Symbioses have long been recognized as close and sustained interactions between different species. For example, many scleractinian corals form a symbiosis with photosynthetic dinoflagellates (Symbiodinium spp.). Although symbiotic relationships are often categorized as mutualisms, parasitisms or commensalisms, symbioses may shift between these categories depending on environmental conditions. In this study, we used a temperate coral species, Astrangia poculata, to investigate whether this classic coral-algal mutualism may have parasitic or commensal tendencies under different environmental conditions. Astrangia corals are facultatively symbiotic, and thus, provide an ideal experimental system for testing the impact of symbionts on host performance. Sixteen colonies of each symbiotic condition, symbiotic (brown) and aposymbiotic (white), were subjected to four different experimental treatments: light/cold, light/warm, dark/cold, and dark/warm (128 colonies total). Colonies were fed twice weekly and maintained under these experimental conditions for a total of three months. We measured the physiological characteristics of both the coral host (tissue thickness and polyp growth), and algal symbiont (Symbiodinium identity, cell density, and maximum photochemical efficiency of PSII) to determine whether mutualism persisted under different treatments.

Results/Conclusions

In all light and in all cold treatments, host colonies performed well, regardless of symbiotic state. It is noteworthy that in the dark/warm treatment, brown host colonies seemed to perform poorly compared to white hosts, suggesting that a typically mutualistic symbiosis may potentially become parasitic in adverse conditions. Symbiont loss (bleaching) was also observed in the dark/warm treatment, which may represent an alternative to parasitism. These results are consistent with shifts in the nature of the symbiosis, and may lend insight into why both symbiotic and aposymbiotic colonies are found in close proximity in the field. Symbiosis is increasingly recognized as an important selective force underlying evolution, but the ecological context of symbiosis may play an equally important, but previously unrecognized, role.