<P> Organisms in aquatic and terrestrial environments must maintain the right concentration of solutes and amount of water in their body fluids; this involves excretion (getting rid of metabolic nitrogen wastes and other substances such as hormones that would be toxic if allowed to accumulate in the blood) through organs such as the skin and the kidneys . </P> <P> Two major types of osmoregulation are osmoconformers and osmoregulators . Osmoconformers match their body osmolarity to their environment actively or passively . Most marine invertebrates are osmoconformers, although their ionic composition may be different from that of seawater . </P> <P> Osmoregulators tightly regulate their body osmolarity, maintaining constant internal conditions . They are more common in the animal kingdom . Osmoregulators actively control salt concentrations despite the salt concentrations in the environment . An example is freshwater fish . The gills actively uptake salt from the environment by the use of mitochondria - rich cells . Water will diffuse into the fish, so it excretes a very hypotonic (dilute) urine to expel all the excess water . A marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt . It actively excretes salt out from the gills . Most fish are stenohaline, which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to . However, some fish show a tremendous ability to effectively osmoregulate across a broad range of salinities; fish with this ability are known as euryhaline species, e.g., Flounder . Flounder have been observed to inhabit two utterly disparate environments--marine and fresh water--and it is inherent to adapt to both by bringing in behavioral and physiological modifications . </P> <P> Some marine fish, like sharks, have adopted a different, efficient mechanism to conserve water, i.e., osmoregulation . They retain urea in their blood in relatively higher concentration . Urea damages living tissues so, to cope with this problem, some fish retain trimethylamine oxide . This provides a better solution to urea's toxicity . Sharks, having slightly higher solute concentration (i.e., above 1000 mOsm which is sea solute concentration), do not drink water like fresh water fish . </P>

What does the kidney do to help a marine fish control internal water levels
find me the text answering this question