The Collaborative International Dictionary of English v.0.48:

Sea fan \Sea" fan"\ (s[=e]" f[a^]n`). (Zool.)
   Any gorgonian which branches in a fanlike form, especially
   Gorgonia flabellum of Florida and the West Indies.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sea fan
    n 1: corals having a treelike or fan-shaped horny skeleton