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Pleurobranchus varians  Pease, 1860

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this species was described by William Harper Pease in 1860. Pease' original text (the type description) reads as follow:
"Oval, rather rugose, convex above. Mantle rounded behind, deeply sinuose in front, and margins slightly undulated. Tentacles arising from the head, curving laterally, deeply grooved below, truncated, cylindrically tapering, transversely lamellated. Eyes at their posterior bases. Oral veil large, convex in front, and much dilating laterally, where it is deeply grooved. Mouth proboscidiform. Branchial plume simple, pinnate on the middle of the right side. Foot large, reaching the edge of the mantle laterally and behind. Colour varying ; some bright red, others lemon-yellow, or purplish brown, others again variegated with whitish ; beneath paler than above. Shell on the anterior half of the body, concealed, small, fragile, pellucid, oblong-ovate, convex, and ornamented with wrinkles of growth. Nucleus posterior, more or less brownish."
let me quote Goodheart, et. al. (2015) on it:
"External morphology and coloration: Body size up to 70 mm long. Rhinophores with multiple horizontal striations along the length. Foot does not project from the mantle. Gill rachis tuberculate at the base of the pinnae. Tubercles polygonal in shape and larger, more flattened than those in P. albiguttatus. Tubercles similar in size in some specimens, whereas in others tubercles become smaller towards the edge of the mantle. Background colour orange to dark red. Opaque white pigment on some of the tubercles in some specimens, similar to P. albiguttatus. Oral veil and rhinophores same colour as the mantle." and:
"Our phylogenetic analyses show that a subset of animals from the Central Pacific, but mainly from the Hawaiian Islands, are genetically distinct from the other species of Pleurobranchus included in the analysis. These specimens externally match the original description of P. varians; therefore, we reinstate the species name P. varians for this subset of specimens as they are genetically distinct from other species of Pleurobranchus."
The idenfication of the above specimen was suggested by the photographer Martin Burman and confirmed by the corresponding author on the Goodheart, et al. (2015) paper on Pleurobranchus Ángel A. Valdés. He writes pers. comment:"I cannot be completely sure without a specimen but this animal looks like P. varians to me. We found this species in the Philippines before and other locations across the India-Pacific."
Very similar are two variable species of Pleurobranchus: P. peronii  Cuvier, 1804 and P. albiguttatus (Bergh, 1905), I do not know how to distiguish them, they occur together, so there is not a geographical separation.
References:
Pease WH. 1860. Descriptions of new species of Mollusca from the Sandwich Islands.
Proceedings of the Zoological Society of London 28: 18-36.
Goodheart J., Camacho-García Y., Padula V., Schrödl M., Cervera J.L., Gosliner T.M. & Valdés Á. (2015). Systematics and biogeography of Pleurobranchus Cuvier, 1804, sea slugs (Heterobranchia: Nudipleura: Pleurobranchidae).
Zoological Journal of the Linnean Society. 174: 322-362., available online at https://doi.org/10.1111/zoj.12237
More information on Pleurobranchus varians are on the website of Cory Pittman & Pauline Fiene Sea Slus of Hawai'i!
How to cite:
Köhler, E. (2017), published 3 July 2017, Pleurobranchus varians  Pease, 1860
available from https://www.Philippine-Sea-Slugs.com/Pleurobranchomorpha/Pleurobranchus_varians.htm