GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 23. Nekton
POWERPOINT SLIDE SHOW NOTES
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Topic 23. Nekton
Nekton are the true swimmers. Fish dominate the nekton, but there are plenty other
critters that swim as well.
Most of the nekton share similar adaptations
There are three major nekton groups
There are four groups of fish; one is extinct, three living.
The three classes of modern fish
Agnatha: the jawless fish
Chondrichthyes are the cartilaginous fish. They lack true bone; there skeletons are made
of the same material as our nose and ears. Sharks, skates, and rays populate this class of
fish. They are primitive fish, evolving over 400 million years ago. Chondrichthyes are
rarely fossilized because the cartilage decomposes. Usually only their teeth are
preserved.
Osteichthyes are the bony fish, with skeletons of bones. This is the dominant class of
living fish.
Characteristics of Most Fish
Effect of body shape on drag. Drag is a frictional force tending to inhibit the movement
of an object through a fluid. There is an optimum width-to-length ratio (called the
profile thickness) that creates minimum drag as a fish moves through the water. The
curved line on the graph shows how much drag occurs versus the profile thickness. The
lowest drag occurs when the profile thickness is about 0.22-0.23. Look at the profile
thickness of the fish listed. Even something as huge as the blue whale has a perfect
shape for minimum drag. The same concept is applied to ship design.
Swim bladder helps a fish control buoyancy to control its vertical position in the water
column. Without buoyancy control, a fish would have to expend a lot more energy to
maintain its vertical position.
Not all fish have swim bladders. Mostly gas is exchanged into and out of the swim
bladder through the blood stream. However, some fish have the ability to gulp air at the
water surface to add to their swim bladders. Swim bladders in fish may have evolved
into lungs in air-breathing animals.
Eel-like fish must move their entire bodies for locomotion. Advanced fishes are more
efficient and only need to move their tail fins.
Cartilaginous Fish, the Chondrichthyes
Cartilaginous Fish
Bony fish, the Osteichthyes.
Bony fish are broken into two large groups, the lobe finned fish and the ray finned fished.
The Coelacanth is a primitive bony fish.
Ray finned fish are the dominant type of nekton
Schooling in fish
Instead of ears, fish detect sound through their lateral lines.
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Fish fins come in many shapes
Two common types of fins
Body and fin shape control a fish’s cruising, maneuverability, and acceleration.
Yellowfin tuna have rigid and forked tail fins.
Groupers have rounded and flexible tail fins.
American and European eels spawn in the Sargasso Sea. Larva migrate to fresh-water
rivers, lakes, and ponds to live. They return to the Sargasso Sea as adults to reproduce.
Salmon are the opposite of eels. They live in salt water as adults and return to fresh
water to spawn.
Fish with life cycles like salmon are called anadromous fish. Fish with life cycles eels are
called catadromous fish.
Bony fish are major food resources. Most are epipelagic.
Flounder begin their lives as upright swimming fish, but change living habits. One of
their eyes migrates as the fish changes to live on its side, flat on the ocean floor.
The cyclothone may be the most common fish in the sea.
Deep water fish
Many deep water fish have large eyes to gather any available light
A deep water fish
Deep sea angler fish. The appendage at the front has a photophore at the end which the
fish dangles in front of its mouth trying to attract potential prey.
This tiny angler fish has all the classic characteristics of a deep-water fish.
Deep sea gulper can unhinge its jaws.
Fish are dominant, but there are many other types of nektonic animals.
Green sea turtle
Marine iguana
Sea snakes
Many types of birds are so connected to the ocean that some consider them to be
nekton.
Penguins are very well adapted for life in the ocean. They are very streamlined with
wings that have evolved into paddles.
Flamingos
The polar bear is the world’s large land predator. Seals make up a large portion of their
diet.
There are even a few invertebrates that swim.
Squid
A review of osmosis
Osmoregulation in fresh water fish versus marine fish.
Another view of osmosis
Marine mammals evolved from mammals that had lived on land. Humans are also
mammals, which is probably part of the reason why so many people are attracted to
whales, dolphins, seals, marine otters, and so forth. There were advantages to living in
water that were great enough to adapt to an aquatic lifestyle. Advantages included such
things as a more reliable food supply and a lack of predators.
There are four groups of marine mammals.
Sea otters
Pinnipeds
Seals versus sea lions
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True seals. Earless means they have no external ears.
Elephant seal
California Sea Lion
Galapagos Sea Lion
Fur seal
Pacific Walrus
Manatee
Cetaceans—two groups
Some toothed whales. Note the scale.
Moby Dick was a sperm whale, a type of toothed whale
Killer whale, or Orca
Dolphins are small toothed whales
The teeth of a harbor porpoise
Some baleen whales. Note the scale. In general they are much larger than toothed
whales. The blue whale is the largest animal that has ever lived. Larger than dinosaurs.
Baleen is used as a sieve to strain food from the water. Baleen whales gulp a large
mouthful of water with plankton and fish, then close their mouths and with their
tongues force the water out through the baleen, straining out the food. Baleen is often
called whalebone, although it is not true bone. It is made of material like our fingernails.
It is bristled like a brush and hangs like a curtain from the upper jaw. Baleen is strong but
flexible and had such uses as inserts for collar stiffeners, buggy whips, parasol ribs, and
as corset stays. It has now been replaced by plastics and fiberglass.
Humpback whale
Convergent evolution means that morphologic features serving the same function
evolved in unrelated species. A whale’s flipper looks similar and performs the same
function as a fish’s fin, but they are anatomically quite different. The flipper is like our
hands, with bone and fingers. The fish fin is made up of spines covered by skin and are
controlled by muscles in the body of the fish. Perhaps the most common example of
convergent evolution is wings. Birds, bats, bees, beetles, and many other things have
wings and fly. The wings all perform the same function and look very similar, but
anatomically they are quite different.
Diving in cetaceans.
Adaptations for diving
Decompression sickness
Nitrogen narcosis
Cetacean migration and navigation. Recall Sofar channels from Topic 8
Some toothed whales have such good echolocation that they can be trained to identify
objects, and trained to perform underwater tasks for the military and other applications.
Whales often migrate large distances between winter feeding and summer calving
grounds.
Whaling. Matthew Maury’s charts helped whalers find whales and led to the beginning
of their decline.
An old whaling ship
Whaling was a rough way to make a living in the sailing ship days. Voyages could last
three years or more.
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PBS has a timeline of whaling that includes a lot of interesting information:
http://www.pbs.org/wgbh/americanexperience/features/timeline/timeline-whaling/
New Bedford, Massachusetts, was one of the centers of whaling in the mid-1800’s. The
New Bedford Whaling Museum web site offers a vast amount of information:
http://www.whalingmuseum.org/learn/research-topics/overview-of-north-americanwhaling/american-whaling
The whale-ship Essex was sunk by a sperm whale in 1820. Owen Chase was first mate on
the ship, and one of only eight survivors. The tragic story became the basis of Herman
Melville’s book, Moby Dick. By the way, if you have heard of the musician Moby, his real
name is Richard Melville Hall. He is a grand-nephew of Herman Melville.
In the Heart of the Sea, by Nathaniel Philbrick, is a recent book about the whale-ship
Essex. The sketch is by cabin boy Thomas Nickerson.
Modern whale ships are floating factories.
Scrimshaw is artwork carved, painted, or inked onto whale teeth. Whalers had a lot of
free time during their long voyages.
A list of marine mammals. Don’t worry about the scientific names
Some whales