Marine Organisms
Marine vs. Terrestrial Life
Marine
Terrestrial
Organisms – similar density as
Organisms – much higher density than
environment (salt water) less energy to air … than to walk or fly.
float/swim… small effect of gravity High gravity impact (fall down)
Water supports bodies, no need to put Need strong skeletal material (animals:
energy in skeletons
bones; trees: trunks)
Plenty of water for life
May become water limited
Temperature variation low
Temperature varies strongly
Light limited: reflection of light at sea Light energy substantially higher than in
surface and rapid light absorption with aquatic systems, low absorption by air
water depth
Nutrient limited: nitrate, phosphate,
silicate, iron
High nutrient concentrations in natural
soils
Major part of nutrient regeneration in Nutrient regeneration in soil close to plant
the dark deep-sea
uptake
Physically unstable habitats
Physically stable environment
Classification by habitat
Plankton are those organisms that live suspended in the water column and are too
small to be able to swim counter to typical ocean currents, they include
phytoplankton, zooplankton and bacteria.
Nekton (‫)اﻟﺴﻮاﺑﺢ‬: Those animals that move in the water column, but they are
capable of more powerful swimming and can move against a current or through
turbulent water. They range from small fish, to the largest whales.
Benthos:(‫ )اﻟﻘﺎﻋﻴﺔ‬Include those animals and plants that live attached to the bottom
or on or in the bottom.
ƒ Infaunal: Animals that live in the bottom within the sediment such as clams.
ƒ Epifaunal: Animals that live on or attached to the surface of the bottom, e.g.
oysters and barnacles.
ƒ Semi-infaunal: Those animals that are partially burrow in the bottom e.g
Artina.
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Neuston
Nekton
Plankton
Epifaunal
Boring
Benthos
Semi-faunal
Infaunal
Plankton
Phytoplankton are photosynthetic planktonic protists and plants, and usually
consist of single-celled organisms or of chains of cells.
Zooplankton all heterotrophic plankton, their size range from 2 µm
(heterotrophic flagellates, protists) up to several meter (jellyfish)
Mixoplankton are photosynthetic protists, but also can ingest other plankton.
Meroplankton are plankton for only part of their life cycle. They include the
planktonic larval stages of many benthic invertebrate groups.
Holoplankton, are those planktonic organisms that spend all their life cycles as
plankton, i.e., Planktonic throughout life.
Neuston are the plankton associated with the water surface, such as bacterial
films.
Pleuston: Planktons that have a float protruding above the sea surface, e.g.
Portuguese man-of-war.
Classification of Planktonic organisms according to their size
•
•
•
•
Ultraplankton or picoplankton (<2 µm),
Nanoplankton (2-20 µm),
Microplankton (20-200 µm),
Mesoplankton (0.2-20 mm)
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• Macroplankton (2-20 cm),
• Megaplankton (20-200 cm).
Plankton is traditionally sampled by plankton nets with a mesh size of various
pore size.
Fig.
Paired plankton nets are placed overboard. The length of wire and angle of entry are
measured to find the depth of sampling. A calibrated propeller gives the volume of water sampled.
Plankton Life
As we have mentioned before, Plankton are the plants and animals that drift
around on the oceans’ currents. Phytoplankton are the microscopic plants that
convert sunlight and nutrients to starch and organic matter. Not only do
phytoplankton form the base of the oceans’ food chain, they also produce at
least 80% of the oxygen that we breathe.
Zooplankton on the other hand, are the animal members of the marine planktonic
community. Most zooplankton occupy the second or third trophic level of the
marine food web. As such, these herbivores and small carnivores play an
exceptionally important role in marine food webs.
Planktonic organisms usually depend upon the surface waters for survival.
Phytoplankton will die unless they are near a source of sunlight for photosynthesis.
Zooplankton on the other hand, depend upon phytoplankton, or at least upon
animals that consume phytoplankton. Zooplankton, therefore, also must remain in
the surface waters.
How do plankton keep near the surface or avoid sinking?
A particle will remain suspended in water if it is less dense than seawater. Many
planktonic organisms however are somewhat denser than seawater and will
therefore sink in a quiet water column.
Plankton reduce their density relative to that of the surrounding water and avoid
sinking to the bottom or to depths greater than a depth at which they can
photosynthesize (in case of phytoplankton) or survive by one or more of the
following mechanisms:
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1. By storing (accumulating) oils and other fluids of low density
• Diatoms and other organisms such as copepods, and fish eggs and larvae,
contain droplets of oil which reduce their density and can serve as food
reserves.
• Some blue-green bacteria have vacuole-like structures that contain lowdensity gaseous nitrogen.
2. By having flattened bodies and appendages, spines, and other body
projections that increases their surface area and reduces settling velocity.
• The bell-shaped jellyfish has flat bottom and tentacles combine to slow
sinking relative to a sphere of the same shape and bulk composition.
• Some diatoms, such as Chaetoceros, form twisted chains, which spiral as they
sink slowly.
3. By having Flotation structures
• Portuguese man-of-war Physalia for example have a large gas-filled sac that
acts as a float from which the rest of the colony is suspended.
4. By replacing heavier ions with lighter ones
• Many zooplankton are replacing dense magnesium, calcium, and sulfate ions
with lower-density ammonium, sodium, and chloride.
5. By accumulating ions of low specific gravity
• The dinoflagellate Noctiluca, for example, accumulates ions of low specific
gravity, which reduces their density.
6. By having the ability to swim
• Pteropods (‫( )ﻣﺠﻨﺤﺔ اﻷﻗﺪام‬sea butterfly) for example, has two lateral winglike
projections, and the snails flap through the water.
• Jellyfish move in pulses, through rapid compressions of circular muscles,
which compress the bell and force water backward.
• Crustaceans such as copepods use assorted appendages to push against the
water to move forward.
• Arrow worms swim by undulation.
All these swimming methods are effective means of countering sinking.
7. By depending upon water turbulence which keeps organisms suspended in
the water column.
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