VEXATIOUS VARIABILITY – No. 1
Hypselodoris jacksoni
There are a number of common species of nudibranch that can exhibit a variability of presentation. These differences can sometimes be put down to geographical location but more than often, not necessarily so. At first it can be confusing but once it is realised that the variations belong to the one species the initial disappointment at not having recorded a different species is replaced with the satisfaction of knowing that the range of variability is being cataloged.
The images below could speak for themselves in displaying the different colour patterns that Hypselodoris jacksoni exhibits upon its mantle even within the one small locality, in this instance, the offshore reefs of the Sunshine Coast of southern Queensland, Australia.
In this composite image we have four variations in the mantle presentation of Hypselodoris jacksoni. The basic theme is the same but the variations are worthy of remark.
– The top left image is what I would call the “standard” presentation where the background colour is white overlaid with a dark pigment infilled with large yellow patches that create the effect of a mantle reticulation.
– The top right image loses the reticulum effect due to the yellow patches being broken up into spots that are significantly separated.
– The bottom left image goes even further in that regard.
– In the bottom right image the yellow spots are joined up to form longitudinal lines creating a longitudinal striped effect of alternating dark and yellow stripes.
The specimens of Hypselodoris jacksoni together in this image exhibit significant variability. The specimen on the right is close to what I have referred to as the “standard” presentation, but with the yellow patches being surrounded by a great deal of additional grey pigment such that a more narrow and therefore distinctive reticulation is created. The specimen to the left could be simply described as having numerous small yellow spots on a dark background (i.e. no reticulation) and with the lateral white scallops being almost eliminated.
A quite pale specimen of Hypselodoris jacksoni displaying yet another variation where the pale yellow spots are closely sited together and shaped such as to create the effect of a tight reticulation.
In the species description of Hypselodoris jacksoni by Wilson & Willan, 2007 the variability in colouration and pattern was discussed and a number of images included to support the comments. The variation in gill rachis markings was also covered in that original description.
David A. Mullins – August 2021
References:
– Wilson, N. G. & Willan, R. C. (2007). Hypselodoris jacksoni, a new species from the south-western Pacific Ocean (Nudibranchia: Chromodorididae), with a discussion on intraspecific variation in mantle glands in Chromodoris willani Rudman, 1982. Zootaxa, 1549: 29-42