Nutritional ecology of the giant clams Tridacna tevoroa and T. derasa from Tonga: influence of light on filter-feeding and photosynthesis

Abstract

This study compares the nutrition of 2 species of giant clam (Tridacnidae) from Tonga: the rare Tridacna tevoroa, which inhabits relatively deep waters (9 to 33 m), and 7: derasa, a more common and widespread species usually found in shallower habitats (1 to 20 m). The principal aim of the con)parison was to determine how T tevoroa survives at greater depths than other tridacnid species. Rates of filter-feeding, respiration and the photosynthesis-irradiance response were measured in clams of a wide size range (ca 20 mm to ca 500 mm) which had been acclimated to 4 levels of shading, simulating depths of 1, 5, 15 and 28 m. Carbon utilisation by the tissues of the host, partitioned into growth and respiration components, were compared with the supply of carbon via filter-feeding and photosynthate from zooxanthellae. Rates of respiration, growth, filter-feeding and maximum photosynthesis did not vary between species or level of shading. Only T tevoroa significantly increased its photosynthetic efficiency with increasing depth. Consequently, ~t was able to rely on phototrophy while at greater depth (ca 10 m deeper) than T. derasa. At their normal depths, phototrophy provides most ( 1 tevoroa: 70% at 28 m, 105% at 15 m). if not all ( T derasa). of the carbon required for growth plus maintenance, and filter-feeding is a relatively minor source (8 to 14 %). These 2 species contrast with the largest tridacnid species, 7: gigas, in which filter-feeding is a major source of carbon, at least in small clams, and the relative allocation of carbon for growth compared to respiration is much higher.