Cool things you didn't know - 1 - statocysts and decapods

Crabs, lobsters, and crayfish - all members of the decapods

You probably know about our equilibrium organs making up the inner ear.  Called the vestibular system, the three loops of tubing (vestibules) give us a sense of balance, each tube corresponding to each of the three dimensions of our reality.  It turns out that balance systems are not unique to life - many other creatures have a similar system.

Many aquatic creatures use statocysts, somewhat analogous to our vestibule canals.  However, instead of there being three individual organs, there is a single organ that essentially does the same thing.  The one I want to discuss today in my blog is found in decapods - crabs and their relatives.

I copied the image below from this source - it shows the statocyst of a lobster.  It is essentially a round ball of cells, empty in the center - with hair-like (cilia) projections facing the center.  A small, hard object, the statolith, has its position within the sphere affected by gravity.  When it is at the bottom of the organ, the creature knows it is upright.  It also uses it to determine acceleration forward and other movements.

The interesting thing here is what the statolith is and how it gets there.  It is located near the antennae, a pair of them, and is closely associated with the crustacean's exoskelton.  When they shed their outer shell, the statocyst is destroyed.  It is remade during the creature's reformulation of a new exoskeleton.  What's lost too is the statolith; they need a new one.

The solution is simple - the creature pours sand upon its head where the antannea are located.  A small grain of sand enters the developing statocyst and becomes the new statolith.  Crayfish do it, many crabs do it, and lobsters do it.  The cool thing is that you can substitute iron filings for sand in the environment.  The statolith then is attracted to magnets.  If you put a magnet above a so-affected creature, the organism will turn upside down as the statolith is at the top of the aquanited organ, making the animal think it is inverted.

How cool is that!

Thanks for reading.

Eric Svendsen     www.ericspix.com


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