New Research in Sea Horse Excretion

 
Function Specific to the Sea Horse Excretory System
     Sea horse excretion differs from that of most other vertebrates in that they do not have glomeruli.  Lacking glomeruli prohibits the sea horses from filtering their waste (Schmidt - Nielson 1998).  The way lacking filtration is positive for sea horses goes back to their being hypoosmotic to sea water. Since they are hypoosmotic they must drink sea water to stay hydrated.  They secrete sodium and chloride ions through their gills, but they must secrete the divalent ions magnesium and sulfate in their urine.  To avoid the energy required to reabsorb the ions having a glomeruli would require, the sea horses completely removed the glomeruli (Hickman and Trump 1969).
Structure of the Sea Horse Excretory System
     The sea horses have a type IV marine teleost kidney.  This means that in the sea horse kidney the head kidney is not discernible from the trunk kidney.  Also only the extreme posterior portion is fused (Hickman and Trump 1969).  Research done by Hentschel and Meyer (1982) focused on using oxidative enzyemes to determine the roles of different structures in fish kidneys.  In regard to sea horses, their research showed that the nephrons of sea horses are reduced in comparison to non-Teleost fish.  The most important structures in the sea horse kidney are the proximal segment II and the archinephric duct system.  These two structures are responsible for the secretion of the divalent ions magnesium and sulfate.  Their research also confirmed Hickman and Trump's conclusion (1969) that the sea horse kidney functions in a gland-like way.
This is a picture of the sea horse kidney.
Picture courtesy of Hickman and Trump (1969).

 

This picture shows the structure and function of a nephron in a sea horse kidney.  Solid arrows indicate active transport and open arrows indicate diffusion.  PSII is the second proximal segment, CT is the collecting tubule, and CD is the collecting duct.
Picture courtesy of Hickman and Trump (1969).

 
 
 
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