Larvae can be grown in the same flat-bottomed tanks used for embryo development or in conical-based fibreglass tanks fitted with bottom drains (see Figure 52). Tanks may be of relatively small volume (200 to 1 000 l) for experimental purposes and smallvolume production or much greater in both size and volume in high volume output commercial hatcheries. They may be operated as static systems or on flow-through. Water is changed in static systems on a periodic basis, whereas in flow-through culture, it is continuously introduced, a fixed volume being exchanged and replaced daily. This topic is discussed in depth in section 5.1.4.2.
D-larvae of hardier species (including Crassostrea and Tapes) can be grown at densities of 15 000 to 20 000 per l, but growth and survival are generally improved at lower densities (Table 10). Reduced densities are recommended for scallop species of the genera Pecten, Patinopecten, Placopecten and species of Chlamys and Argopecten, where between 5 000 and 10 000 early-stage larvae per l is appropriate. The larviparous flat oyster, Ostrea edulis is generally grown at 2 000 to 5 000 larvae per l because of the large size of the intial D-larvae. Some species can be successfully grown more intensively than above using high-density culture techniques (see section 5.1.4.1).
Rearing tanks are aerated - most commonly by a single, central air outlet located just off the tank bottom - at flow rates ranging from a slow bubble rate for D-larvae increasing to 200 l per hour for later stage larvae. The source of pressurized air needs to be free of carbon and oil. Low pressure, high volume, regenerative air blowers are ideal for the purpose. The air is filtered at source to 0.22 or 0.45.m particle size by a series of cartridge filters of decreasing porosity. This is to reduce air-borne contaminants which may include harmful micro-organisms. It is also advisable in humid conditions to dry the air before it enters the tanks by passing it through a sealed unit, such as filtercartridge housings, containing either anhydrous calcium chloride or silica gel. These drying agents need to be replaced as they become saturated to be effective.