How do marine organisms cope with life in the sea?

Most show a variety of adaptive responses to conditions within the marine environment

Today, consider general adaptations to physical and chemical environmental (abiotic) conditions.

Examples will include: temperature, salinity, oxygen, and light.

Later in the semester, we will consider adaptations by specific groups of organisms to the biotic and abiotic environment.

Types of adaptive responses:

Behavioral (movements or taxa, panting, shivering)

Biochemical (e.g., enzymes, proteins)

Physiological (e.g., changes in membrane transport, increased circulation)

Morphological (thin vs. thick shells, flat vs. erect body forms)

Assessing adaptation by measuring performance: the tradeoffs between growing, reproducing, and staying alive.

Metabolic rates, growth, death, respiration, reproductive output, etc.

Regulators vs. conformers: two adaptive means for coping with environmental change

Acclimation often allows organisms to perform under a broader range of environmental conditions

Consider some specific environmental factors

Temperature (= metabolism)

Salinity (= osmotic gradients)

Oxygen (= respiration)

Light (= photosynthesis and vision)

Viscosity and flow (= drag)

Temperature (a relatively constant environmental feature)

Temperature and metabolic rate

Temperature and physiological performance:

to freeze or not to freeze

to bake or not to bake

Temperature and reproduction

Endotherms (regulators) vs. Ectotherms (conformers):

the costs and benefits of changing vs. maintaining temperature

 

Salinity (an often rapidly changing environmental feature)

Osmosis "the pressure of being different"

Diffusion "let's not hang out together"

Coping with a body full of less-than-salty fluids in a sea full of salts:

Drinking water and expelling salts can compensate for osmotic differences

Oxygen

Oxygen is needed for respiration but is toxic at high concentrations/pressures

Absolute O₂ consumption generally increases with body mass (although the rate of consumption slows): Size can matter

O₂ consumption generally increases with activity rate: to move or not to move

Getting enough O₂ is a function of surface area to volume ratios: larger animals require gills

Using different O₂ binding pigments can help with life in O₂ poor environments.

Intertidal organisms may face oxygen starvation

Rembember those Oxygen minimum layers that can occur in mid-water, often near thermoclines

Light

Light is essential for vision (finding prey, avoiding predators, detecting mates, etc.) and photosynthesis

Light levels diminish rapidly with depth and many marine organisms have very simple eyes

Organisms in high light environments can have very good vision

Light, nonetheless, may provide important cues for behavioral adaptation

Bioluminescence: making your own light in the dark

 

Viscosity (Life in the sea is a drag)

Relative to air, water is a dense, viscous fluid.

Water is more supportive than air.

Water prevents desiccation relative to air (duh!)

 

Viscosity is important for a variety of other reasons 

Viscosity increases drag, which can profoundly influence the physical forces acting on organisms  

Viscosity causes water to flow within "streamlines", particles tend to move within, not across streamlines

 

Laminar vs. Turbulent flow

Two organismal perspectives on movement in water: "movement" in the sea is relative.

A stationary object in a current is hydrodynamically the same as an object moving in calm water.

Movement within water can be interpreted as a function of two forces:

Inertial force.

Viscous force.

The Reynolds number (R_e) reflects the relative importance of inertial and viscous effects within a fluid

When R_e is large, inertial forces dominate. Translation: large organisms can swim.

When R_e is small, viscous forces dominate. Translation: very small organisms go with the flow.

R_e is a function of velocity, size and the density of an object, divided by a fluid's viscosity.

In sea water, we can effectively take density and viscosity to be constants...

Thus velocity and size primarily determine R_e.

Marine organisms smaller and slower than zooplankton (R_e < 1000) are dominated by viscous forces...

Translation: No swimming allowed at microscopic scales. 

When water flows past an immobile object, a boundary layer exists between the object and the main flow.

At low R_e there may be very little exchange.

Biological consequences:

Reaching the bottom can be a problem for small larvae

Fertilization may be difficult for sperm.

"Filter feeders" must actually pluck their food out of the water column.

Bernoulli's Principle: Pressure varies inversely with the velocity of a fluid

Translation: Speed can be uplifting (think of an airplane wing)

Examples of how organisms use Bernoulli's principle: Bottom fish and burrowing worms

Water motion past an object creates total drag which, in turn, is a function of skin friction and pressure drag

At low R_e skin friction dominates and the amount of area exposed to flow the primary determinant of drag forces

At higher R_e values, pressure drag dominates, and an objects orientation (shape) relative to the direction of flow is important.

Teardrop shapes minimize drag, and many fish show this body plan.

Many sessile organisms either contract or are flexible under high flow regimes, reducing the potentially deleterious effects of drag.

Not surprisingly many of the behavioral and morphological adaptations we see in marine organisms are ultimately driven by the physical aspects of life in a flowing, viscous medium