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Pretty similar is Ercolania sp. 4 which differs by small solid rhinophores, and is as 'Unknown Family' "Caulerpa mimic" in Gosliner et al. (2015) page 74 top right.
The original description of Sacoproteus smaragdinus is from Japan, now there are records from many locations,
from West to East: from Bali Island, Indonesia, Singapore, Australia, and from the Marshall Islands , to Hawaii. Apte et al. (2016) extend the range
of S. smaragdinus to India far westwards from its present distribution.
This species was posted on 22 October 2017 at these websites as Stiliger smaragdinus Baba, 1949 with the comment:
"The correct generic placement of this species is not clear, Kathe Jensen is "preparing a description of a new genus for it" that was 20 years ago (1997)". Krug et al. (2018) erected the genus Sacoproteus for the sea slugs that masquerade as toxic algae Caulerpaspp. with Sacoproteus smaragdinus (Baba, 1949) as type species.
I changed it on 2 December 2018 into Sacoproteus smaragdinus (Baba, 1949) [Stiliger].
Gosliner, T.M, D.W. Behrens & Á.Valdés. 2015. Nudibranch & Sea Slug Identification Indo-Pacific.
New World Publications Inc. Jacksonville, FL. 408 pp.
Apte D.A, Kamboj R.D & Bhave V. (2016). First record of Stiliger smaragdinus Baba, 1949 (Mollusca: Sacoglossa: Limapontiidae) from India.
Journal of Bombay Natural History Society 112 (2): 114-117.
Krug, P. J.; Wong, N. L. W. S.; Medina, M. R.; Gosliner, T. M.; Valdés, Á. A. (2018). Cryptic speciation yields remarkable mimics: A new genus of sea slugs that masquerade as toxic algae (Caulerpa spp.).
Zoologica Scripta. , available online at https://doi.org/10.1111/zsc.12310
More information on Sacoproteus smaragdinus as Stiliger smaragdinus are on Bill Rudman's Sea Slug Forum!
How to cite:
Köhler, E. (2018), published 2 December 2018, Sacoproteus smaragdinus (Baba, 1949)
available from https://www.Philippine-Sea-Slugs.com/Sacoglossa/Sacoproteus_smaragdinus.htm