The love child of a Zoanthid and a Hammer Coral [updated]
All the tentacles are arranged in two clearly demarcated circular rows, and only the inner row develops the hammer-like/split-ends appearance. All polyps in this colony exhibit this mutation, but colonies of this exact specimen in other aquariums do not. The owner does not know what caused this unique gene expression. As such, the owner is uncomfortable marketing this zoanthid as the next pricey "designer coral" (good for him!) This is a clear case of ecomorphology.
Absolutely beautiful specimen and photography!
Read the original thread at Nano-Reef.com
Thanks to the Red Robot for the link!
UPDATE: The owner has also observed this phenomenon in another zoanthid colony in the same tank. Something in this specific aquarium is causing the polyps to develop these unique split tentacles.