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
Holoepepelagic
thresher shark, mackerel shark, blue shark
Floating eggs
waters of tropics, subtropics
Meroepipelagic
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
squids
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
Locomotion
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
laminar
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
bluish
greenish
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
Echolocation
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
Migrations
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
diving
osmotic regulation
have a large body
insulating blubber beneath skin
circulatory system and extremities
too warm - open mouth
dive depth is great, why?
Bends
storage of oxygen for long periods
bradycardia
myoglobin
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
Echolocation
Baleen
Magnetite internal compass
Caudal peduncle
Gas bladder
Altricial
Precocial
Bradycardia
Pectoral fin
Keel
Sharks
Light
Squids
Induced drag
Anatomical
Frictional
Bends
Physiological
Form
Dark
Rays