The Sargasso Sea isn't actually a "source" of Janthina as such, at least not
in the sense that they are produced there.  Janthina are true pelagic species,
living on the surface of the open ocean, even hundreds of miles from the
nearest land.  They float along, at the mercy of currents and wind (they can't
swim), buoyed up by the bubble rafts they produce, and they can be found in
the tropical and sub-tropical zones of all the world's oceans.  There are only
a few species, and the same species are found worldwide, wherever Janthina
wash ashore, whether in South Africa, New Zealand, Japan, Polynesia, Hawaii,
Oregon, Florida, England, or wherever.  The Sargasso Sea is often a "source"
of shells that wash ashore in the northern Atlantic, simply because:
1.  The Caribbean is one of the many places Janthina are found
2.  Sargassum is also found there in unusual abundance
3.  Sargassum often acts as a "net", entrapping floating Janthina
4.  Sargassum, with our without entrapped Janthina, often breaks away and
gradually floats ashore.
 
Paul M.