Understanding colour and chemical diversity in marine molluscs

A project undertaken at the School of Biological Sciences, The University of Queensland, and supervised by Karen Cheney

Summary
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.
Background

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).

Outcomes

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:

  1. 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;
  2. Identified 45 new compounds that have not previously been reported;
  3. 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; 
  4. 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;
  5. Highlighted the significance of this intriguing, emerging model system to the fields of evolution, ecology and sensory neurobiology.
  6. Supported research support for 4 PhD students and 5 honours students
  7. Appeared on TV shows Scope (Channel 10) and Totally Wild (Channel 11) to explain our work on nudibranchs.
Publications from this grant

Below is a list of publications that have been published as a result of this grant. We antipicitate that we have at least another 8 publications that will be submitted from this project (as per July 2017):
 
Winters, A. E., Green, N. F., Wilson, N. G., How, M.J., Garson, M. J., N. Justin Marshall, N. J., & Cheney, K. L. Relaxed selection on individual pattern elements may allow phenotypic divergence of aposematic signals (accepted, Proceedings of the Royal Society, Lond B)
 
Forster, L. C., Pierens, G. K., White, A. M., Cheney, K. L., Dewapriya, P., Capon, R. J., and Garson, M. J. (2017), Cytotoxic spiroepoxide lactone and its putative biosynthetic precursor from Goniobranchus splendidus ACS Omega 2 (6): 2672-2677. doi 10.1021/acsomega.7b00641.

Giordano, G., Carbone, M., Ciavatta, M. L., Silvano, E., Gavagnin, M., Garson, M. J., Cheney, K. L., Mudianta, I. W., Giovanni Fulvio Russo 2, Guido Villani, G., Zidorn, C., Cutignano , A., Fontana, A., Ghiselin, M. T., Mollo, E. (2017) Volatile secondary metabolites as aposematic olfactory signals and defensive weapons in aquatic environment Proceedings of the National Academy of Sciencesdoi: 10.1073/pnas.1614655114

Forster, L., Winters, A., Cheney, K. L., Dewapriya, P., Capon, R., Garson, M. (2016). Spongian-16-one Diterpenes and their anatomical distribution in the Australian nudibranch Goniobranchus Collingwoodi Journal of Natural Products 10.1021/acs.jnatprod.6b00936

Dewi, A.S., Cheney, K.L., Urquhart, H.H., Blanchfield, J.T. and Garson, M.J. (2016) The sequestration of oxy-polybrominated diphenyl ethers in the nudibranchs Miamira magnifica and Miamira miamirana. Marine Drugs, 14 11: doi:10.3390/md14110198
 
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, 8 3: 16.1-16.8. doi:10.3390/d8030016
 
White, A. M., Dewi, A. S., Cheney, K. L., Winters, A. E., Garson, M. J. (2016) Oxygenated diterpenes from the Indo-Pacific nudibranchs Goniobranchus splendidus and Ardeadoris egretta. Natural Product Communications, 11 7: 921-924.
 
Cheney, K. L., White, W., Mudianta, I. W., Winters, A. E., Quezada, M., Mollo, E. Garson, M. J. (2016) Choose your weaponry: selective storage of a single highly toxic compound, latrunculin-A, by closely related nudibranch molluscs PLoS ONE, 11 1: 1-16. (doi:10.1371/journal.pone.0145134).
 
White, A. M., Pierens, G. K., Forster, L. C., Winters, A. E., Cheney, K. L., Garson, M. J. (2015). Rearranged diterpenes and norditerpenes from three Australian Goniobranchus mollusks. Journal of Natural Products. 79 3: 477-483, DOI: 10.1021/acs.jnatprod.5b00866
 
Yong, K.W. L., Mudianta, I.W., Cheney, K. L., Mollo, E., Blanchfield, J.T. and Garson, M. J. (2015) Isolation of norsesterterpenes and spongian diterpenes from Dorisprismatica (= Glossodoris) atromarginata. Journal of Natural Products, 78 3: 421-430. doi:10.1021/np500797b
 
Mudianta, W. I, White, A. M., Suciati, Katavic, P. L., Krishnaraj, R. R., Winters, A. E., Mollo, E., Cheney, K. L. and Garson, M. J. (2014) Chemoecological Studies on Marine Natural Products: Terpene Chemistry from Marine Mollusks Pure and Applied Chemistry 86 (6), 995-1002

 

Figure 1. Nudibranchs, Phyllidia ocellata, in the laboratory to be used for colour analysis

 

Figure 2. Secondary metabolites from nudibranchs that provide chemical defense from predators (Mary Garson, unpublished data).

 

Figure 3. Fieldwork in Nelson Bay, NSW collecting and photographing nudibranchs

 

Figure 4. A calibrated digital camera used to collect images for colour pattern analysis

 

Figure 5. Goniobranchus splendidus in the field