Can starfish tol­er­ate fresh wa­ter?

Science Illustrated - - ASK US -

Starfish live in salt wa­ter, and their bod­ies have the same salt con­tent as ocean wa­ter. Vary­ing salt con­cen­tra­tion in the wa­ter can be a lifethreat­en­ing chal­lenge to a starfish. If there is less salt in the sur­round­ing wa­ter than in the crea­ture's body flu­ids, its cells might burst.

This is due to os­mo­sis, which means that wa­ter will al­ways balance ar­eas with dif­fer­ent salt con­tents. In salt wa­ter, there is os­motic equi­lib­rium, as the salt con­cen­tra­tion is the same on both sides of the starfish’s cells. Equal quan­ti­ties of wa­ter pass in and out of the cells. In fresh wa­ter, there is no equi­lib­rium, as the salt con­cen­tra­tion is higher in­side the cells than in the

sur­round­ing wa­ter. So, wa­ter seeps into the cells, and they could burst.

Un­like fish, starfish have no os­mo­sis reg­u­la­tion, and con­se­quently, long pe­ri­ods in fresh wa­ter could be fa­tal.

If a starfish is left in fresh wa­ter, it will die, as its cells burst.

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