Nudi Notes

VEXATIOUS VARIABILITY – No. 2 Goniobranchus geometricus

Sep 7, 2021


Goniobranchus geometricus

Goniobranchus geometricus, to the uninitiated, can appear very much like a phyllidiid nudibranch if the gill is not displayed – all black and white and lumpy. However there are, from time to time, other variations in the mantle appearance that make you take a second look. All though have the characteristic green-tipped rhinophores and gills. Can any be called typical?


Goniobranchus geometricus: This composite image shows four minor variations.
Upper left: The tubercles are low and spreading with each surrounded by a significant amount of off-white pigment. There are no small tubercles located in the meandering black region. The tubercles are essentially in two locations – medially between the rhinophores and gills and laterally around the mantle edge.
Upper right: The tubercles are higher and better defined. The black region contains smaller individual tubercles with reduced surrounding off-white pigment. The tubercles in the medial region are all linked by the off-white pigment surrounding the tubercle bases.
Lower left: This example is not unlike the Upper right specimen except the tubercles in the black region are much reduced to just a few very small spots
Lower right: The tubercles are very well-defined, separated and accentuated by the significant reduction of the accompanying off-white pigment around their bases. This reduction causes the black region to be much larger and dominant thus giving both the medial and lateral mantle regions a much darker appearance.


Goniobranchus geometricus: This composite image shows four more variations that present with many smaller tubercles in different arrangements.
Upper left: Tubercles are smaller and much greater in number. The off-white pigment is a light brown instead. The medial region exhibits a double diamond appearance created by the meandering of the black region that itself carries many small tubercles.
Upper right: The double diamond is very well defined. The black region is large and spreads almost to the mantle edge with fewer large tubercles. An overall darker appearing specimen.
Lower left: A profusion of small white tubercles all linked by their surrounding off-white pigment – brown closer to the mantle margin. The black region is reduced to a quite narrow meandering line.
Lower right: This specimen exhibits dark brown in lieu of black with light brown surrounding the tubercle bases. The medial tubercles are large and crowded forming a ridge.  The submarginal tubercles are also large but mostly form a continuous line.


Goniobranchus geometricus: This final example exhibits black rays radiating from the black meandering line to the very edge of the mantle and even transverse lines across the top of the notum.


David A. Mullins – September, 2021

– Rudman, W. B. (1973). Chromodorid opisthohranch Mollusca from the Indo-West Pacific. Zoological Journal of the Linnean Society, 52:175-199.

– Willan, R. C. & Coleman, N. (1984). Nudibranchs of Australia, Neville Coleman, AMPI: 26-27

– Marshall, J. G., Willan, R. C. (1999). Nudibranchs of Heron Island, Great Barrier Reef: A survey of the Opisthobranchia (sea slugs) of Heron and Wistari Reefs; Backhuys: Leiden, The Netherlands, 1999.

– Rudman, W. B. (1999 January 15). Chromodoris geometrica Risbec, 1928. [In] Sea Slug Forum. Australian Museum, Sydney. Available from and associated messages.