The larval rearing section of a hatchery does not require the very low light levels of the broodstock component of the hatchery, but direct strong sunlight should be avoided.
Mud crab larvae have been found to eat more at light levels of 1 000–6 000 lux, while below 1 000 lux, larvae both eat less and have increased mortality rates. As crab larvae exposed to 24-hour light exhibit decreased survival, compared with a 12-hour light/dark cycle, natural lighting should be the primary light source for larval rearing areas.
The larval rearing areas should be well ventilated, with a reasonably high ceiling to minimize humidity (Figure 4.7). There should also be adequate space between tanks and equipment to facilitate operation and provide access for equipment to be removed or serviced.
Adequate physical separation from broodstock and feed production areas is required in order to minimize aerosol drift and maintain a high standard of biosecurity. Appropriately located walls, or barriers of some sort of sheeting, can assist in separation. When tanks are not being examined, they can be covered with plastic or similar sheeting (Figure 4.8). This reduces temperature fluctuation in tanks, in addition to controlling aerosol sprays.
Larval rearing tanks of various designs have been used to culture mud crab larvae, including circular tanks with a conical base, hemispherical round tanks, parabolic tanks and rectangular tanks.
The colour of the tank in which mud crab larvae are grown has been shown to have a major impact on survival. Larvae grown in black tanks have significantly higher survival rates than any other tanks, with increasing larval survival recorded in increasingly darkly coloured tanks.
