question about sea star article??

[chicki]

Read with interest the article about sea stars written by Dr. R. Shimek, and must say I really injoyed it.
The question that I have is....you mentioned that they like about 10 ppm more salinity then the normal 1.027 . I don't understand how that can be? Are there different areas in the ocean that have that high, if so then why don't the corals and fish also like that salinity?
Thank you
Lynn

 

[rshimek]

Hi Lynn,

I think you are mixing up salinity and specific gravity.

Salinity is measured in parts per thousand (ppt), and oceanic salinity ranges from 35 ppt to 37 ppt.

Depending on the temperature, water at these salinities will have a specific gravity of 1.025 to 1.027. I used the measurement in parts per thousand because 1) it is actually a measure of salinity (specific gravity is simply a ratio of how much a given amount of water weighs compared to pure water - so if the specific gravity of a water sample is 1.025, that means that that water sample weight 1.025 times as much as a sample of pure water at that temperature would weigh), and 2) salinity - as a measurement - is temperature independent, while specific gravity varies with the temperature.

Now, to your question - all coral reef animals do best at normal reef salinities - typically 36 ppt to 37 ppt.