Guardians of the clams: a decadal monitoring effort of endangered giant clams by citizen scientists

ABSTRACT Giant clams are highly conspicuous marine bivalves in the tropical oceans because of their large shells and colourful outer mantle tissues. This makes them an appropriate iconic species for both trained and untrained citizen science surveys. In Singapore, because of good public awareness of giant clams, they are often reported by citizen scientists, who document marine species using photography. Furthermore, these opportunistic records are captured in a comprehensive public blog (Wild Shores of Singapore; https://wildshores.blogspot.com) with specific details of the field surveys. This study presents the first feasible effort in establishing a long-term dataset based on citizen science observations to report giant clam species-level population status and trends over time. Between 2011 and 2020, citizen scientists covered 8.53 km2 of intertidal reef areas and consistently recorded only two giant clam species (Tridacna squamosa and T. crocea) annually. Despite the more comprehensive coverage and sampling of reef areas, the citizen scientists did not encounter the other native species: Hippopus hippopus, T. gigas and T. maxima. Also, the respective densities of T. squamosa and T. crocea were 0.00120 and 0.00034 individuals per 100 m2, which makes them the smallest known giant clam populations in the Indo-Pacific region. Regardless, the sampling efforts of citizen scientists were generally similar across the years, but future surveys could adapt different survey protocols for the respective giant clam species. Findings from this study also support the marine conservation efforts in Singapore, such as using active intervention measures to protect their giant clams. Additionally, lessons learnt from these citizen science observations can be harnessed to encourage monitoring of giant clams throughout the Indo-Pacific region.