MadSci Network: Zoology

Re: why all large exoskeleton animals are aquatic?

Date: Mon Sep 27 11:15:11 2004
Posted By: David Richman, Staff, Entomology
Area of science: Zoology
ID: 1095921357.Zo

Exoskeletons imply internal connections for muscles, as well as a rigid 
body that has to be molted periodically.  The former may cause some 
problems for the organism as it gets larger (molting requires a period 
when the organism is soft and vulnerable and the muscles will not work 
well with a soft exoskeleton), but the mere weight that a heavy 
exoskeleton adds as size increase makes it impossible for land-dwelling 
arthropods to get much larger than 250-300 mm.  In order to be as strong 
as the exoskeleton for smaller arthropods a very large arthropod would 
have to have a much thicker exoskeleton.  The associated weight would make 
movement difficult and above a certain size, virtually impossible.  
Another constraint to large size is the inefficient oxygen transport 
method through tracheae.  Arthropods do not need to invest much energy in 
oxygen transport simply because of their size constraints associated with 
an exoskeleton.  Thus every thing is connected to everything else. 

Water is more supportive than air (it is heaver and non-compressible) and 
thus arthropods can push the size limitations further.  Even the largest 
living animal with an internal skeleton (the blue whale) and the largest 
without a skeleton (e.g. the lionís mane jellyfish) are aquatic, so the 
support provided by water helps ALL creatures reach larger sizes, not just 
those with exoskeletons.  The larger dinosaurs may have been partially 
aquatic, but even if not they were able to support their huge masses with 
huge internal skeletons, especially noticeable in their massive leg bones.

The arthropod leg size varies, but even heavier legs with an exoskeleton 
and internally attached muscles would not help much because of the 
problems associated with thickness of the cuticle, oxygen transport and 
muscle function that occur with larger size.  

All of these systems work well at small sizes and arthropods are quite 
successful as evidenced by the well over one million species.  They just 
cannot get huge.  


Price, P. W. 1997. Insect Ecology 3rd ed.  John Wiley & Sons, New York.


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