Conserving threatened species during rapid environmental change: using biological responses to inform management strategies of giant clams

Abstract

Giant clams are threatened by overexploitation for human consumption, their valuable shells and the aquarium trade. Consequently, these iconic coral reef megafauna are extinct in some former areas of their range and are included in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species and Convention on International Trade in Endangered Species of Wild Fauna and Flora. Now, giant clams are also threatened by rapid environmental change from both a suite of local and regional scale stressors and global change, including climate change, global warming, marine heatwaves and ocean acidification. The interplay between local- to regional-scale and global-scale drivers is likely to cause an array of lethal and sub-lethal effects on giant clams, potentially limiting their depth distribution on coral reefs and decreasing suitable habitat area within natural ranges of species. Global change stressors, pervasive both in unprotected and protected areas, threaten to diminish conservation efforts to date. International efforts urgently need to reduce carbon dioxide emissions to avoid lethal and sub-lethal effects of global change on giant clams. Meanwhile, knowledge of giant clam physiological and ecological responses to local–regional and global stressors could play a critical role in conservation strategies of these threatened species through rapid environmental change. Further work on how biological responses translate into habitat requirements as global change progresses, selective breeding for resilience, the capacity for rapid adaptive responses of the giant clam holobiont and valuing tourism potential, including recognizing giant clams as a flagship species for coral reefs, may help improve the prospects of these charismatic megafauna over the coming decades.

Differential photo-physiological responses of two giant clam species to elevated temperature stress from Rodrigues Island, Western Indian Ocean

“…Thermal stress is known to be one of the leading causes of bleaching. Similar to corals, giant clams have been largely affected by global ocean warming (Watson and Neo 2021). An increase in seawater temperature can also negatively affect giant clams since they also host and strongly depend on symbiotic Symbiodiniaceae to survive (Blidberg et al 2000).…”

“…Unlike corals, giant clams host the symbiotic Symbiodiniaceae extra-cellularly, within zooxanthellal tubes in the mantle and stomach (Norton et al 1992). Nonetheless, similar to corals, giant clams have also been affected by global ocean warming (Watson and Neo 2021) since they depend on symbiotic Symbiodiniaceae (Blidberg et al 2000). Exposure of giant clams to elevated temperatures has led to their mass mortality around the world (Junchompoo et al 2013).…”

Assessing taxonomic, functional and phylogenetic diversity of giant clams across the Indo‐Pacific for conservation prioritization

“…This exceptional group of large bivalves consists of 12 recognized species in two genera ( Hippopus and Tridacna ) living in subtropical and tropical coral reefs across the Indo‐Pacific region (Fauvelot et al, 2020; Neo et al, 2017). Broadly, all species are threatened by various anthropogenic stressors including overfishing, habitat degradation and ornamental trade (Apte et al, 2019; Larson, 2016), and these stressors are likely to be amplified by projected global climate change (Watson & Neo, 2021). The combination of past and present threats has led to some species becoming highly imperilled in various parts of their ranges; for instance, the largest species Tridacna gigas is reportedly severely depleted or locally extinct in 26 of 31 of its known localities (Neo et al, 2017).…”

“…We can therefore expect that the loss of certain giant clam species may lead to substantial shifts in reef ecosystem functioning. Despite the notable challenges for the giant clams (Watson & Neo, 2021), resources and funds supporting their conservation in the recent years remain limited and short‐term (i.e. less than five years).…”

Fluted giant clam (Tridacna squamosa) restocking experiment in an urban turbid reef environment

“…Although large‐scale harvesting is currently less prominent than in past decades (Hester & Jones, 1974; Hirschberger, 1980), giant clams remain under pressure from subsistence and semi‐commercial fishers (Neo et al, 2019; Ramah et al, 2019). Environmental impacts, such as habitat loss and climate change‐driven bleaching, have also contributed to their threatened status (Watson et al, 2012; Andréfouët et al, 2013; Watson & Neo, 2021). As populations thin, reproductive success through broadcast spawning becomes less effective, and subsequent natural recruitment diminishes (Neo et al, 2013; Neo, 2020).…”