Environmental influences on growth and survival during the ocean-nursery rearing of giant clams, Tridacna gigas (L.)

The effects of temperature, emersion, light and contrasting localities on growth and survival of juveniles of Tridacna gigas were studied at Orpheus Island, North Queensland, Australia. Shell lengths of ocean-nursery phase juveniles increased by an average of almost 10 mm per month over the 17-month study period. However, there was a strong seasonal component to growth rate, which varied from highest levels in late summer to almost zero in late winter when water temperatures were near 20°C. Juvenile clams tolerated up to 10 h per day mean emersion, but were completely stunted in growth. Periods of up to 3 h emersion during daytime, but not at night, had a positive effect on growth, suggesting that photosynthesis continues during emersion. Growth and survival were poorer at a more oceanic locality than at a more turbid, protected locality, apparently due to disturbance from turbulence at the oceanic site. Clams in 90% shade showed poor growth and survival compared to those in 50% shade and full sunlight. This species is an obligate phototroph. Thus, juveniles of T. gigas have conflicting environmental requirements in their need to be exposed to high light levels but concealed from predators. It is suggested that the complex structure of coral reefs provides juvenile clams with microhabitats where they are both cryptic and exposed to intense light, and this is the particular feature linking giant clams to coral reefs. In providing juvenile clams with protective cages, as in ocean-nursery culture, it should be possible to use environments other than coral reefs for their culture.