A project undertaken at the School of Biological Sciences, The University of Queensland, and supervised by Karen Cheney
Animals frequently use dazzling colour patterns to find food, attract mates and avoid predation. Aposematic signals are used to warn predators that a species contains chemical or other defense mechanisms. Nudibranchs (marine shell-less molluscs) store secondary metabolites from dietary sources for their own chemical defenses, and also exhibit a variety of colour patterns, ranging from those that are highly camouflaged against their background to those that display highly conspicuous colour patterns. Using a multidisciplinary approach, we investigated the evolution of visual signals in this intriguing model system by evaluating the conspicuousness of colour patterns. We also identified and determined the relative strength of chemical compounds used to deter predators in a range of nudibranch species.
Theoretical and experimental models predict that warning signals in animals evolve from defended inconspicuous (cryptic) species that mutate to produce conspicuous morphs (Leimar et al., 1986). Secondary defenses deter predators during an attack, and once a predator has associated bright coloration with unprofitability of prey, they avoid similarly-coloured prey in future encounters. However, in evolutionary terms, it is unclear whether visual signals in aposematic species are ‘quantitatively honest’, in the sense that conspicuousness correlates positively with levels of toxicity and thus conveys reliable information. Or whether signals reach a threshold over which predators avoid all signals, irrespective of how conspicuous a signal is (Speed et al., 2010).
One intriguing, understudied model system to study how aposematic signals have evolved are nudibranchs, which appear strikingly exposed to predation, yet they are avoided by most predators. The majority of nudibranchs have evolved to sequester secondary metabolites from dietary sources, or produce chemicals de novo, which they then use as chemical deterrents. Many nudibranchs also use cryptic color patterns or camouflage to blend into their surroundings, opting to avoid detection altogether rather than promote predator recognition (Marin et al., 1997; Cheney et al., 2014). This variation in predator avoidance strategies makes nudibranchs an ideal model system to test theories about the evolution of visual signaling and chemical defenses.
However, we know little about the identity and relative strength of chemical defenses in most nudibranch species. Indeed, the identity and level of chemical defense is known for less than <1% of all mollusc species even though marine molluscs, are an important source of diverse natural products, and investigations into their biological and chemical properties have lead to the discovery of many biologically potent chemicals with FDA approval due to their analgesic, anti-inflammatory, antiviral and anticancer activity (Gerwick and Moore, 2012). Indeed, the antitumour depsipeptide Kahalalide F was also isolated from the opisthobranch mollusc Elysia rufescens, which is used by both the mollusc and its dietary alga Bryopsis spp. (the true source of Kahalalide F) as a chemical defense from predation. Kahalalide F is currently in clinical trials for the treatment of advanced cancerous tumors, including those related to melanoma (Salazar et al 2013).
Using our combined expertise in marine and visual ecology (Karen Cheney), natural product chemistry (Prof. Mary Garson), and animal vision and signaling (Prof. Justin Marshall). Over the course of this grant, we have:
- Collected over 80 species of nudibranch, and have identified the chemistry in approximately 50 species. We have conducted experimental assays on fish and shrimp to examine the strength of these chemical defenses for over 35 nudibranch species;
- Identified 45 new compounds that have not previously been reported;
- Tested key evolutionary hypotheses on the relationship between the conspicuousness of visual signals and the strength of their chemical defenses. We found that those which have the strongest visual signal are generally the most toxic and camouflaged nudibranchs have little chemical defense;
- Developed new ways of analyzing colour patterns using calibrated cameras and a new analytical framework so we can estimate how visual signals are seen through the eyes of potential predators;
- Highlighted the significance of this intriguing, emerging model system to the fields of evolution, ecology and sensory neurobiology.
- Supported research support for 4 PhD students and 5 honours students
- Appeared on TV shows Scope (Channel 10) and Totally Wild (Channel 11) to explain our work on nudibranchs.