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Very similar is Elysia tomentosa Jensen, 1997. Let my quote Bill Rudman:"Jensen compares E. tomentosa with E. expansa (O'Donoghue, 1924), which was also described from the Abrolhos Islands, but in that species there are fewer papillae, they are never branched, and there is a black line at the parapodial edge [see Jensen 1993]."
And pers. comment of K. R. Jensen: "I think this is more likely Elysia expansa, which also occurs on Caulerpa, sometimes together with E. tomentosa. E. expansa has that dark brown, almost black stripe along parapodial margins and some glandular patches at the tips of the parapodial lobes. In E. tomentosa parapodial margins are pale red and usually more papillose than E. expansa."
The above specimen was misidentified and posted on 14 August 2013 at these websites as Elysia tomentosa Jensen, 1997. I realized my error
on 12 September 2017 and changed it into Elysia expansa (O'Donoghue, 1924).
More pictures of Elysia expansa from several locations, some of them with identification confirmed, are at NUDIPIXEL, let me quote: "Most importantly, there is a pale pink band at the parapodial margin edged with a narrow black line above and below."
O'Donoghue, C. H. (1924) Report on Opisthobranchiata from the Abrolhos Islands, Western Australia, with description of a new parasitic copepod.
Journal of the Linnean Society, London 35: 521-579, Pls.27-30.
Jensen, K.R. (1993) Sacoglossa (Mollusca, Opisthobranchia) from Rottnest Island and central Western Australia. [In]:
Proceedings of the 5th International marine Biological Workshop: The marine Flora and Fauna of Rottnest Island, Western Australia.
(Eds: Wells, F.E.; Walker, D.I.; Kirkman, H., Lethbridge, R.) Western Australian Museum, Perth, 207-253.
How to cite:
Köhler, E. (2017), published 12 September 2017, Elysia expansa (O'Donoghue, 1924)
available from https://www.Philippine-Sea-Slugs.com/Sacoglossa/Elysia_expansa.htm"