There is egg on my face.
I was looking at your anemone again ... and .... I don't think you have
S. gigantea. Looking at the pictures here
http://www.geocities.com/fish_boy1986/anemone.html it's quite apparent the carpet has tentacles much, much longer towards the centre of the oral disk versus the periphery of the oral disk, and this is a classic ("textbook") trait of
S. mertensii.
Not to alarm you, but
S. mertensii is perhaps the largest of all the hosting anemones. They can easily get to a meter in diameter in the wild
But don't get me wrong, it's a VERY cool anemone still, whatever species it is.
