Chapter 6
Marine Vertebrates
Although occupying a single subphylum, vertebrates rival nearly all other phyla in terms
of diversity and ecology. They occupy all marine habitats and are important players in
nearly all marine food webs.
Chapter Concepts Outline
6.1 VERTEBRATE FEATURES
All vertebrates possess the standard chordate features as well as
ƒ segmental ossification of the notochord
ƒ a musculature that is segmented into myomeres
ƒ and a closed circulatory system with hemoglobin contained within red blood cells
that cannot leave the blood vessels.
6.2 MARINE FISHES
Fishes, which include about 50% of all living vertebrates, are a diverse assemblage that
is difficult to characterize but which usually live in water, swim with fins, possess scales,
and use gills supplied with a countercurrent circulation for gas exchange.
Agnatha—The Jawless Fishes
Ö Agnathans are living jawless fishes that are distinguished from other vertebrates by
their lack of a jaw, paired fins, and scales.
Ö Hagfishes are marine benthic scavengers that possess unique slime glands,
whereas lampreys are anadromous creatures that often parasitize teleosts as adults.
Chondrichthyes
Ö The sharks, rays, and chimeras of class Chondrichthyes are mostly marine fishes
that often grow to large sizes, retain metabolic waste products (urea and trimethylamine
oxide [TMAO]) to achieve osmotic equilibrium with seawater, and possess a
characteristic heterocercal caudal fin.
Ö Chondrichthyans reproduce via internal fertilization
Ö They possess internal embryos that develop inside egg cases
Ö The egg cases are deposited in the environment or are born live after completing
their development inside their mothers, both with and without the benefit of a placental
attachment to her blood supply.
Osteichthyes—The Bony Fishes
Ö Many thousands of species of bony fishes are marine, and all but one are ray finned.
Ö The only exception is the lobe-finned coelacanth, first collected in the western Indian
Ocean but now known from Indonesia as well.
Ö Fishes swim via thrust generated by highly coordinated movements of many different
fins and fin combinations, including
ƒ lateral movements of the caudal fin,
ƒ flapping or undulating pectoral fins,
ƒ
ƒ
fanning of the pectoral fins,
or sculling movements of the dorsal and anal fins.
6.3 MARINE TETRAPODS
Air-breathing tetrapods that have returned to a sometimes full-time life in the sea
includes representatives of reptiles, birds, and mammals.
All are hypoosmotic to seawater, and two groups are homeothermic.
Marine Reptiles
Ö About 100 species of sea snakes, seven species of sea turtles, and one
representative each of iguana and crocodile are truly marine (other reptiles are
estuarine).
Marine Reptiles and Birds
ÖAll marine reptiles, and the closely related sea birds, eliminate excess salts via their
kidneys and specialized salt glands contained in their mouths, nostrils, or orbits.
Marine Birds
Ö “Marine” birds range from those that are nearly full-time residents of the sea, such as
penguins, to other species that simply visit the shoreline to feed, such as some ducks,
geese, and coots.
Marine Mammals
Ö Three orders of mammals can be found in the sea, including
ƒ Carnivora (sea otters and pinnipeds),
ƒ Sirenia (manatees and dugong),
ƒ and Cetacea (whales, dolphins, and porpoises).
ƒ Carnivora (sea otters and pinnipeds),
ƒ Sirenia (manatees and dugong),
ƒ Cetacea (whales, dolphins, and porpoises).
Breath-Hold Diving in Marine Mammals
Ö All air-breathing tetrapods are capable of diving deeper and longer than humans, and
some species, such as elephant seals, sperm whales, and emperor penguins, can dive
to more than 1000 m for longer than an hour.
6.4 VERTEBRATE SENSORY CAPABILITIES
Chemoreception
Ö Chemoreception is very important to marine fishes, and they may possess some of
the most sensitive noses of any animal.
Ö Tetrapods all close their nostrils while under water and therefore rely on their sense
of smell very little.
Electroreception and Magnetoreception
Ö The ability to detect weak electric and electromagnetic fields has been demonstrated
in bacteria, cartilaginous and bony fishes, some birds, and possibly some whales.
Vision
Ö Aquatic vision requires alterations to terrestrial visual systems to compensate for the
quality and quantity of light available under water.
Ö Marine vertebrates focus by moving the lens within the eye (rather than changing its
shape) and detect light with altered ratios of photoreceptor cells and unique visual
pigments.
Equilibrium
Ö Almost all vertebrates have two types of receptors to help maintain equilibrium. One
responds to the pull of gravity, and one detects acceleration forces.
Sound Reception
Ö Fishes. Most marine fishes use lateral lines and labyrinth organs for detecting waterborne sounds.
Ö Tetrapods. Most marine tetrapods use anatomic systems for hearing that are very
similar to those seen in terrestrial species.
Because sound travels about five times faster in water, cetaceans have evolved
alternative routes for sound conduction to the sensory hair cells in their cochlea.
Objectives
1. To describe the generalized features of all vertebrates and then apply these features
to marine vertebrates.
2. To survey the marine fishes in taxonomic order including the jawless fishes, the
cartilaginous fishes, and the bony fishes.
3. To survey the marine tetrapods in taxonomic order including the reptiles, birds, and
mammals.
4. To examine adaptations that enable marine tetrapods to exist in the sea.
5. To review the respiration of marine mammals and fishes.
6. To examine sensory adaptations in nektonic animals, specifically chemoreception,
vision, equilibrium, sound reception, electroreception, and magnetoreception.