Re: What role in nature do Cnidarians have?

Cnidarians are an ecologically diverse group that includes predators such 
as many anemones, many jellyfish, northern corals, colonial hydroids and 
hydra.  They capture their prey with tentacles loaded with tiny poison 
darts called nematocysts.  Other cnidarians, mostly corals and some 
anemones and jellyfish in shallow, warm waters may in addition, or 
exclusively, rely on microscopic algae embedded in their outer tissues to 
provide them with nutrients generated by algal photosynthesis.  An 
interesting question is why this difference (predominantly predatory 
versus photosymbiotic, respectively) between cold and warm seas?  The 
answer in part concerns the relative need to recycle nutrients in these 
different environments.  The waters around coral reefs are clear because 
they are very poor in nutrients.  Thus to derive sufficient nutrients, 
many cnidarians (and some clams and sea slugs) harbour algae in their skin 
to provide them with carbohydrates.  The algae benefit too by receiving 
nitrogenous wastes from the host animal useful in building proteins.  
Conversely, in cold regions the waters are cloudy with suspended matter 
and dissolved nutrients and many more organisms there, including 
cnidarians, can make a living filtering the water with outstretched 
tentacles.