Giant clams occupy an unusual position in reef ecology, deriving nutrition both from symbiotic algae living in their tissues and from filter-feeding on particles in the water. Researchers at the Swire Institute of Marine Science and the School of Biological Sciences at HKU, with collaborators at the University of the Philippines Diliman and the Semirara Marine Hatchery and Laboratory, set out to quantify how six species divide their reliance between the two.
Carbon and nitrogen isotope measurements from the clams and from their algal symbionts provided the data, and a model the team calls HERS translated it into estimates of dependence on photosynthesis versus filter feeding. The species proved to occupy different points along a feeding spectrum rather than clustering around a single strategy, which is itself notable for a group often treated as ecologically interchangeable.
The pattern that emerged carries a conservation implication. The fastest-growing and largest clams depend more heavily on their algal partners, and that dependence may make them more vulnerable to climate change, since the algae are sensitive to warmer seas. Larger size also makes them easier targets for over-harvesting, so the same species face two compounding pressures. The study was published in Communications Biology.
The authors note that some species appear plentiful yet may already be in decline and under-protected, a mismatch that abundance-based assessment would miss entirely. A framework that predicts vulnerability from feeding physiology offers conservation planners something more diagnostic than population counts, and it is the kind of trait-based approach that reef management has been slow to adopt.