The Benthic Zone
Benthic zone is the
substrate (floor) on
both the shallow
continental shelf
and the abyss.
The substrate is composed of 4
types of sediment:
1. Biogenous or organic
- greater than 30% organic
material
- Oozes
- Contain skeletal & fecal remains
2. Terrigenous or lithogenous
- greater than 30% inorganic
products of weathering &
volcanic ash.
3. Hydrogenous or authogenic
- minerals precipitated from
chemical compounds in
seawater
4. Cosmogenous
- materials that fall from space.
• These different types of
sediments, called
patchiness, determine
where animals will live
on the ocean floor.
Particle Sizs and Settling Rate in Sediment
Type of
Diameter
Particle
Boulder
>256 mm (10 in.)
-
Cobble
64-256 mm(>2.5 in.)
-
Pebble
4-64 mm(1/6 - 2
1/2in.)
-
Granule 2-4 mm(1/12 - 1/6 in.)
• Grains of sediment
become sorted
depending upon how
long they have been
around and the
transportation
mechanisms.
Settling Velocity
Sand
-
Time to Settle
4 km (2.5 mi)
-
0.062-2 mm
2.5 cm/sec ( 1in/sec)
Silt
0.004-0.062 mm
0.025 cm/sec ( 1/100
in/sec)
6 months
Clay
<0.004 mm
0.00025 cm/sec
50 years*
1.8 days
1. Porosity
- the amount of pore spaces for air,
food, and water held between
grains.
- More spaces = better environment
2. Sorting
- measure of uniformity of grain size
- Poorly sorted grains are many
different sizes and have better
porosity
- Well-sorted grains are less porous,
are nearly the same size and
provide poor habitat.
- Poorly-sorted grains are good
because they provide better
habitats, hold more food & water.
3. Maturity
-
-
measures the age of
the sediments
Older sediments will
have been exposed to
heavy erosion
Younger sediments
show less erosion
Characteristics of the
Benthic Zone:
• 90% of organisms on
continental shelf
• 10% in the abyss
• Mesopelagic zone - 99% of
the organisms are
bioluminescent (produce
light by chemical reaction)
• Very stable environment increases with depth.
• Lack of light is a major
limiting factor - reducing
food, predation, and mating.
• Diversity increases
with depth - more
niches are created
because resources
are limited.
• Oxygen is supplied
from cold, saline
waters of the poles.
• Biomass decreases
with depth - this low
population density is
directly related to
food scarcity.
• Communities entirely composed of consumers and
scavengers
• Producers are found only in photic zones
• Decomposers like bacteria are more common in
mid-water mesopelagic zone.
Hydro-Thermal Vent Communities
• The exception in benthic
communities’ producers
are the chemosynthetic
bacteria around
hydrothermal vents.
• 1977 - scientists (DSV Alvin
- Project FAMOUS)
discovered unique
hydrothermal vent
communities.
• Vent communities found in
all oceans at depths 1-2
miles down.
• Bacteria living near the vent
use hydrogen sulfide
dissolved in seawater to build
organic molecules in a
process called
chemosynthesis.
• Animals clustered near the
vents grow to huge sizes; can
withstand temperature
differences from 36o to 662o F.
• The community also has
many suspension-feeders
attached to the hard rock
bottom which is unusual in the
deep sea
• Vents last 100 years when supply of H2S is
exhausted, the ecosystem
dies.
• Cold Seeps - methane
and sulfur-rich fluids seep
into the ocean floor where
symbiotic bacteria use
sulfur oxidation for
survival.
• Cold Seeps are home to
millions of benthic worms,
crabs, and mollusks.
Summary Questions
1. How is biomass and diversity related to an
increase in depth?
2. Why is the herbivore trophic level absent in
deep sea benthic pyramids?
3. How do the sizes of organisms living around the
hydrothermal vents compare with other benthic
organisms?
Adaptation of Benthos
Benthos is the name of
organisms living on the
substrate.
They obtain food by:
1. Suspension feeding -
-
filter food suspended in
water column
Sandy & course sediments
where currents contain
large quantities of food and
stable (don’t clog feeding
mechanism)
Corals living below the light
2. Deposit feeding
-
Eat food particles lying
in sediments
Dominants in muddy,
fine sediments
3. Scavengers or
Croppers
-
-
Quickly locate dead
animals falling from
above.
