Anatomical complexity allows for heat-stressed giant clams to undergo symbiont shuffling at both organism and organ levels

• Published in: Marine biology 2025, Vol.172 (1), p.4, Article 4
• Main Authors: Mies, Miguel, Garrido, Amana G., Zilberberg, Carla, Sumida, Paulo Y. G., Pang, Caryn Z., Ip, Yuen K., Chew, Shit F.
• Format: Article
• Language: English
• Subjects: Biomedical and Life Sciences; Bleaching; Body organs; Chlorophyll; Chlorophylls; Clams; Complexity; Coral reefs; Corals; Dinoflagellates; Ecophysiology; Freshwater & Marine Ecology; Heat; Heat stress; Life Sciences; Mantle; Marine & Freshwater Sciences; Microbiology; Mollusks; Next-generation sequencing; Oceanography; Organisms; Organs; Original Paper; Population decline; Population studies; Symbiodiniaceae; Symbionts; Thermal stress; Upper mantle; Zoology
• ISSN: 0025-3162; 1432-1793
• DOI: 10.1007/s00227-024-04567-3

Giant clams are photosymbiotic molluscs, hosting Symbiodiniaceae dinoflagellates. Serving as an alternative model organism for ecophysiological studies within reef environments, giant clams differ from corals due to their anatomical complexity, with extracellular symbionts present in multiple organs. We aimed to determine if clams, under thermal stress, exhibit symbiont shuffling both at the organism level and across individual organs. Therefore, the fluted giant clam, Tridacna squamosa , was exposed to control and heat-stress temperatures of 26 and 30 ºC, respectively, for 45 days. Subsequently, the degree of bleaching was assessed through quantification of symbiont cells and chlorophyll- a loss via fluorometric detection and photometric analysis. The relative composition of Symbiodiniaceae ITS2 rDNA profiles across ten different organs was determined using metabarcoding by next-generation sequencing. Findings show that the outer mantle of heat-stressed clams lost approximately 30% of its symbionts and 45% of the chlorophyll- a content. Extensive shuffling took place at the organism level, with the downregulation of thermally-sensitive Durusdinium phylotype D4/D5, and upregulation of thermally-tolerant, homologous and generalist phylotypes belonging to Symbiodinium and Cladocopium genera. At the organ level, shuffling took place only in the outer mantle, the only organ directly exposed to light. The other organs did not undergo compositional changes in symbiont phylotypes and may potentially serve as symbiont reservoirs. These results illuminate the complexities of symbiont shuffling within an anatomically intricate organism, offering perspectives for other photosymbiotic reef organisms. Additionally, this study advances the knowledge regarding bleaching in giant clams, a relevant resource that has experienced substantial population declines.