Isotopic and elemental compositions reveal density‐dependent nutrition pathways in a population of mixotrophic jellyfish

Nicolas Djeghri,Stephan Behl,Maria Stockenreiter,Gerda Ucharm,Thomas Hansen,Sharon Patris,Philippe Pondaven,Jessica Y T Huang,Herwig Stibor

doi:10.1002/ecs2.3295

Abstract

AbstractMixotrophic organisms are increasingly recognized as important components of ecosystems, but the factors controlling their nutrition pathways (in particular their autotrophy–heterotrophy balance) are little known. Both autotrophy and heterotrophy are expected to respond to density‐dependent mechanisms but not necessarily in the same direction and/or strength. We hypothesize that the autotrophy–heterotrophy balance of mixotrophic organisms might therefore be a function of population densities. To investigate this relationship, we sampled mixotrophic jellyfish holobionts (host, Mastigias papua etpisoni; symbiont, Cladocopium sp.) in a marine lake (Palau, Micronesia) on six occasions (from 2010 to 2018). Over this period, population densities varied ~100 fold. We characterized the nutrition of the holobionts using the δ13C and δ15N signatures as well as C:N ratios. δ13C values increased and δ15N values decreased with increasing population densities (respectively, R2 = 0.86 and 0.70, P < 0.05). Although less distinct, C:N ratios increased with increasing population densities (R2 = 0.59, 0.1 > P > 0.05). This indicates that the autotrophy–heterotrophy balance tends toward autotrophy when population densities increase. We propose that the availability of zooplanktonic prey is the main driver of this pattern. These results demonstrate that the autotrophy–heterotrophy balance of mixotrophic jellyfishes can be tightly regulated by density‐dependent mechanisms.

Highlights

Density-dependent processes are key mechanisms that have profound impacts on the biology and ecology of organisms (e.g., May et al 1974)
We characterized medusae nutrition through the use of their stable isotope signatures (δ13C and δ15N) and C:N ratios. In photosymbiotic organisms such as Mastigias papua etpisoni holobionts, δ13C values tend to increase with the relative contribution of autotrophy and decrease with the relative contribution of heterotrophy, whereas the reverse is true of δ15N
We hypothesize that density-dependent process would affect the mixotrophic nutrition of the Mastigias papua etpisoni holobiont leading to changes in its autotrophy–heterotrophy balance

Summary

Introduction

Density-dependent processes are key mechanisms that have profound impacts on the biology and ecology of organisms (e.g., May et al 1974). To investigate the potential link between population density and mixotrophy, we sampled Mastigias papua etpisoni medusae from Ongeim’l Tketau over six different years (between 2010 and 2018) including large variations in population densities.

Results

Conclusion