5.1.5 Growth and survival of larvae

The growth and stages in development of larvae of the Pacific oyster, Crassostrea gigas, and the sand scallop, Pecten ziczac, from the D-stage to metamorphosis are shown in Figure 65.

Figure 65: Photomicrographs of the growth and development of Pacific oyster, Crassostrea gigas, (A) and sand scallop, Pecten ziczac, (B) larvae.

Larvae of the different groups of bivalves grow at different rates. Those of scallops and clams have initially larger D-larvae and reach settlement size and metamorphosis at a considerable smaller shell length (210 to 230 ?m) than do larvae of the oviparous oysters (320 to 340 ?m). Comparative growth rate of a number of species cultured at 24+2oC is shown in Figure 66. Some species, including the calico scallop, are slower in reaching an exponential rate of growth. They tend to have a lag phase before rapid growth commences. Others like the Manila clam and Pacific oyster grow rapidly from the initial D-stage. Growth rate slows in all species as they approach the settlement stage.

Figure 66: Comparative growth of larvae of some warmer water bivalve species (European flat oyster, Ostrea edulis, Pacific oyster, Crassostrea gigas, Manila clam, Tapes philippinarum and the calico scallop, Argopecten gibbus) from the D-larva stage to metamorphosis when cultured at 24+2oC.
Day 0 denotes the day that eggs were fertilized.
The blue arrow shows the day that flat oyster larvae were liberated by brooding adults.

Similarly, survival of larvae from the D-stage to metamorphosis is variable among different species. It may be as high as 50 to 70% on average in some of the oyster and clam species and as low as 15 to 30% in scallops. Much depends on culture protocols and the extent of the culling (removal) of slower growing larvae during the rearing process. A considerable part of losses in larval numbers through the culture period is often associated more with culling and discarding of slower growing individuals than it is to the death of larvae. The proportion of larvae that reach metamorphosis is also related to culture conditions including diet and ration, temperature and salinity and to relatively uncontrollable factors such as seawater quality and disease (see 5.3).