Trophic ecology of Caribbean octocorals: autotrophic and heterotrophic seasonal trends

Abstract

Studies over the past decades indicate that octocorals are becoming the dominant group in some areas of the Caribbean. Yet, basic knowledge about the trophic ecology of these organisms and their seasonal and speciesspecific variability is still scarce, though this might play a key role in determining their importance in benthic–pelagic coupling processes and, consequently, their role in carbon cycles. In the present study, two Caribbean gorgonian species (Plexaurella nutans and Pterogorgia anceps) were studied during an annual cycle, to assess seasonal variations in their reliance on heterotrophic versus autotrophic energy inputs. Zooplankton capture rates and bulk tissue stable isotopes were measured on a monthly basis to assess heterotrophic energy input, while autotrophic contribution was quantified monthly by Symbiodiniaceae cell densities and pigment contents, accompanied by seasonal measurements on Symbiodiniaceae (Breviolum sp.) photosynthetic performance and host respiratory demand. The results show that while autotrophy was the main energy source for both species, there was also a non-neglectable input through zooplankton that accounted for 0.2–0.8% and 0.7–3.4% of the energy demands in P. nutans and P. anceps, respectively. Our data further demonstrate that there are species-specific and seasonal differences in the contributions of these two nutrition modes, though there is no indication of shifts in the predominant mode during the year in either species. The energy inputs resulted in a positive energy balance throughout the year, with an energy surplus available for somatic growth, gonads, and/ or energy reserves (e.g., lipids). However, the seasonal patterns differed between species, a feature that is most likely related to the different reproduction periods of the octocorals. Altogether, the information gathered here serves for a better understanding of the trophic ecology of mixotrophic octocorals and the seasonal variability of the nutritional modes that will define their potential impact in the carbon cycle and benthic–pelagic coupling processes of coral reefs.