Ecophysiology of two mesophotic octocorals intended for restoration: Effects of light and temperature

Light and temperature are driving forces that shape the evolution and physiology of mesophotic organisms. On the Mississippi-Alabama continental shelf, octocorals dominate the mesophotic seascape and provide habitat for many fish and invertebrate species. Gaps in knowledge regarding the fundamental physiological responses of these species to light and temperature are of particular interest to restoration activities following the Deepwater Horizon oil spill. To address these gaps, the photobiology and thermal tolerance of Swiftia exserta and Muricea pendula were assessed in the field and laboratory. Pulse amplitude modulated fluorometry, histology, light microscopy, and epifluorescence imaging revealed low densities of photosynthetic endobionts in samples of S. exserta and none in samples of M. pendula collected near the determined bottom of the euphotic zone (51.45 m). Response to the recorded monthly mean habitat temperature range (18.5–25.4°C) was assessed using respirometry and polyp activity data from live corals exposed to temperatures between 18°C and 26°C. There was no significant difference in oxygen consumption for either species between 18°C and 26°C, and calculated Q₁₀ values were not significantly different from 1, thus suggesting that both species have a low sensitivity to the local thermal environment. However, a negative correlation between temperature and polyp activity suggests that M. pendula is more sensitive to higher temperatures than S. exserta. This study improves the understanding of the effects of light and temperature on mesophotic octocoral physiology and lays the foundation for future work to explore the thermal thresholds of each species and the endobiont–host relationship in S. exserta.