Investigation into the functional role of chromoproteins in the physiology and ecology of the Hawaiian stony coral Montipora flabellata in Kāne‘ohe Bay, O‘ahu
Bright purple Montipora flabellata colonies in Kāne‘ohe Bay are found exclusively in shallow, high water flow areas where solar irradiance is commonly super-saturating. The uniform purple colony coloration is separate from the algal symbionts and distinct due to the presence of protein-pigment complexes (chromoproteins) within the coral host tissue that are believed to serve a range of functions including immunity and symbiont photoprotection. My investigation aimed to fill large knowledge gaps regarding the distribution, ecology, and physiology of this important coral species with a focus on elucidating the functional role of the chromoproteins and their potential to act as photoprotectants. Surveys were conducted across Kāne‘ohe Bay to map the species’ distribution and the in situ environmental conditions were examined to determine whether the resident M. flabellata colonies require or simply thrive in high water motion conditions. At a much finer scale, light and confocal microscopy were utilized to locate, identify, and describe cell constituents—particularly chromoproteins—to determine whether their physical location within coral epithelia would permit them to “screen” algal symbionts from excess light energy. Additionally, light energy absorbance and photosynthetic performance measurements were used to determine whether presence of these putative photoprotective pigments affects the photosynthetic apparatus. Contrary to expectations, no linear relationship between chromoprotein absorbance and photosynthetic performance was found signifying the possibility of another, important function for these pigments. The results further suggest that the thick sheaths of chromoproteins found at the apical surface are well-positioned to screen excess light, but at a cost. Mucocytes in M. flabellata epidermis are greatly reduced compared to other species, potentially hindering production of sufficient surface mucus, believed essential for healthy function. In high water flow areas, however, a permanent layer of surface mucus would likely be unnecessary. This unique example of a functional biological trade-off supports the hypothesis that M. flabellata requires high water flow to survive and provides supporting evidence that this coral species may be endemic to Hawai‘i.