<P> An oyster reef can increase the surface area of a flat bottom 50-fold . An oyster's mature shape often depends on the type of bottom to which it is originally attached, but it always orients itself with its outer, flared shell tilted upward . One valve is cupped and the other is flat . </P> <P> Oysters usually reach maturity in one year . They are protandric; during their first year, they spawn as males by releasing sperm into the water . As they grow over the next two or three years and develop greater energy reserves, they spawn as females by releasing eggs . Bay oysters usually spawn from the end of June until mid-August . An increase in water temperature prompts a few oysters to spawn . This triggers spawning in the rest, clouding the water with millions of eggs and sperm . A single female oyster can produce up to 100 million eggs annually . The eggs become fertilized in the water and develop into larvae, which eventually find suitable sites, such as another oyster's shell, on which to settle . Attached oyster larvae are called spat . Spat are oysters less than 25 mm (1 in) long . Many species of bivalves, oysters included, seem to be stimulated to settle near adult conspecifics . </P> <P> Oysters are considered to filter large amounts of water to feed and breathe (exchange O and CO with water) but they are not permanently open . They regularly shut their valves to enter a resting state, even when they are permanently submersed . In fact their behaviour follows very strict circatidal and circadian rhythms according to the relative moon and sun positions . During neap tides, they exhibit much longer closing periods than during the spring tide . </P> <P> Some tropical oysters, such as the mangrove oyster in the family Ostreidae, grow best on mangrove roots . Low tide can expose them, making them easy to collect . In Trinidad in the West Indies, tourists are often astounded when they are told, in the Caribbean, "oysters grow on trees". </P>

How long can oysters survive out of water