This is a species that has been looking for a genus. As we have noted in NSSP 2nd Edition on page 407, there have been a small group of sacoglossids previously assigned to Stiliger, that are genetically distinct from that group. S. smaragdinus was the only described species in that group of slugs associated with the toxic algae, Caulerpa. Because of the shape of the cerata, resembling branches of Caulerpa, in NSSI we even refer to them as the Caulerpa mimics.

Kikutaro Baba originally placed this species in the genus Stiliger based on its rhinophores but was confused about the shape of the cerata.

Krug et al (2018), upon a careful genetic analysis resolved the issue by resurrecting the genus Sacoproteus, placing all the Caulerpa mimics there.

You may have swum by this critter a dozen times and missed it due to its cryptic nature. NSSI describes S. smaragdinus as being uniform pale green. Bill Rudman's Sea Slug Forum shows numerous examples of variation. So, if Gordon's critter is really S. smaragdinus, it is certainly a wild variation.

My diving career started in the cold lakes of northern Wisconsin during summer camp in 1964. I have now logged over 3000 dives. My passion for this hobby/sport kicked into high gear around 2006 when I got my first underwater camera and went to Taveuni, Fiji. It was a Canon SD 550 point and shoot. Certainly limited for wide angle, but it took amazing pictures of Nudibranchs! It ignited an obsession for underwater photography and nudi hunting that has taken me to all the corners of the Coral Triangle.

I retired and moved to the Philippines in 2008 to avoid those long international flights and have been blessed to be here for the last 14 years. I chose the island of Negros Oriental for many reasons, but mostly for the easy access to great macro/critter photography. Also being the frogfish capital was the bonus round. And the 600 plus species of nudibranchs that call it home. Being in the center of the best diving on planet earth is a dream come true.

So, as they say " I'm living the life".

Webmaster's Notes:

Gordon certainly has a knack for locating and photographing one of the most difficult sea slug species (Stiliger) to find.
A prevous Stilger submission by Gordon which morphed into a BOW was posted as Stiliger sp.5. Interesting enough, Stiliger sp. 5 was also photographed at Romblon, Philippines!
Let's see, what is my travel agent's number?

Send Gordon email at gtillen@mac.com