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Oceanic Nekton

Chapter 3

Nekton Characteristics

Developed power of locomotion

Large animals

Fast moving

Predominantly vertebrates

Fish are most numerous nekton

Found in epipelagic zone of open ocean

Difficult to study in sea

Difficult to capture under natural conditions

Scientists forced to infer

Most direct contact is with seals, sea lion porpoise

Importance to ecology

Tunas major world fishery

Most popular are whales - controversy

Composition of the Oceanic Nekton

Marine mammals, reptiles, birds

Only invertebrates cephalopod mollusks


thresher shark, mackerel shark, blue shark

Floating eggs

waters of tropics, subtropics


Only part of the time in epipelagic

More diverse group

Spawn inshore waters

Environmental Conditions

Light, temperature, density, and currents

perception of environment

three dimensional

transparent medium - water

no shelter for predators

evolution of adaptation

great mobility

nervous and sensory systems development

navigate, capture food, avoid predators

faster swimming speeds

development of camouflage

Adaptations of Oceanic Nekton

Must stay afloat

gas or swim bladders

two types

physostome - open duct between gas bladder and esophagus

physoclist - no duct

rete mirable network - small blood vessels branch to large blood vessels

oval- special gas absorptive section

Denton and Marshall 1958 - fast swimming fish lack gas bladder

enough lift from pectoral fins

marine mammals with additional air sacs

figure 3.6, p. 99

air trapped under feathers, hollow bones, (excluding penguins) replacement of heavy chemical ions in body fluids with lighter ones


increase buoyancy - lay down lipids ( fat, oil ) instead of swim bladder.

Lipids in the liver - pelagic sharks

skin or blubber

hydrodynamic mechanisms

pectoral fins or flippers


Two groups

groups necessary to create the propulsive force

reduction of the resistance of the body to passage through the water

fins from side to side

amplitude of lateral movements

muscle contraction

caudal peduncle, figure 3.8, p. 101

short, stubby fishes ( tuna) are faster than long, narrow fishes ( eels ) fig. 3.9, p. 101

figure 3.10, p. 102 - locomotion of fin movements

rays, certain squids, ocean sunfish fig. 3.10. P. 102

paddling movements - marine air breathing vertebrates

marine turtles, seals, sea lions, penguins fig 3.11, p. 103

jet propulsion - squids

Surface of Resistance and Body Shape

Shape reduce the surface of resistance to a minimum

frictional resistance- fig. 3.12

form resistance - long and thin

induced drag or turbulence


swordfish and rough skin

tuna and scales

best shape blunt in front and tapered to the posterior end

whales, dolphins, tunas fig. 3.13, p. 104

body structures are recessed

Defense and Camouflage

Very important, but in large nektonic animals not that important. Not many predators.

Transparency of body ( not in nekton )

cryptic coloring



bicolored - countershading figs. 3.16, 3.17, p. 107

alternation of body shape ( cryptic body shape )

as long it does not interfere with fast movement

ventral keel

properties of light in water, fig. 3.15, p. 106

flying fish

Sensory Systems

Lateral lines

ampullae of Lorenzini - electroreception

eyes lateral positioned

lack of overlap of fields of view

binocular vision is small or absent


Sense of hearing

sound travels four times faster in water than in air

farther range of communication than sight

sonar and sound waves

figure 3.18, p. 109

melon in toothed whales

how is sound developed

figure 3.19, p. 110

Reproduction and Life Cycle

No special reproduction mechanisms are apparent

tuna, marlin spawn eggs that float

egg production is huge over many months

albacore tuna 2 - 2.6 million eggs

striped marlin over 13 million eggs

ocean sunfish spawn 300 million eggs

sharks a few large eggs or embryos

short lives

marine birds and turtles- like terrestrial relatives - shelled eggs, nest on land

seals, sea lions walruses - birth to live young

harbor seal - swims immediately

breeding grounds of pinnipeds


For breeding

common for air-breathing vertebrates

fish horizontal migration

salmon - from open ocean r back to freshwater

sea turtles - smell of beach, Ascension Island, Atlantic Coast of South America

magnetic fields

Tuna - all over the world oceans

water temperature

elephants migrate by sex

food availability

Special Adaptations of Marine Birds and Mammals

Maintaining temperature


osmotic regulation

have a large body

insulating blubber beneath skin

circulatory system and extremities

too warm - open mouth

dive depth is great, why?


storage of oxygen for long periods



fresh water

salt release

albatrose and DMS

Ecology of Nekton

Little known

Feeding Ecology and Food Webs

baleen, figure 3.34. P. 123

Figure 3.35, p. 124

Figure 3.36, p. 125

Food Webs

Figure 3.37, pages 127, 128, 129

Ecological Significance of Marine Mammals

What they eat and where they are located

Things to Know for the TEST



Magnetite internal compass

Caudal peduncle

Gas bladder




Pectoral fin





Induced drag








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