Author: Gosliner & R. Johnson in Epstein et al, 2018

Order: Nudibranchia  Family: Chromodorididae

Maximum Size: 25 mm

Sightings: Sunshine Coast

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Hypselodoris melanesica Gosliner & Johnson in Epstein et al, 2018

Hypselodoris melanesica grows to 30 mm in length and has a body colour of translucent purple with a thin white mantle margin that is repeated upon the foot. Well defined narrow bands of deep violet pigment (more magenta to my eye) encircle the base of each rhinophore and gill pocket. The clavus of the rhinophores is orange with redder tips, as are the gill branches that are usually five in number arising from a common base. There are no mantle glands. It is true that some specimens of Hypselodoris bullockii can present as light purple in colour and have darker bands around the base of rhinophore and gill pockets however those coloured bands are wider and much more diffuse rather than well defined. One characteristic feature of Hypselodoris melanesica is the presence of a distinct break in that deep violet encircling band located on the posterior face of the rhinophore pockets. All of these external features are exhibited by the specimen in the main image above.

Although the molecular evidence (DNA) suggests that H. melanesica and H. bullockii are on the boundary of being conspecific the authors argue that there are differences in the radula and reproductive system as well as the consistent external differences noted above.

This species was given the name melanesica because the authors believed its distribution was limited to Papua New Guinea and the Solomon Islands – i.e. Melanesia. Based on that belief the authors stated that the two species are geographically isolated. The specimen above was photographed on the Sunshine Coast, southern Queensland, Australia as part of the Sea Slug Survey of the Sunshine Coast, indicating both a larger range extension than previously believed and, that these two species are not geographically isolated, as Hypselodoris bullockii is commonly sighted on the Sunshine Coast.

Perhaps the finding of H. melanesica in an overlapping distribution with H. bullockii adds further support to the theory of Mullerian mimicry by contributing additional evidence of convergent colour patterns among sympatric species.

References:
– Epstein,H.E., Hallas,J.M., Johnson, R.F., Lopez, A., Gosliner, T.M. (2018) Reading between the lines: revealing cryptic species diversity and colour patterns in Hypselodoris nudibranchs (Mollusca: Heterobranchia: Chromodorididae). Zoological Journal of the Linnean Society, 2018, XX, 1–74.

– This Species Profile has been modified from a previously published article in Dive Log Magazine’s – Critter ID with NudiNotes Column, Issue: #369 (April 2019): 12 by David A. Mullins

David A. Mullins, July 2019