Visual nudibranch signals

Naomi Green at Lizard Island

Nudibranchs are slug-like marine animals.  Many nudibranch species use extraordinary colours and patterns to warn predators they are toxic. Research by scientist Naomi Green at the Australian Museum’s Lizard Island Research Station is expanding our understanding of nudibranch warnings and how fish respond.

Naomi states, ‘Visual signals are used by animals to attract mates and prey, defend territories and deter predators. The sensory and psychological traits of signal receivers can drive the evolution and design of the signals.

Naomi and her team investigated whether particular components of conspicuous warning signals (colour, pattern, borders) improve the ability of a predator to learn to avoid toxic prey.

An adult PicassoTriggerfish – © Andy Lewis

Naomi conducted behavioural experiments with Picasso Triggerfish (Rhinecanthus aculeatus). The fish were trained to approach and tap at different coloured stimuli to receive a palatable or unpalatable food reward.  The stimuli designs were based on those displayed by nudibranch Chromodoris annae colour patterns, many of which have a yellow border, with red or blue highly contrasting internal patterns. A video of this can be seen below:

Triggerfish performing behavioural test by Naomi Green

The research Naomi conducted showed that ‘the external yellow border strengthened avoidance learning by predators. This was also an essential component of the signal for fish to make generalisations between learnt colour patterns and novel (but similar) colour patterns’.

Chromodoris annae – © Barbara Banks

These results help explain diversity within internal patterns of nudibranch signals both within and between species.  Avoidance learning seems to be achieved through the highly conserved yellow border. The study provides intriguing insights into the importance of pattern in animal visual signals to predators, the relative role of each signal component, and how variation in warning signals can be maintained in the marine environment.

“The complexity and psychology of fish, their ability to learn and their diversity is the most intriguing part of my research. After snorkeling in the Great Barrier Reef and seeing the amazing colours I was completely inspired”  – Naomi Green.