Effects of 0.2mm membrane-filtered seawater as a culture medium on fertilized eggs and larvae of the pacific oyster, Crassostrea gigas

Early stages of C. gigas were reared in the laboratory at 25.5 C in 500-ml culture vessels devised to exclude contaminating microorganisms, and were fed axenic microalgae. Raw seawater or 0.2-m membrane- filtered (m-f) seawater was used as the culture medium, changed at 2-day intervals. Eggs fertilized in raw seawater transformed at normal rates into trochophore and D-shape larvae when suspended in m-f seawater. However, the use of m-f seawater in larval culture was counterproductive in controlling bacterial populations. Compared to animals reared successfully in raw seawater, D-shape larvae failed to develop normally in m-f seawater. The animals grew minimally, exhibited shell and velum abnormality, and most were dead by 9 days of age. During the 2-day periods, total bacterial levels (loglo units) increased from < 1 to >6 ml-1 in m-f seawater, but from 3.95 to < 6 ml-1 in raw seawater. Fatal disease of oyster larvae was due to mixed bacterial populations rather than to Vibrio strains.