The Pelagic Zone
Section 1: Life Near The Surface and Life In The Deep
Pelagic Zone: the water region of the ocean that is above the
ocean floor and away from the shore.
– In order to survive in the pelagic zone species have
evolved adaptations for defense, hunting and mating.
Hunting Adaptations in the Pelagic Zone
How do organisms stay above the
ocean floor?
• Larger organisms are more dense
than water.
– Swimming stay above the ocean
floor requires a lot of energy.
– Many organisms use gas-filled
organs to stay afloat.
– Gas is less dense than water.
Hunting Adaptations in the Pelagic Zone
•
Swim Bladder: an internal, flexible, gas-filled structure.
– Gases are added and removed through the blood which
will cause the swim bladder inflate or deflate.
• Some fast moving fish do not have any gas-filled organs to
stay afloat.
– Some fish are strong enough to continuously swim to stay
off the ocean floor.
– Sharks have a specialized tail fin shape which provided lift
as they swim.
Hunting Adaptations in the Pelagic Zone
•Mobility of a fish depends on the quantity and type of muscle
•There are 2 types of muscles: red and white
– Red Muscle: Contains
myoglobin which is a red
pigment that carries oxygen.
More oxygen allows for long
periods of muscle use.
– White Muscle: Provides great
acceleration for short periods of
time but fatigues much more
rapidly than red muscle tissue.
Hunting Adaptations in the Pelagic Zone
Bony fish can be split into 2 groups based on mobility:
1. Cruisers: Hunt by actively swimming and seeking
prey.
– Contain mostly red tissue to swim long distances
2. Lungers: Sit and wait for food.
– Contain mostly white tissue because they only
travel short distances in quick bursts.
Hunting Adaptations In The Deep
• There is very little food available in the
deep sea.
• Deep sea fish have developed
adaptations to conserve energy
– Weak and flabby muscles (they mostly
sit and wait for prey to some by)
– Deep water fish have swim bladders
filled with fat instead of gas which
does not compress under pressure.
– Why couldn’t a fish from the deep
sea have a swim bladder that is filled
with air?
Hunting Adaptations In The Deep
• Deep sea fish have large mouth and
expandable stomachs to consume large prey.
• Anglerfish use bioluminescence to lure in prey.
Section 2: Defense Adaptations In The
Pelagic Zone
Countershading: Fish living near
the surface usually have a . . .
• Back that is black or blue.
– A predator that is looking down will
not see the fish against the dark
ocean floor.
• Belly that is white or silver.
– A predator that is looking up will
not see the fish against the bright
that is filter into the ocean.
Defense Adaptations In The Pelagic Zone
• Photophores: Light producing organs.
• Organism use photophores to match the
amount of light in the environment.
• Similair to countershading.
Sex in the Pelagic Zone
Reproduction
• 3 basic types of reproduction
1. Oviparous Reproduction:
animals release eggs that
develop and hatch outside the
body.
– Most fish produce thousands of
eggs because most will be
consumed by predators.
– Some fish will only produce a
few eggs, but they viciously
defend their eggs.
– In seahorses, the male is
responsible for carrying the eggs.
Sex in the Pelagic Zone
2. Viviparous reproduction – mothers directly nourish their
unborn young through, and give birth to live young.
– Some sharks, rays and all marine mammals reproduce
in this way
– This is the way that is closest to human reproduction
Sex in the Pelagic Zone
3. Ovoviviparous: animals develop and hatch from eggs,
inside the female, which gives birth to live young
– Organisms are not directly nourished by the mother,
they consume egg yolks for nourishment.
– This is the reproductive cycle of most cartilaginous fish
– Once the sand tiger shark consumes all its egg yolk it
eats its brothers and sisters in the womb, then it
begins to eat its mothers unfertilized eggs directly out
of her ovaries.
Sex in the Deep
• Finding a mate in the deep
sea can be more difficult than
finding food.
• Challenge 1: Finding the
same species
– Different species use different
patterns of bioluminesence
– Male Parasitism: male angler
fish bite onto the female and
remains attached for the rest of
his life.
Sex in the Deep
• Challenge 2: Finding the opposite sex.
– Hermaphodites – many deep sea
organism produce both eggs and sperm.
– Females release a chemical that males
can smell.
Schooling and Migration in the
Pelagic zone
Schooling: a well defined social group
of fish or squid.
• Common among many cruiser fish,
hammerhead sharks and manta
rays.
• Use vision, smell, and lateral line to
form schools and move in unison
Schooling and Migration
• Reasons for Schooling
– Protection from predators
• Predators are unable to target single individual
• To intimidate or confuse predators
– Makes swimming easier by reducing drag.
– Makes hunting or feeding easier.
– Makes mating easier.
(Different fish have different reasons to school)
Schooling and Migration
Migration: regular mass movements from
place to place daily, annually, or once in a
life time
• Reason to Migrate
– Feeding
Ex: Tuna live in tropical waters but
migrate to temperate waters to feed.
