Buoyancy and flotation

Unlike terrestrial organisms, where density of atmosphere << bodies and land organisms need structural tissue (skeleton and muscles in animals) to give body shape and ability to move. Density of water is similar to density of bodies of marine organisms. Buoyancy of water helps keep floating organisms at surface.
- supports bottom living organisms
- lowers energy spent by swimming organisms
However, tissue is slightly more dense than surrounding SW so without special adoptions, organisms would sunk.

Portuguese man - of - war and By - the - Wind - sailor secrete gases into a float so they float - also acts as sail.

Snails float by producing gas bubbles in intestine or form bubble rate to float at surface - shell very thin, light and fragile.

Algae secrete gas bubbles or have gas filled floats to keep them in sunlit surface waters to maximize photosynthesis.

Cephalopods chambered nautilus and cuttle fish regulate buoyancy.

Nautilus Secretes Chamber in shell - lives in outer chamber - Chambers filled with N2

Cuttle fish has soft internal shell - controls buoyancy by regulating the amount of gas and water in shell.

Fish - gas filled swim bladder -
Continuously swimming predators, mackerel, tuna, sharks - have no swim bladder - neither do bottom fish.

Small phytoplankton and zooplankton synthesize oil droplets incorporated in cells to decrease density and slow sinking

- also have large surface area : volume
- dependent on small size
- also developed cellular processes - spines, appendages that increase surface area and slow sinking.

Large Marine animals - tissue more dense than SW.

Whales, seals increase flotation by storing large amount of blubber ( low density fat).

Sharks and some other fish store oil in liver and muscle.

Seabirds - fat deposits, light bones and air sacs, water proof feathers.


- salt content of SW provides increased buoyancy but differs from salt content of body fluids of osmoregulators, e.g. fish - have salt concentration between FW and SW

- in SW, Lose water by osmosis from high water conc. inside to low water conc. outside.

Fish must constantly use energy to maintain ionic homeostasis.

SW fish drink continuously and excrete excess ions across the gills and in concentrated urine.

Sharks and rays have same osmotic concentration in body fluids as in SW

- Maintain high urea levels - allows tissue to retain water, prevents salt uptake


- most inverts
- body fluids in equilibrium with salt content over wide range.

Salinity may limit distribution of organisms
- since salinity is constant in deep waters, species living below surface layer are widely dispersed.

Surface organisms often find salinity barriers in coastal waters
        - salinity in bays and estuaries may fluctuate
        - most successful estuarine animals are good osmoregulators.        
        - salmon - anadromous fish.


Density of SW is affected by temp and salinity. In polar waters, surface water is cold and dense but in tropical waters, warm water is less dense.

- tropical organisms usually hade more appendages, produce more gas bubbles, have larger surface areas than similar species in polar waters.

Temp has a large effect on metabolism
- cold blooded animals - body temp varies with environmental conditions.

cold water forms in general grow slower, live longer and grow larger than warm water forms

- also distribution of temp. in the oceans limits distribution of marines organisms - both vertically and horizontally.

Tuna - exploits thermocline to feed because of heat conservation and generation in muscles and brain.

Pressure may alter metabolic rates but deep living organisms have no gas - filled chamber to maintain a high pressure

Air breathing mammals

- dive deep for a long time with no problems
- blood absorbs more O2 and tolerates more CO 2 than land mammals.
- vascular shut directs blood plow to hear and brain - away from periphery
- lungs collapse during dive, forcing our air out and preventing blood form absorbing gas at high pressure.


- Vertical distribution Pelagic environments

Neritic ( inshore )

Pelagic (offshore)

- Organisms / living in water column - not on the bottom

- ties to water column - depend very little on substrate

Neritic province - in shore

- covers continental shelf and water above it
- extends form MLW to end of shelf
- max depth 200 m - but varies with oceans because of variations in width of shelves
e.g. California shelf - few miles
west coast of FL - 60 miles.


- offshore from edge of continental shelf to >7,000 - 10,000 m
- total diurnal and seasonal movement of pelagic organisms into neritic realm due to fluctuations in current strength.

Epipelagic. Mesopelagic (200m to 1000m)
Bathypelagic - 100 - 400 m - uniform condition most of the ocean. Abyssopelagic ( below 400m) trenches - similar physically to bathypelagic. Small populations of swimming inverts; food is scarce.

Benthic environments.

- on, in, or near the bottom
- unlike pelagic, benthic habitats involve characteristics of water column and underlying sediments.


a). supralittoral
b). littoral - intertidal zone - depth depends on tidal range.
inner (above 150 m) and outer (150 - 200 m) zone based on depth of photic zone
SLOPE - BATHYBENTHIC- continental slope, rise and part of ocean floor to 4000m

ABYSSAL ZONE -abyssal plains below 4000m - 6000m

Hadal zone (6000 - 10, 800 m) trench