The oystermen of Arcachon likely don't really care to argue where an oyster starts and stops being called a "naissain". All that matters are "the dots" - the more the merrier!
Two species of oyster live in the Arcachon Bay, the Pacific oyster and the European oyster. The Pacific oyster is by far the commercially more important species. Sometime in March or early April, when the water reaches a temperature of about 13° C, the Pacific oysters start getting a little "romantic". Nature's wisdom decides how many males and females will exist among these hermaphrodites. Sometime between May and June, the Pacific oysters get "milky". Each oyster is now filled either with sperm or eggs - by the millions. In the summer months, when the water in the bay reaches about 19° to 22° C, the oysters spawn. The Arcachon Bay is a huge and complex marine environment. The pacific oysters may thus spawn a little earlier in one area of the bay than in another.
Unlike European oysters, Pacific oysters don't take
care of their offspring at all. They are scientifically referred
to as "broadcast spawners". Enormous masses of sperm
and eggs are released almost simultaneously into the sea. Purely
by chance, "some" sperm will find "some"
eggs and oyster larvae will begin to form. Since we are talking
billions of sperm and eggs emitting from a bed of Pacific oysters,
"some" can easily translate into millions of Pacific
oyster larvae. However, the chance of survival of a Pacific oyster
larva is not very good. It is just slightly better than that
of a snowball in hell. After about twenty days of floundering
around in the plankton layer of Arcachon Bay, running the gauntlet
of countless natural enemies and other dangers, likely only one
percent will live long enough to find a suitable place to settle
down on and survive the radical metamorphosis into a "baby
oyster" - which soon is visible to the naked eye as a tiny
brownish "dot," no bigger than a pin head.