Bleaching reduces reproduction in the giant clam Tridacna gigas

Bleaching or the expulsion of the photosynthetic symbiont Symbiodiniaceae ('zooxanthellae') caused by the increase in seawater temperatures causes massive mortalities in giant clams. After a bleaching disturbance, population recovery is heavily dependent on reproduction and recruitment success. However, how bleaching directly affects the reproductive performance of giant clams is, to date, unknown. Here, we examined the direct impacts of bleaching on the reproduction of the giant clam Tridacna gigas between 2020 and 2021 in the Philippines. A continuous 6 mo monitoring of egg concentration, egg stage, egg size, recovery, and survival in prolonged bleached (moderate or severe) and non-bleached hatchery-bred T. gigas maintained in the wild was conducted. The probability of producing eggs and the proportion of intact eggs (developing and mature) were highest in healthy giant clams, followed by moderately bleached giant clams, and lowest in severely bleached giant clams in which >50% individuals had empty gonads. The oocyte mean diameter was similar among giant clams, regardless of degree of bleaching. Only 5 severely bleached giant clams showed partial color recovery, whereas 2 healthy and 12 severely bleached giant clams died. This study shows that apart from the lethal effects of severe bleaching, the reproductive processes of surviving bleached T. gigas such as gametogenesis and egg production can be also negatively affected. Results suggest that the increasing frequency and intensity of bleaching due to thermal stress can lower the reproductive potential of giant clams and likely other zooxanthellate organisms, resulting in reduced recruitment and population decline.