Ok, I’ll be honest.  As plant biochemists we usually cheer for the guys in green (in this case, the seaweeds).  But even we can make an exception when fleshy seaweeds attack corals, which are already in serious decline from coral bleaching, warming sea temperatures, and other aspects of climate change.  In a recent article in Science magazine, Dixson and Hay describe one way corals fight back against seaweeds that threaten to overgrow them.  In short, some corals can call in grazing fish – in this case gobbies – to remove the invaders.  The communication is chemical, involving waterborne signals.  Here’s the reference:

D.L. Dixson and M.E. Hay. Corals chemically cue mutualistic fishes to remove competing seaweeds. Science, Vol. 338, November 9, 2012, p. 804. doi:10.1126/science.1225748.

and a summary from Scientific American:

http://blogs.scientificamerican.com/science-sushi/2012/11/08/coral-recruits-goby-bodyguards-against-seaweed-assasins/