Since our lecture on coral diseases, I was curious about the coral immune system. This paper focused on variables commonly associated with invertebrate immunity and investigated their relationship with regards to coral susceptibility (bleaching and disease) with both hard (Scleractinia) and soft (Alcyonacea) corals spanning 10 families from the Great Barrier Reef. Whilst this paper mainly focuses on coral bleaching, it mentioned several times how the mechanisms of defence against coral bleaching are similar to the defence strategies employed during infection.
This paper regarded four main variables, which were used to make up a Constituent immunity number. These variables were presence/frequency of melanin, size of melanin containing granular cells, phenoloxidase (PO) activity, and concentrations of fluorescent proteins (FPs.)
- Presence/frequency of melanin
- Size of melanin containing granular cells
The melanin-synthesis pathway is a key invertebrate defence mechanism, triggered by physical injury or invasion by foreign organisms. When activated, it enables the deposition of encapsulating melanins and produces highly cytotoxic intermediates such as reactive oxygen species (ROS).
The 15 species investigated all contained melanin-containing granular cells in the free body wall tissue, however the distribution and density of melanin containing granular cells varies within species. Melanin frequency was negatively correlated with disease susceptibility (deducted from field studies of disease prevalence) – ergo corals with the lowest melanin frequency showed the highest disease prevalence. The spatial distribution of melanin is also important in the effectiveness of coral defence e.g. having UV- and visible-light-absorbing melanain in epidermal cell layers may protect zooxanthellae from excess UV/light in the environment.
It has been proposed melanin-containing granular cells are potential amebocytes, a mobile possible phagocytic defence mechanism which characteristically aggregate in area of injury or invasion. A mobile defence mechanism is further beneficial as entry points of attack are unpredictable.
- Phenoloxidase (PO) activity – which indicates melanin pathway activity.
This is another indication of melanin pathway activity, and once again was present in all coral families, while showing a significant variation of activity dependent on species.
- Concentrations of fluorescent proteins (FPs)
Known to remit light at different wavelengths, the biological role of FPs is still not fully understood, however they do show reactive oxygen scavenging properties which would provide protection during high oxidise stress conditions such as coral bleaching and pathogen invasion. FPs are present in unbleached but still compromised tissues suggests that they are involved in immunity against not just coral bleaching, but also infection. Like other variables investigated, concentrations varied significantly among coral families.
Immunity parameters were strongly interdependent with size of melanin-containing granular cells, PO activity and FP concentrations. There was a significant positive correlation between Constituent immunity and both bleaching and disease susceptibility.
Consistent differences in disease susceptibility across families suggest taxa-specific levels of disease resistance and investment in immunity mechanisms. Immunity was found to explain over 78% of interfamily variation in bleaching susceptibility, and 83% of disease susceptibility, providing evidence that the coral host itself plays a major role in pathogen and bleaching resistance.
Another interesting point raised by the paper, is the idea that Immunity varies among individuals due to species specific differences in allocation of energy to certain traits. This paper uses life history theory to predict the species which have higher immune functions, will have allocated less energy into growth and/or reproduction. This idea was backed up by the fact that the fast growing/branching species of Acropora (which requires a vast amount of energy to promote rapid colonial growth) was seen to demonstrate the lowest immune rank, which in turn contributed to a high susceptibility to bleaching and disease.
The paper also mentioned the necessity of a holist approach in understanding bleaching and disease with regards to corals – due to the combined effects of several base defence mechanisms in these species: which I though was a point that needed reiterating, as often we get so focused in science we find ourselves drawing conclusions between A and B – without taking into consideration the whole network of mechanisms which are going on in the microscopic world!
Palmer, C. V., Bythell, J. C., & Willis, B. L. (2010). Levels of immunity parameters underpin bleaching and disease susceptibility of reef corals. The FASEB Journal, 24(6), 1935-1946.