Sargassum is a well known macroscopic floating algae in the Sargasso Sea,
but they are relatively restricted in locality. This seaweed, forms an
extensive habitat for unique association, just like Art said, of more than
50 species of animals. The total wet weight of Sargassum has been estimated
at between 4 and 10 million tonnes (quit incredible !!). Some of the
endemic species, have developed special camouflage protections by coming to
resemble Sargassum weed both in shape and in colour.
         I too am curious in knowing what molluscs live on the these weeds ...
 
 
        Another interesting note on Janthina I just pick up:
 
        Janthina are strikingly coloured with violet and blue pigments, that have
been suggested that provide protection from the high levels of ultraviolet
light, but such coloration may also be effective as camouflage from
predators because it blends closely with in the blue colour of tropical
oceanic water. Janthina also exhibits counter-shading; that is, those body
surfaces that are directed downward in the water are lighter in colour then
areas close to the surface, and this too may be a predator defence. Marine
predators like fish, which approach them from below, would view the lighter
undersides of these molluscs against a background of lightly coloured sky
whereas, to aerial predators, the darker blue upper surfaces of prey would
blend with the dark blue colour of the water.
 
Adeus,
 
        Carlos