Gisela Dionísio1,2*, Catarina Santos1,3*, Ricardo Cyrne1*, Mariana Hinzmann2,4, Bernardo A. Duarte5, Ana Lopes1, Vanessa Madeira1, Lidia M. García6, Sara H. García-Morales 6, Jorge Machado4, Rui Rosa1
1MARE – Marine and Environmental Sciences Centre, Laboratório Marítimo da Guia, Faculdade de Ciências da Universidade de Lisboa, Cascais, 2750-374, Portugal
2Departamento de Biologia & CESAM, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
3ICBAS‐UP– Abel Salazar Biomedical Sciences Institute, University of Porto, Porto, 4050-313, Portugal
4CIIMAR– Centre of Marine and Environmental Research, University of Porto, Porto, 4050-123, Portugal
5MARE – Marine and Environmental Sciences Centre, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Campo Grande, Portugal
6 UCM – Facultad de Biología, Universidad Complutense de Madrid, Madrid, 28040, España
Giant clams are iconic tropical bivalve molluscs enrolled in a rare symbiotic relationship with photosynthetic dinoflagellates (Symbiodinium spp.). Besides their important ecologic role, they have a great cultural and economic value to local human communities. However, populations have been declining over the past decades and their conservation is of considerable management importance. Nonetheless, there is considerable knowledge gap regarding the effects of climate change, with rising temperatures and increasing pCO2 levels, over these species.
The present study represents a comprehensive assessment of the physiological responses of the holobiotic system (Tridacna maxima clams and respective symbiotic dinoflagellates) to the foreseen near-future conditions of warming (∆ 3ºC) and high pCO2, with concomitant acidification (ΔpH=0.4). After a two-month acclimation period in a cross-factored design, we evaluated an array of endpoints including: i) respiration and productivity, ii) Symbiodinium cellular conditions, iii) haemolymph cellular and biochemical conditions; iv) shell ultrastructure; v) heat shock response; vi) lipid peroxidation and vii) antioxidative enzymatic activity.
The exposure to the experimental conditions triggered deleterious effects in most endpoints. Temperature appeared as the main driver of stress, although effects of acidification were also present and interaction between factors was evident in several cases. The overall fitness of the holobiont was undermined with an evident decline in Symbiodinium populations and cellular damage in the clams, as the defence mechanisms appeared to be insufficient to cope with the new conditions.
Anthropogenic pressure is already responsible for decline of giant clam populations worldwide and climate change will most likely impose additional stress, undermining the conservation efforts taking place. On the other hand, their unique and charismatic nature may act as a beacon for the conservation of coral reef systems regarding the global change issues.