Organismal Physiology Lecture No. 19: Water, Salt & Fish
Thursday November 15 , 2012
-The fact that human beings are approximately 75% water means that it is important for us to
discourage any form of water loss.
Major Constituents Found In Seawater:
- - 2- +
-Referred to in parts per thousand, Chloride (Cl) , Sodium (Na), Sulfate (SO ), Magn4sium (Mg ),
Calcium (Ca ), and Potassium (K ) are the primary constituents found in seawater and make up most of
the ions found there (in that order). (Note that minor elements are not considered in this course).
Various Animal Physiologies Adapted To Seawater:
-Many invertebrates such as the Cnidarians and Annelids possess physiological membranes that are very
close or similar to the salt content in sea water. They are said to be isosmotic to sea water salinity (their
osmolarity is very similar to sea water). Sharks and cartilaginous fish are very much like the mentioned
invertebrates as they are, for the most part, isosmotic with sea water. The hagfish is a primitive, jawless
(no paired fins) vertebrate that is quite isosmotic to its saltwater environment.
-Bony fish however, are very different from sharks, rays and invertebrates because of their being
hypoosomotic to salty seawater. In essence, they are bags of very dilute water living in a saline
environment. This presents a physiological problem in maintaining adequate water balance. We are
strangely much like these fish in that we need to keep from dehydrating.
The Issues Of Bony Fish In Saltwater:
-Salts are ingested and absorbed across the gill surface, which results in a major increase in the body’s
concentration of salts. The issue lies in how to get rid of the excess salt. Essentially salts must be
excreted in a higher concentration than the concentration of salt that is ingested. The fish kidney cannot
solve the problem since the urine of saltwater fish is limited (they pee very little) and mostly get rid of
divalent ions. The fish therefore requires the assistance of another organ to solve the problem.
Gills As Exchange Organs In Bony Fish:
-Although gills are usually known for gas exchange (CO and O 2, they 2re also used to remove the salts
that are ingested with food and water (gill surfaces are the major site for absorption in marine teleosts).
Gills create the problem because they are a permeable membrane that lets in salt, however they have
also evolved to be part of the solution (expelling salt concentration).
Chloride Cells & The Export Of Chloride:
-In the longitudinal cross-section of a lamella, a chloride cell is present at the base. Also known as
mitochondrion-rich cells (MRC), these crucial cells are embedded in the epithelium