It is truly amazing how organisms have evolved to fill every tiny niche that you can imagine. Sargassum is a member of phaeophyceae (phaeophyta). UC Berkeley has an excellent introduction to the phaeophytes at this link: UC Berkeley
Explorers during the Age of Exploration dreaded getting becalmed in the subtropical Atlantic Ocean and surrounded by vast areas of Sargassum.
Here's a great little video from National Geographic on the Sargassum fish.
