Could a different way of thinking be some corals saviour?

As we are all aware, the increasing frequency and severity of bleaching events and pathogen invasion is contributing to the global decline of corals and many studies focus on microbe communities as the main defense. For example, my group’s paper from the coral seminar (Mao-Jones et al. 2010) doomed corals to a life under pathogenic rule following thermal stress, yet did not consider how the coral host defends itself against pathogen attack. This paper however, focuses on the corals own immunity to disease and the mechanisms it employs, rather than assuming that it’s all down to epidermal microbes.

Importance is placed on this immunity as coral species appear to be declining at differing rates. The paper looks to species-specific immunological responses as the explanation but also mentions that in many cases symbiont differences are used as an explanation as zooxanthellae diversity allows for differences in tolerance to pathogen, thermal and UV stress. However, I tend to agree with this paper as immunity differences would provide much more variation in coral survivorship at species level than the differences between 4 species of zooxanthellae.

The recent discovery of corals possessing invertebrate-like immune mechanisms allowed 4 parameters to be used in this study to determine baseline immunity in several coral species. By comparing the immunity levels between species like this, we can enhance our understanding of the immune pathways utilised to maintain coral health, which could allow us to better predict reef health through environmental changes both in the short and long term, and to reduce those effects by enhancing coral immunity.

The findings suggest, importantly, that the parameters used are accurate measures of immunity as significant positive correlations were found between immunity and susceptibility to both bleaching and disease. However, they also seem flawed as to me; a positive correlation suggests that a higher level of immunity would correlate to a higher level of susceptibility. Although, in the discussion they go on to talk about the inverse correlations of these factors, so it seems unclear as to what they mean. From these correlations though, the authors were able to determine which coral species would be most threatened by a bleaching/disease event.

This finding has important implications as it suggests that some species have naturally have less chance of surviving an event, which leads us to infer that some corals may be predisposed to bleaching or disease. The authors explain this in terms of trade-offs, whereby some corals invest the majority of their energy in other areas of life history than immunity, meaning that they are lacking in this department. Conversely, other corals invest highly in immunity and so are lacking in other departments, e.g. reproduction, but at least survive disease better.

In my opinion this idea and way of thinking makes so much sense and could allow us to manage the issue of bleaching and disease much more efficiently. It would allow us to target specific species rather than managing corals generally, consequently allowing us to put more resources into the species that are most likely to need help. Interestingly, melanin levels had the most influence on coral immunity which could be attributed to its antimicrobial activity and photoprotection properties. This could be an area of research which could help us develop ways to protect corals and their diversity better.

A review of: Caroline V. Palmer, John C. Bythell and Bette L. Willis (2010) Levels of immunity parameters underpin bleaching and disease susceptibility of coral reefs The FASEB Journal Vol.24 1935-1946