The Sargasso Sea starts about 100 miles southeast of Bermuda, and occupies an
area roughly 2/3 the size of the United States.  It is made up of floating
Sargassum seaweed.  This stuff is designed to float - it has little hollow
spherical floats attached, which is how you can recognize it if you find it
washed up on the beach.  It is yellow-brown in color, and the "leaves" are
rather stiff, feeling almost like plastic.  There are many kinds of animals
specifically designed to live there, including the sargassum fish Sue
mentioned (a type of small angler fish, scientific name Histrio histrio), as
well as seahorses, pipefish, and specialized crabs, shrimp, anemones, and
others.  Many of them are the color of Sargassum, and some have structures
that make them look even more like Sargassum. As far as mollusks go, the
purple sea snails (Janthina) can be found there, but Sargassum isn't the
habitat of Janthina as such.  Janthina lives floating on the surface of the
open ocean, and so does Sargassum, so inevitably they sometimes get tangled
together, and the snails may be carried ashore when portions of the weed break
off and are driven ashore by currents and winds.  There is however a snail
which actually lives on the Sargassum, and nowhere else, namely Litiopa
melanostoma (common name - Sargassum snail).  It is a small (under a half
inch), thin-shelled, brown snail.  It somewhat resembles a fresh water Lymnaea
shell.  I have found them on Sargassum weed beached on the south shore of Cape
Cod (Massachusetts).  That's a long way from the Sargasso Sea, but the Gulf
Stream comes right up the Atlantic coast, and often carries Sargassum with it.
One other interesting fact about the Sargasso Sea - it is the breeding ground
for the common eels of the north Atlantic, both European and American.  They
migrate down the Gulf Stream to the Sargasso Sea to breed; then the Gulf
Stream carries the planktonic larvae back north.
 
Paul M.