– Breeding
• Salmon : live in saltwater but
migrate to fresh water to spawn.
Explain why deep sea fish do not form
schools or migrate.
Section 2:
Cartilaginous Fish vs. Bony Fish
Section 2: Cartilaginous vs. Bony Fish
Cartilaginous fishes: Have a skeleton made of cartilage
which is a material that is lighter and more flexible
than bone
• Belong to the Chondrichthyes class of organisms
• There mouth is below there head
• They have rough sand paper like scales
• Ex: sharks skates and rays and ratfish’s
Cartilaginous vs. Bony Fish
Bony fishes: Have a skeleton made of bone
• Belong to the Osteichthyes class of organisms.
• There mouth is usually at the front end of their head
• They have smooth scales covered with a thin layer of
mucus
• The great majority of fish in the pelagic region
Cartilaginous vs. Bony Fish
Locomotion
Both bony and cartilaginous fish have 4 sets of fins:
– Caudal fin: provides
forward movement.
– Pectorals: helps in
steering and breaking
– Pelvic fins: helps in
steering and keeping
balance
– Dorsal/anal: helps the
fish stay upright
Cartilaginous vs. Bony Fish
Locomotion in Cartilaginous Fish
• sharks use their fins for locomotion and buoyancy
(staying afloat)
• bony fish use their fins only for locomotion because they
use swim bladders to stay afloat.
• Sharks tend to sink because they don’t have a swim
bladder.
• Heterocercal caudal fin - an asymmetric fin where the
top lobe is larger than the bottom lobe which provides
lift.
Cartilaginous vs. Bony Fish
Locomotion in Bony Fish
• 4 different caudal fin shapes
– Rounder: good acceleration and
maneuverability, but not good at
prolonged swimming
– Truncate: Similar to the rounder
shaped caudal fin.
– Forked: not good at acceleration and
maneuverability but ok at prolonged
swimming.
– Lunate: worst at acceleration and
maneuverability but best at
prolonged swimming.
Cartilaginous vs. Bony Fish
Feeding in Cartilaginous Fish
• Sharks eat by taking bites out of large
organisms, not swallowing smaller
organisms
• Most sharks mouths are adapted to have
rows of teeth that help them bite and
tear off peaces of larger prey
• Only Whale sharks, Basking sharks, and
Megamouth sharks are suspension
feeders
– Suspension feeding sharks have large
mouths with small teeth that is good
for straining through large quantities
of water
Cartilaginous vs. Bony Fish
Feeding in Bony Fish
• Most bony fish are totally or mostly carnivorous and
so they have moths adapted to help them capture,
grasp, and hold their pray which is usually swallowed
whole
• What do they eat?
Cartilaginous vs. Bony Fish
Respiration in Cartilaginous Fish
• Sharks need to swim in order
to breath
• Sharks open and closer their
mouth while swimming
which helps force water over
their gill slits
• Since rays and skates are not
constantly swimming they
use a modified pair of gill
slits called spiracles to help
move water over their gills
Cartilaginous vs. Bony Fish
Respiration in Bony Fish
• Boney fish have gill covers
that circulate water over the
gills without swimming.
• The fish sucks in water
through the mouth while the
gill covers are closed.
• Then the mouth closes, the
gill covers open, and the
water is forced over the gills.
Cartilaginous vs. Bony Fish
Respiration in Bony Fish
• Countercurrent system of flow: blood flows over the
gills in the opposite direction that water flows over
the gills.
– allows the most oxygen deprived blood to meet
the most oxygen rich water so the fish gets more
oxygen.
Cartilaginous vs. Bony Fish
Osmoregulation: the ability of organisms
to regulate the amount of salt and
water in their bodies
• Osmosis: the movement of water
molecules through a membrane from
high concentration to low
concentration.
• Why can’t you drink saltwater if you
are stranded at sea?
Cartilaginous vs. Bony Fish
Osmoregulation in Cartilaginous Fish
• To prevent dehydration sharks and other
cartilaginous fish concentrate urea (one of the
main components of urine) in their blood.
• Makes the concentrations of solutes in their blood
close to or even higher than that of sea water
• Excess salt is secreted via the gills, skin and urine
Cartilaginous vs. Bony Fish
Osmoregulation in Bony fish
– Urea is toxic to bony fish
– Due to osmosis and diffusion, water leaves the fish
and solutes enter the fish causing dehydration.
– Bony fish use kidneys to prevent dehydration by
excreting only a little water and a lot of solutes
through urine
Cartilaginous vs. Bony Fish
Sensory Structures in Cartilaginous Fish
• Have a week sense of vision with little to no color
vision
• Ampullae of Lorenzini: a special sense organ located
in the head of cartilaginous fish that can detect week
electrical signals given off by the nervous systems of
their pray
Cartilaginous vs. Bony Fish
Sensory Structures in Bony Fish
• Have a good sense of vision but can
only focus by physically moving closer
or further away
• Lateral line – a system of small canals
that run along the body of the fish
and allow it to sense vibrations in the
water