I'm no expert in osmoregulation, and you should know there is a vast body of literature on it, so here is a simple view of it: Most marine organisms have physiological adaptations for dealing with fluctuations in salinity. Some fish have salt glands that help regulate osmotic balance, crustaceans have specialized cells in their gills and antennal glands for osmoregulation, and some some animals such as echinoderms just conform to small changes (3-5 psu), but can otherwise be affected by large changes in salinity. It takes energy to osmoregulate so metabolic demand increases during periods of increased regulation. That can impose stress to an animal. Some crabs, for example blue crabs, osmoregulate at low (0-24 psu) and very high salinities (>40), and osmoconform at moderate salinities (24-37 psu).

I agree with Artur, Salinity change happens in coastal water and it is very stable in offshore waters. Most marine invertebrates are isosmotic (same salt conc. in their body fluids as their outer environment). Osmotic equilibrium is attained by transfer of water, and swelling or shrinkage of the body ensues. Higher marine organisms (osmo-regulators) maintain their body fluid content through gills and kidney (example fish). If you wood like to understand more please read the book "The biology of Marine Animals by J.A.COLINNICOL @ https://archive.org/details/biologyofmarinea00nico

Organisms have two strategies to deal with salt:

1) Salt-in strategy, in which they accumulate KCl or other salts in the cytoplasm

2) Organic-osmolyte in strategy, in which they can accumulate an organic osmolyte like glycine betaine, ectoine or trehalose etc

@Jeffery:Sir thanks for the reply.I basically asked the question about the survival of osmotic conformers and stenohaline i.e they are not capable of mainataining the ion content .I wonder they survive.Although mostof the organisms in salty water are osmo regulator which are exactly opposite of osmotic conformers.