Re: how do marine plants and animals survive in sea water

Dear salimander pelican,

How do plants and animals survive in the ocean?  This is a pretty huge 
question so I am going to have to guess at what you specifically mean. I am 
going to assume you are interested in survival mechanisms particular to 
life in a salty aqueous medium.

Of course the plants and animals in the sea are subject to stresses similar 
to land-dwelling organisms. For example, they must find mates, obtain food, 
avoid being food. and adapt to temperature and light conditions (seasons, 
etc.). They deal with these in many of the same ways land plants and 
animals do, i.e. avoidance, camouflage, chemical signals, etc., etc., etc.. 

There are a few environments that are unique to the sea such as the high 
pressure environments at extreme depths. Organisms there have adapted to 
life at depths that would crush us by adjustments in structure and 
biochemistry. Also there are deep-sea vent communities that live in deep 
marine 'hot springs' with no light for photosynthesis to base a food chain 
on. These communities are based on chemosynthesis which makes organic 
matter from inorganic matter using chemical energy rather than light 
energy.

As far as the aqueous salty medium is concerned the concern there is 
regulation of osmotic pressure in the body. This is a concern for creatures 
in freshwater and on land as well, which have things in common, 
integumenatry systems for example.  There are some differences though, 
because of the direction of osmotic pressure. In freshwater for example, 
the problem is keeping too much water from entering body tissues while the 
fresher water outside 'pushes' in. In freshwater fish, this means that they 
don't drink water (that would make the problem worse) and they excrete lots 
of dilute urine that dumps water out of the body that is being forced in. 
In salt water, the osmotic pressure is in the opposite direction. The high 
concentration of salts in seawater means that ,arine creatures need to kepp 
water from leaving tissues (or continulally replace it). So marine fish do 
drink water and excrete small volumes of concentrated urine. 

That is the basic stiory for fish. And you can see that the challenge for 
marine animals is more like the situation in land animals where the water 
balance tips in favor of losing too much water to the environment. So you 
can understand that other marine animals such as marine mammals have many 
of the same adaptations as land animals, and whales also drink water, for 
example.)

As far as this challenge for most simpler animals and marine plants, that 
is pretty simple. Most of the invertebrates like copepods, squid, 
and jellyfish in the sea are essentially isotonic. This is alos true for 
most phytoplankton (algae) and seaweeds. That means that the water content 
of their tissues is essentially the same as the surrounding sea water. They 
do not expend energy to control their water balance. Incidentally, that is 
why there are so many more types of invertebrates in the sea than on land.

I hope this is what you were looking for.