These larvae are then known as eyed larvae or pediveligers. When the larvae reach 250–300 µm, a foot develops, as does a light-sensitive organ that forms a black spot. These veliger larvae develop over 2 to 3 weeks depending on the rearing temperature and the quality and quantity of phytoplankton available: the hinge and hook appear around the eighth day when the larvae measure 130–160 µm on average. These planktonic larvae measure 60 µm and have a velum, which they swim with and use to feed on small phytoplanktonic algae (3 to 4µm). Then, by about 24 hours, shells with two equal valves form to produce D-larvae. Six to eight hours after fertilisation, shell-less trochophore larvae appear. However, there is a possibility that they can become male again over the course of their lives, depending on environmental conditions. Oysters are generally male at their first sexual maturation (known as protandrous hermaphoditism), they then become female for their second or third reproductive season (according to site). The Pacific oyster is an alternate hermaphrodite. Bacterial development can be rapid and fatal for larvae and postlarvae due to the rearing techniques used : fine seawater filtration, organic matter input from phytoplankton and a relatively high temperature (25 ☌). While the level of natural settlement in the wild is dependant on the environment in the broadest sense (biological water quality, temperature during larval development, quantity and quality of nanoplankton), success in the hatchery is highly dependent on the microbiological quality of the rearing water. Hatchery production must be accompanied by systematic prophylactic sanitary measures. The form of rearing used in the field is decided by the initial size. Future possibilities of genetic selection on criteria such as growth, resistance, shape and taste quality.Choice of product: diploid or triploid oysters.Choice of size (T2 to T10 or more), and batches of homogeneous size.individuals, not attached to a substrate or requiring physical separation work. Spat from hatcheries come ‘one by one’, i.e.Production is spread over a longer period of the year.Hatchery spat is different from wild-collected spat in the following ways : The culture and production of Pacific oyster Crassostrea gigas in France depends on natural collection from the wild for 70-80 % of spat and on hatchery production for between 20 and 30%, depending on years. This info-card was updated in 2011 but not yet translated. Hatcheries, the case of the Pacific oyster
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