Project Results

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What is the context of this research?

Project Background
Aposematic animals use conspicuous color patterns to promote avoidance learning in predators. These eye-catching color displays are regularly associated with defended prey because the cost of conspicuousness is thought to be too great for undefended individuals (Ruxton, Sherratt et al. 2004). Nudibranchs often have bright body patterns that appear strikingly exposed to predation, yet they are generally avoided by most predators (Edmunds 1987; Rudman 1991). Many of these soft-bodied sea slugs have evolved to sequester secondary metabolites from dietary sources, which they then use as chemical deterrents (Thompson 1960; Karuso 1987; Avila 1995). Other nudibranchs have cryptic color patterns or camouflage to blend into their surroundings, opting to avoid detection altogether rather than promote predator recognition (Rudman 1991; Cortesi and Cheney 2010). This variation in predator avoidance strategies makes nudibranchs an ideal model system to test theories about the evolution of aposematism. However, poor resolution in nudibranch phylogenies currently hinders our ability to test such theories.

Project Methods
I will photograph and collect nudibranchs from the reef and transport them to the lab for processing. Spectral reflectance of colour patches will be measured using a spectrophotometer. These measurements will then be analyzed using the Vorobyev Osorio predator vision model to estimate conspicuousness from the perspective of a reef fish predator. Photographs will be analyzed using 1st and 2nd order image statistics to determine how the brightness contrast and spatial frequency of the pattern compares to the nudibranch’s natural background. Tissue samples will be collected for DNA sequencing to be used in population genetic and phylogenetic analyses. Next, I will extract secondary metabolites from nudibranch tissue. The chemical content and molecular structure of each extract will be determined using analytical tools such as H NMR, HPLC, and chemical databases. I will then prepare artificial pellets containing nudibranch extracts at varying concentrations and feed these picaso triggerfish and pufferfish in order to assess the relative deterrence of nudibranch chemicals to these generalist reef predators.

What is the significance of this project?

Marine organisms, in particular 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). For example, investigations into the conotoxin venom from fish-hunting cone shells (genus Conus) resulted in drugs being developed to manage severe and chronic pain (Molinski et al 2009). 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). Whilst many natural products are too rare to be collected in large quantities for the pharmaceutical industry, they may act as lead compounds that can be precursors for synthetic compounds. However, the identity and level of chemical defense is known for less than <1% of all mollusc species. This research will aid in the discovery of novel compounds, while also helping put such chemicals into a natural context so that we can understand their ecological function and evolution.

Also, While nudibranchs are widely theorized to be aposematic, little evidence is available to link unpalatability to conspicuousness, mostly for lack of quantitative pattern measurements. To my knowledge, the combination of unpalatability assays and predator vision models have not yet been used to examine aposematic traits in the marine environment. With this new level of understanding, I will be able to inform and shape future theoretical models, which will be applicable to a range of ecological and evolutionary processes. These findings will be pertinent to variety of organisms ...including us!

What are the goals of the project?

1. Is conspicuousness correlated with defense? Aposematic prey are most likely to avoid attack with memorable color patterns, but they are most likely to survive an attack with potent chemical defenses. It is energetically costly for animals to invest resources into both of these traits, and it is unclear if it is most efficient to invest more in conspicuousness, more in defense, or equally in both. I predict that aposematic coloration will act as an honest signal amongst nudibranch species with a positive correlation between defense and conspicuousness.
2. Which evolved first? I also predict that defense will evolve before conspicuousness, as the cost of conspicuousness is thought to be too great for undefended prey.
3. Why are patterns so variable within a species? Chromodoris splendida is characterized by red spots on a white mantle with a yellow to orange rim. It is theorized that consistency in pattern is beneficial for aposematic species, so that they will be more easily recognized and avoided by predators. However, these spots can range in size from large blotches to small spots and range in color from bright red through to maroon and even black. This same species has also been known to vary in the quantity and composition of chemical defenses. I will test whether population genetic variation correlates with phenotypic variation. I will also test whether pigmentation and defensive chemicals depend upon compounds sequestered from the diet.