Go for long periods
without food.
Will eat deposit
feeders
Other Adaptations include:
- Special respiratory &
feeding appendages that
keep animals above the
sediment to prevent
clogging or smothering in
the substrate.
- Expandable mouth and
bodies to accommodate
large but infrequent meals.
- Air bladders are small
or missing as buoyancy
is not needed.
Pressure is required for
proper enzyme
functioning.
- Evisceration
(expelling) internal
organs to provide
predator with a meal
while they escape to
regenerate lost parts.
- Lights and lures to
attract prey, mates or
frighten predators.
- Blind creatures occur
where sight is of no
advantage.
Reproductive Strategies:
1. Dispersal of planktonic larvae
-
common only on the shelf.
Adv: chance for larvae to land on
the best substrate
Disadv: heavy predation
2. Direct development
(brooding)
-
-
internal fertilization is
common in cold, deep
water where food is
scarce.
Adv: protection of an
adult
Disdv: is adult is
eaten, all the offspring
are lost
3. Asexual
reproduction
-
-
Budding
Adv: solves the
problem of finding a
mate
Disadv: lacks the
adaptability
provided by variety
in a gene pool
Summary Questions
1. How do benthic organisms reproduce?
2. Name 4 adaptations for benthic marine life.
3. Why are benthic organisms difficult to study?
Phylum Mollusca
Second largest phylum
behind Arthropoda
General
Characteristics:
-
Many are benthic
Complete digestive tract
Open circulatory system
Usually separate sexes
Cephalization (having a
head)
- Soft bodies; mantle
secretes a shell
Class Gastropoda
• Gastro = stomach
• Pod = foot
• Univalvia (one shell)
• Ex: snail, abalone,
conch
• Most are benthic
herbivores
• Dog welk is carnivirous
• Radula (tongue) with a
ribbon of flexible teeth
works like a mini-chain
saw
Class Pelecypoda
• Bivalvia (two-footed,
also two shells)
• Ex: clams, oysters,
scallops
• Benthic planktivores
• Filter sea water through
an inccurrent &
excurrent pore.
• Sessile (oyster)
• Swim by opening &
closing both shells
(scallop)
Class Cephalopoda
• Cephalo = head
• Pod = foot
• Squid, octopus,
cuttlefish, chambered
nautilus
• Have the most
advanced brain of all
invertebrates.
• Mouth has a sharp
beak for eating worms
& fish
• Moves by jet-propulsion mantle fills with water
which can be squirted out
to propel them backwards.
• Chromatophores in the
skin change colors when
the animal is alarmed and
for camouflage.
• Ink gland is used as a
“smoke screen” for
quick escape.
• Eyes have a cornea,
lens and retina;
excellent vision.
Squid
• Internal shell (pen) that
is clear and flexible. The
only hard part of their
body is their beak.
• Has 2 tentacles with flat
paddles.
Octopus
• A soft, flexible body allows the
octopus to hide in very small
spaces.
• Very shy & do not attack
humans nor eat ships
• Enjoy human contact in
captivity
• Octupus’ head has 8
regenerative arms
Octopus
• Octopus reproduction is
by internal fertilization.
• During the 30 days until
hatching, the female
carefully aerate, cleans
and watches the eggs never leaving (even to
eat).
• The female usually dies
from exhaustion and
starvation soon after
they hatch.
Cuttlefish
• The cuttlefish has an
internal shell that is
sold in pet stores as a
source of calcium for
birds.
Chambered Nautilus
• The spiraled shell is lined with
mother-of-pearl and perfectly
proportioned.
• The walls (septa) divide a series of
compartments in the shell and are
pierced by a tube (siphuncle)
connected to the nautilus.
• Gas & liquid exchange occurs
through the siphuncle walls and
allows the nautilus to regulate it
buoyancy.
Summary Questions
1. Identify three members of the phylum Mollusca.
2. Why do squids ink?
3. Which type of mollusk has bilateral symmetry?