Growth of cultured giant clams (Tridacna spp.) in low pH, high-nutrient seawater: species-specific effects of substrate and supplemental feeding under acidification

Four species of giant clams, Tridacna maxima, T. squamosa, T. derasa and T. crocea, were cultured in outdoor raceways for 364 days at the Waikiki Aquarium and the Oceanic Institute on the island of O'ahu, Hawai'i, USA. Growth of each species was compared among individuals grown with and without supplemental phytoplankton feeding, and directly on the substrate or mounted on concrete plugs in low pH, high nutrient seawater. Among clams cultured with and without supplemental phytoplankton (Chaetoceros spp.), feeding resulted in significantly lower mortality in all species but T. deresa, whereas growth was significantly higher among fed clams for all species except T. squamosa. Tridacna derasa showed roughly a threefold increase in growth when fed (88.5 g +/- 4.4 SD) than when unfed (26.0 g +/- 2.1 SD), whereas T. maxima growth was substantially lower, but nearly 10-fold greater in response to feeding (9.0 g +/- 1.9 SD). The overall mortality rate of juvenile clams was significantly lower in the fed (44.4 +/- 10.0%) than the unfed (71.8 +/- 9.6%) trials, with the greatest effect observed in mortality of T. maxima (fed 15% versus unfed 80%) and T. squamosa (fed 65% versus unfed 95%). None of the T. squamosa remained on concrete plugs for the duration of the experiment. Among the remaining three species, there was no difference in either wet weight or shell length for T. maxima and for wet weight only in T. derasa on (186.5 g +/- 16.1 SD) and off (147.0 g +/- 6.0 SD) the concrete plugs. In contrast, T. crocea had significantly greater shell growth off the plugs (14.3 mm +/- 1.0 SD versus 8.5 mm +/- 1.7 SD) but significantly greater gain in wet weight on the concrete plugs (26.3 g +/- 1.5 SD versus 58.5 g +/- 2.5 SD). The seawater wells used for this study are well characterized with elevated levels of inorganic nutrients and higher pCO(2) relative to tropical ocean waters, roughly approximating predictions for future oceanic conditions under IPCC IS92a emission scenarios. In comparison to previous studies in natural seawater, T. derasa had a significantly higher shell growth rate in the high-nutrient, low-pH well water. In contrast, T. maxima and T. squamosa had significantly lower growth rates in low pH, whereas growth of T. crocea was not significantly different between low pH and ambient seawater. These experiments demonstrate species-specific differences with each treatment, which cautions against making broad generalizations regarding the effects of substrate type, feeding effects, nutrient enrichment, and ocean acidification on tridacnid culture and survival.