VEXATIOUS VARIABILITY – No. 3

Goniobranchus splendidus

Goniobranchus splendidus is a common and conspicuous species of nudibranch, though endemic to the east coast of Australia, from southern New South Wales to central Queensland. It exhibits variation in presentation, some of those attributed and limited to specific regions within its range.

Composite image of four different presentations of Goniobranchus splendidus.
Upper left: A specimen with port-wine colouration to the dorsal blotches. The colour of the rhinophores and gills match the port-wine of the dorsum. The mantle margin is tending to orange. The tails carries an orange tip.
Upper right: This specimen has dorsal blotches of an orange hue as are the rhinophores and gills. The mantle margin is yellow. The tail carries a yellow tip.
Lower left: The dorsal markings of this specimen present as a broken medial line, port-wine in colour. The tail carries a translucent colourless tip.
Lower right: Just to confuse us further here is a specimen without any dorsal blotches at all. The rhinophores and gills are port-wine in colour and the mantle margin is bright yellow. The tail carries a translucent colourless tip.

Variation exists in the size and arrangement of colour on the white dorsum. That colour ranges from port-wine to light orange or to being non-existent. The colour may be arranged as regular spots, large blotches or a single patch of varying size and shape. There is no one arrangement that could be considered typical. In its southern range Goniobranchus splendidus displays a more spotted arrangement of mantle markings compared to the large single or several smaller blotches of those sighted further north. At the extreme northern extent of its range though, the blotches again revert to spots.The colour to the rhinophores and gills may or may not match that of the dorsal blotches/spots. The coloured marginal band to the mantle can also vary from orange to bright yellow and this band may or may not extend to the tail portion of the foot and in varying extent when it does.

Goniobranchus splendidus – This example has most of the dorsum covered in light blotchy orange. The rhinophores are port-wine in colour with the gills a dark orange. The mantle edge is bright yellow and the tail too carries an edge of bright yellow.

Goniobranchus splendidus – This specimen was photographed from Nelson Bay, in central New South Wales and shows the “regional pattern” of smaller, almost regular sized spots on the dorsum. (Image by kind courtesy of Terry Farr.)

Goniobranchus splendidus – Another specimen from Nelson Bay, in central New South Wales also exhibiting the “regional pattern” of smaller, almost regular sized spots on the dorsum. (Image by kind courtesy of Julie Schubert.)

A Splendid Trio – Three specimens of Goniobranchus splendidus feeding together. Top and bottom are port-wine coloured specimens separated by an orange blotched example.

Identification can sometimes be further confused by the presence of several other species in the range of distribution that also possess an essentially white dorsum with red/orange spots. Differentiation can normally be achieved by the combination of colour/s of the band/s on the mantle margin and the colour of gills and rhinophores. These similarities in appearance are attributed to a defensive measure called aposematic colouration whereby a group of animals display similar colour and pattern in order to spread the load of warning (“teaching”) potential predators, mainly fish, that they possess distasteful/poisonous mantle glands.

Note: All of the images shown here were taken in southern Queensland except for two from central New South Wales, as captioned.

David A. Mullins – October, 2021

References:
– Willan, R.C. & Coleman, N. (1984). Nudibranchs of Australia, Neville Coleman, AMPI: 28-29

– Rudman, W.B. (1983). The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris splendida, C. aspersa and Hypselodoris placida colour groups. Zoological Journal of the Linnean Society, 1983, 78, 105–173.

– Angas, G.F. (1864). Description d´éspèces nouvelles appartenant à plusieurs genres de mollusques nudibranches des environs de Port-Jackson (Nouvelle-Galles du Sud), accompagnée de dessins faits d´après nature. Journal de Conchyliologie. series 3(12): 43-70., page(s): 55-56, Pl. 5, Fig. 2

– Rudman, W.B. (1990). The Chrornodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: further species of Glossodoris, Thorunna and the Chromodoris aureomarginata colour group.  Zoological Journal of the Linnean Society, 1990, 100, 263–326.

– Rudman, W.B. (1991). Purpose in Pattern: the evolution of colour in chromodorid nudibranchs. Journal of Molluscan Studies, 57:  5-21.

– Rudman, W. B., (1998, April 22) Chromodoris splendida (Angas, 1864). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/chrosple and associated messages.

– Coleman, N. (2008) Nudibranchs Encyclopedia. Neville Coleman’s Underwater Geographic Pty Ltd, Springwood, Qld.

– Wilson, N.G., Winters, A.E., Cheney, K.L. (2016). Tropical Range Extension for the Temperate, Endemic South-Eastern Australian Nudibranch Goniobranchus splendidus (Angas, 1864). Diversity 2016, 8, 16.

– Nimbs, M.J., Smith, S.D.A. (2017). An illustrated inventory of the sea slugs of New South Wales, Australia (Gastropoda: Heterobranchia). Proceedings of the Royal Society of Victoria 128: 44-113.