How is it done?

[Dreampixels]

Often times while crabbing we will observe a crab swimming on or very near the surface. Many times the swimmer will be a male carrying a female.

This seems like an awesome feat for such a creature. Number one the areas I crab normally have the fresh water on top and it is less dense the salt water and number 2 when a male is carrying a female his has nearly doubled his weight which would double the energy needed to carry this feat out.

Think about yourself swimming. Swimming in Speedos requires so much effort, now jump in fully clothed and the same action requires even more effort. What if you add ¾ of your weight by carrying someone strapped to your belly?

Does this mean a Crab has control over it buoyancy?  How is it done? Yes they can take in more water to become more consistent with the water density, but if they expel the water, what fills the void left by the missing water, a vacuum?  Can they separate the combined gasses found in water to control they density?

What are your ideas?

Here is one possiblity. If you view a crab and consider which way a crab gernally moves and swims his shell design makes a  hyrofoil, all he need to do is control his angle of attack to the water in his movment direction to control his depth. To dive he simply tills his shell with his swimming fins and goes down on and angle, to swim on top simple keep the leading edge of his shell slightly upward and he shali rise. Once he is at the depth he prefers, level out and swim flat.

[tugcapt]

The points of a crab have a chamber that holds sea water and a membrane that separates the blood from it by the gills. This allows neutral buoyancy.

Respiratory System

            The respiratory system consists of the gills located in the two lateral branchial chambers (Figs 1, 11, 19-36).   These large chambers occupy the pointed lateral sides of the body and are bounded on all surfac­es by the skeletal branchiostegite of the carapace.   The branchiostegite is a double fold of body wall enclosing the branchial chamber.   It is the lateral carapace. Of its two layers, the outer is heavy and calcified and part of it has been removed.   The inner layer is thin, uncalcified and unsclerotized and is still intact, covering the gills.   This transparent body wall lying over the gills is no more than a thin chitinous membrane investing the dorsal surface of the branchial chamber.   It is almost invisible but it is all that separates the branchial chamber (which is filled with seawater) from the hemocoel (which is filled with blood).  Find and remove this thin, transparent sheet from the surface of the gills.   The branchial chamber is now open and the gills are exposed for study.   

Complete anatomy at  http://webs.lander.edu/rsfox/invertebrates/callinectes.html

[ACFISHERIES]

Some of us don't wear speedo's

[Dreampixels]

The points of a crab have a chamber that holds sea water and a membrane that separates the blood from it by the gills. This allows neutral buoyancy.

This explains a lot. Now with a propulsion system and steering system its time to GET ER DONE!   Thanks!