The giant clam Tridacna squamosa quickly regenerates iridocytes and restores symbiont quantity and phototrophic potential to above-control levels in the outer mantle after darkness-induced bleaching

Giant clams are ecologically and economically relevant reef inhabitants that host photosynthetic dinoflagellates inside tubules located mainly in their colorful outer mantle. This study examined the effects of exposure to darkness for 30 days and the subsequent 11 days of recovery under a normal photoperiod on the outer mantle of the fluted giant clam Tridacna squamosa. Changes in the abundance of iridocytes and symbionts were assessed by fluorescence microscopy. Light microscopy was applied to quantify symbionts isolated from the outer mantle, while chlorophyll was extracted and analyzed by spectroscopy. The transcript levels of the host’s vacuolar-type H+-ATPase subunit A (ATP6V1A) and symbionts’ form II ribulose-1,5-bisphosphate carboxylase/oxygenase (Zoox-rbcII) were determined by quantitative real-time PCR (qPCR) and used as proxies for iridocyte abundance and phototrophic potential, respectively. The protein abundance of ATP6V1A and Zoox-RBCII was quantified by western blotting. After exposure to darkness for 30 days, the outer mantle of T. squamosa individuals lost the distinct multiple color patterns, and the gene and protein expression levels of ATP6V1A/ATP6V1A and Zoox-rbcII/Zoox-RBCII decreased dramatically. Microscopy assessment confirmed the reduction in iridocyte and symbiont populations, and the chlorophyll level also decreased considerably. However, just 11 days after returning to a normal light:dark regimen, the quantity of coccoid dinoflagellates and the expression of Zoox-rbcII/Zoox-RBCII in the outer mantle increased significantly to levels higher than those of the individuals prior to exposure to darkness (control), while the chlorophyll content returned to the control level. Additionally, the outer mantle regained most of its coloration with partial recovery of the iridocyte population. These results are relevant not only for understanding the phenomena of light deprivation and symbiont loss in dinoflagellate-associated reef organisms, but also signify that the giant clam-coccoid dinoflagellate holobiont is phototrophically plastic and particularly tolerant to bleaching.