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This picture was posted on 3 September 2010 on these websites as Dendrodoris nigra, I changed it on 21 March 2011 into Dendrodoris arborescens (Collingwood, 1881) [Doridopsis].
I don't know if there is some sand on this specimen or if there are white spots on the mantle. The red marginal band looks like in D.arborescens which has no white spots, or maybe it has a few ...
Very similar are:
black variants of Dendrodoris nigra which can be distiguished a diffuse submarginal orange band and have often white specks
black variants of Dendrodoris fumata which has 5 large gill branches, which reach and extend to the edge of the mantle
Dendrodoris arborescens is at the Sea-Slug Forum as a colour variant of Dendrodoris fumata with a comment that "Brodie & Calado, (2006) propose this black form of Dendrodoris should be considered a separate species Dendrodoris arborescens (Collingwood, 1881)."
In IPN is at page 282 a picture of a specimen from Luzon, Philippines with a circle of short gills like those of Dendrodoris nigra:
whilst the black specimens with a red marginal band at the Sea-Slug Forum have a few large gills, like those of Dendrodoris fumata,
NUDIPIXEL has both variants from different locations as Dendrodoris arborescens...
Brodie, G.D. & Calado, G. 2006. Dendrodoris arborescens (Collingwood, 1881) (Mollusca: Nudirbanchia): larval charcterstics reveal a masked porostome species. Records of the Western Australian Museum Suppl. 69: 119-126.
Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books.
Gig Harbor, Washington. 426pp
How to cite:
Köhler, E. (2019), published 30 June 2019, Dendrodoris arborescens (Collingwood, 1881)
available from https://www.Philippine-Sea-Slugs.com/Nudibranchia/Doridina/Dendrodoris_arborescens.htm