Matt_Wandell said:
I sorta remember that S. mertensii had the same longitudinal rows. Or maybe not! How do you differentiate?
Damn you Wandell, this is about gigantea and haddoni
I don't believe S. mertensii has the defined longitudinal rows on the tentacle side - the spots on the bottom yes (the verrucae, orange and pink, which are sticky in this species). They also don't seem to come in blue or red, just green and brown, sometimes orange, with pretty verrucae. The oral disk on S. mertensii is generally not seen and its tentacles are longer close to the mouth, and its not nearly as sticky as the other two. It also gets way way bigger. So, boiled down, I would look at color, rows and stickyness below and lack of stickyness above. However, from what I understand, S. mertensii is rarely collected for the trade, so we aren't all that likely to see it.
Where they are found, what they adhere to, they are blown around in flow, and who hosts in them can also help ID them.
This is a pretty good reference:
http://www.nhm.ku.edu/inverts/ebooks/ch1.html