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.

Many of the adaptations we see in marine organisms are ultimately driven by the physical aspects of life in a flowing, viscous medium