as larvae
– Brook lampreys
• Adults do not feed.
– Life cycles involve open water adult
stages and stream or river larval
stages (figure 18.7).
Figure 18.6 Class
Petromyzontida
(Petromyzon marinus).
Figure 18.7 Life history of a sea lamprey.
Survey of Fishes
• Superclass Gnathostomata
– Jaws developed from anterior pharyngeal
arches.
– Paired appendages
– Classes
• Chondrichthyes
• Actinopterygii
• Sarcopterygii
Figure 18.8 Paired
pectoral and pelvic
appendages of a
member of the
Gnathostomata.
Survey of Fishes
• Class Chondrichthyes
– Placoid scales, cartilaginous
skeleton
– Subclass Elasmobranchii
• Sharks, skates, rays
– Subclass Holocephali
• Ratfish
• Operculum present
Figure 18.9 Class
Chondrichthyes. (a and b)
Subclass Elasmobranchii.
(a) Reef shark (Carcharhinus
perezi). (b) A bullseye stingray
(Urolophus concentricus).
(c) Subclass Holocephali. The
ratfish (Hydrolagus colliei).
(b)
(a)
(c)
Figure 18.10
Scales and teeth
of sharks.
(a)
(b)
Survey of Fishes
• Class Sarcopterygii
– Lobe-finned fishes
• Fins with muscular lobes
– Lungs used in gas exchange.
– Lungfish
• 3 genera
• Australia, Africa, South America
– Coelacanths
• 2 species
• African and Indonesian coasts
– Tetrapodomorpha
• Extinct ancestors of ancient amphibians and all
tetrapods
Figure 18.11 Class
Sarcopterygii. The lungfish,
Lepidosiren paradoxa.
Figure 18.12 Class
Sarcopterygii. The
coelacanth Latimeria.
Survey of Fishes
• Class Actinopterygii
– Ray-finned fishes
• Fins lack muscular lobes
– Swim bladders
– Chondrosteans
• Sturgeons and paddlefish
– Neopterygii
• Garpike (Lepisosteus) and dogfish or
bowfin (Amia)
• Modern bony fish—the teleosts
Figure 18.13 Class
Actinopterygii, the
chondrosteans.
(a) Shovelnose
sturgeon
(Scaphirhynchus
platorynchus).
(b) Paddlefish
(Polydon spathula).
Figure 18.14 Class
Actinopterygii, the
teleosts.
(a) A flounder
(Pseudopleuronectes
americanus).
(b) Yellowtail
snappers (Ocyurus
chrysurus).
Evolutionary Pressures
• Locomotion
– Streamlined shape, mucoid secretions,
buoyancy of water, body-wall muscles, fin
shape all promote efficient locomotion.
• Nutrition and the digestive system
– Filter feeders and scavengers
• Modern filterers use gill rakers.
– Predators (most modern fish)
• Swallow food whole
– External parasites (lampreys)
– Herbivores
– Digestive tract
• Specializations include spiral valve (sharks) and
pyloric cecae (bony fishes).
Evolutionary Pressures
• Circulation
– Closed
– Heart
• 4 embryological enlargements of ventral
aorta
–
–
–
–
Sinus venosus
Ventricle
Atrium
Conus arteriosus
– Most fish have single circuit.
– Lungfish
• Pulmonary circulation
• Pulmonary and systemic circuits
Figure 18.15 Circulatory system of fishes.
Evolutionary Pressures
• Gas exchange
– Water movement over gills
• Opercular and pharyngeal muscles pump
water in most fishes.
• Ram ventilation in elasmobranchs and
open-ocean bony fish
– Gas exchange surfaces
• Visceral arches support gills.
• Gill filaments and pharyngeal lamellae
– Countercurrent exchange mechanism
(figure 18.16)
Figure 18.16 Gas exchange
at pharyngeal lamellae.
(a) Gill arches. (b) Trout
lamellae. (c and d)
Comparison of
countercurrent and parallel
current exchanges.
Evolutionary Pressures
• Swim bladders and lungs
– Pneumatic sacs connect to digestive tract
in nonteleost fish.
• Function as lungs in lung fish, climbing perch and
ancient rhipidistians
• Function as swim bladders in other bony fish
• Buoyancy Regulation
–
–
–
–
Low density compounds
Fins provide lift.
Reduction of heavy tissues
Swim bladders
• Pneumatic duct (gulp air)
• Counter current exchange at rete mirabile
Figure 18.17 Possible sequence in the evolution of pneumatic
sacs. (a) Origin as ventral outgrowths of esophagus.
(b) Primitive lungs. (c) Swim bladders move dorsal in position
and lose connection to gut tract.
Evolutionary Pressures
• Nervous and sensory functions
– Brain and spinal cord
– Sensory receptors
• External nares
• Eyes
– Lidless and round lens
• Inner ears
– Equilibrium, balance, and hearing
• Lateral line system
– Sensory pits in skin detect water movements.
• Electroreception
– Prey detection by chondrichthyians
– Gymnarchus (figure 18.18)
– Electrophorus (electric eel)
Figure 18.18 Electric fishes.
(a) Electrical fields are used to
detect the presence of prey and
other objects in a murky
environment. (b) The electric
fish (Gymnarchus niloticus).
Evolutionary Pressures
• Excretion and Osmoregulation
– Kidneys
• Filter nitrogenous wastes, ions, water, and small
organic compounds at nephrons
– Glomerulus is filtering capillary network.
– Tubule system promotes reabsorption.
• Freshwater fishes
– Excess water must be excreted.
– Ions and organic compounds are
selectively reabsorbed.
• Marine fishes
– Water must be conserved.
– Excess ions excreted.
Evolutionary Pressures
• Excretion and Osmoregulation
– Elasmobranchs
• Sequester urea in body tissues
• Rectal gland
– Diadromous fishes
• Gills cope with both uptake and excretion of
ions.
– Nitrogen wastes
• 90% ammonia (diffusion across gill surfaces)
• 10 % urea, creatine or creatinine (kidneys)
Evolutionary Pressures
• Reproduction and development
– Most oviparous
• Some ovoviviparous (some elasmobranchs)
or viviparous (other elasmobranchs)
– Fertilization
• Most external
• Copulatory structures
– Claspers in elasmobranch males
– Development
• Usually little or no parental care
• Some tend nests or brood young
Figure 18.21 The male garibaldi (Hypsypops rubicundus)
cultivates a nest of red algae, entices a female to lay eggs
in the nest, and defends the nest.
Further Phylogenetic
Considerations
• Two series of evolutionary events
– Radiation of teleost fishes
– Evolution of terrestrialism
• Terrestrialism
– Tetrapodomorpha
• Osteolepiform sarcopterygians
– Common features with amphibians
» Jaws, teeth, vertebrae, limbs
• Tiktaalik (the “fishapod”)
–
–
–
–
–
Fins, gills, scales
Dorsoventrally compressed and widened skull
Tetrapod-like forelimbs
Lacked opercular supports and dorsal and anal fins
Pectoral girdle and freely moveable neck
Figure 18.22 The fishapod Tiktaalik. This 375-million-yearold fossil helps us understand the transition between
sarcopterygian fish and tetrapods. Its tetrapod-like
features were probably used in foraging the water’s edge
for prey.
Box Figure 18.3 The
fossil record provides
clear evidence of the
evolution of the
tetrapod limb. (a) The
sarcopterygian
Eusthenopteron.
(b) The sarcopterygian
Sauripterus. (c) The
forelimb of the tetrapod
Acanthostega. (d) The
hindlimb of the
tetrapod Ichthyostega.