Zoology subject topic: Nektoon

2.  Introduction
 Adaptation
 Distribution
 Importance

3.  Organisms that have developed
powers of locomotion; capable of
movement at will
 These are the animals that we are
most familiar with:
◦ Vertebrates
◦ Invertebrates

4.  Dominated by fishes (bony fishes; cartilaginous
fishes)
 Includes also other members of the vertebrate class
(marine mammals, reptiles, birds)
 Includes also some members of the cephalopod
mollusks

5.  Buoyancy
◦ Gas or swim bladder (possessed by most fishes)
◦ Two types of gas bladder
 Physostome: Open duct exists b/w gas bladder &
esophagus (e.g. Herrings, salmonids, catfishes)
 Physoclist: No duct is present (spiny-rayed fishes)

7.  Buoyancy contd.
◦ Gas-filled cavities (lungs) --- in air-breathing
nektonic animals
◦ Accessory air sacs (in some marine mammals;
birds)
◦ Air trapped under feathers (penguins)
◦ Air trapped in dense wool undercoats (sea otters
& fur seals)

8.  Buoyancy contd.
◦ Replacement of heavy chemical ions (Na+) in the
body fluids with lighter ones (NH4+) --- in squids
◦ Lay down lipid (fat or oil) in the body---in fishes
(muscles, liver, etc), marine mammals (below the
skin as blubber)

9. • Buoyancy contd.
– Hydrodynamic mechanisms for producing
buoyancy during movement
• Lifting surfaces in the anterior region (pectoral
fins, flippers)
• Heterocercal tail (upper lope is larger than lower
lobe)
• More primitive fishes tend to have hydrodynamic
adaptations to create lift
• More advanced fishes appear to evolve static or
passive means to achieve neutral buoyancy

10.  Adaptation to create propulsive force
◦ Undulation of the body from side to side due to
alternate contractions of the body muscles (fishes)
 Tail movement are up and down in whales
◦ Undulation of the fins: body remains stationary
while fins are moved to effect forward motion

11. Tuna Sailfish

12. • Speed, poisonous secretions, mimicry of other
poisonous or distasteful species, camouflage,
countershading, transparency
• Schooling
– Many individuals maneuver as if one
– Safety in numbers
– Appears larger
– Movements confusing to predator

13.  Are well developed in nekton
 Lateral Line System in Fishes: rows of tubes open
to the surface; contain sensory pits sensitive to
pressure changes in water
 Ampullae of Lorenzini in Sharks & Rays: Sensitive to
minute electric currents in water; use
electroreception to find prey
 Geomagnetic sensory system in marine mammals
for long-distance navigation
 Eyes: Well developed
 Olfactory senses (sense chemicals)
 Hearing senses (Inner Ear in fishes)

14.  Most developed in cetaceans
 High Frequency Sounds used to locate objects
in water (prey/predators); navigate around
objects
 Toothed whales possess very large brains
relative to their body size that enable them to
process acoustical information rapidly

15.  Holonektonic bony fishes spawn eggs that
float and undergo development in the open
ocean waters
 Eggs of some spp. have threadlike
structures for adhering to floating debris
 Heavy losses of pelagic eggs occur due to
predation
 Large Nos of eggs are produced to offset
the losses
◦ Tuna produce 2 – 2.6 million eggs
◦ Marlin (13 million eggs)
◦ Ocean sunfish (300 million eggs)

16.  Pelagic sharks produce a few large eggs or
embryos
◦ Thresher shark: produce 2 embryos
◦ Blue shark: up to 54 eggs
 Eggs are large & retained in the female for
much longer period of time so that at birth or
hatching they will have relatively high
probability of survival

17.  Similar to their terrestrial relatives
 All produce shelled eggs; laid on land
 Marine birds often congregate in large #s to
nest on islands or cliffs that are inaccessible
to terrestrial predators
 This ensures that the helpless young (altricial)
bird will survive until it is old enough to fly,
although this makes the birds vulnerable to
human predation or pollution

18.  Practically all members of the nekton are
carnivorous
 Planktivorous
 Toothed whales, tuna, and sharks are general
carnivores eating both other vertebrates and
invertebrates
 Piscivorous

19.  This is a challenging environment to live in
◦ Extreme 3 dimensionality
◦ No solid substrate
◦ No landmarks to use for navigation

20.  Like plankton, nekton also incorporate
lipids into their tissue
 Sharks concentrate high oil levels in a large
liver

21. • Fishes use stiff fin elements with movable spines
• These fins are used as wings to generate lift
• Heterocercal tails are used in sharks to generate
lifts

22.  All nektonic animals are evolving
mechanisms to swim faster
◦ In many cases, speed is what determines if the
prey escapes or if the predator eats
 Adaptations are directed towards
decreasing drag and increasing forward
propulsion
◦ Elimination of protuberances and a streamlined
body shape helps
◦ The caudal fin provides the major component of
thrust

23.  The best swimmers contain a high amount of
red muscle
◦ Red muscle gets its color from a high concentration
of myoglobin which is great at storing oxygen
◦ Red muscle is best used for long periods of
exertion
 A difference seen when you compare a tuna versus a
flounder

24.  Some pelagic sharks
and large tuna have
the ability to conserve
the heat their muscles
generate
◦ This trick of the
circulatory system allows
cold oxygenated blood to
pass closely with the
warm deoxygenated
blood
◦ There is a transfer of heat
to the cold blood before
it enters the muscle
 Benefit: a warm muscle
is more efficient than a
cold muscle

26.  Color and Camouflage
◦ Countershading - dark dorsal side, light ventrally

27.  Behavior
◦ Schooling behavior is a
method of protection for
small fish
 Flying Fish
◦ Use modified pectoral
fins for gliding above the
surface

28.  Why are migrations made?
◦ Many animals undergo extensive migrations
usually for feeding or reproduction
 Salmons
 Sea Turtles
 Tuna undergo extensive migrations to fully exploit food
resources
 Marine mammals (baleen whales) make migrations in a
north and south fashion for feeding and calf rearing

30.  What influences nekton abundance?
◦ Bottom-up influences
◦ Intrinsic (physiological) influences
◦ Top-down influences
 How important are community-level
interactions?

31.  Planktonic community
 Climate change
 Long line fishing
 Seine nets
 Food web

33.  Growth rates may be temperature dependent
 Size influences
◦ feeding success
◦ the number of eggs produced per female
◦ “escape” from smaller predators through growing
too large to be eaten
 Is there evidence for temperature effects on
nekton success?

34.  Nekton-feeding fishes, birds & mammals dominate
the open ocean nekton.
 Their diet includes fishes, squids, large
crustaceans.
 Size of prey consumed increases with size of the
predator
 Largest carnivore in the oceans is the sperm whale
which preys on the giant squid
 The 2nd largest carnivore of the nekton is the killer
whale which preys on fishes, penguins, porpoises,
seals, sea lions, & even large baleen whales
 Smaller toothed whales, seals, & sea lions feed on
squids & fishes
 Pelagic marine birds feed mainly in the surface
waters consuming small fishes and squids.