A "claire" is simply any more or less saline water compound or basin of varying size, usually square or rectangular, where oysters are temporarily deposited for one or more reasons. The size of the water compound is about 1,800 ft² (approx. 500 m²), some are larger and some considerably smaller. They are usually quite shallow, the depth rarely exceeding 20 inches (about 50 cm). A water compound of this type can be located in the shallow portion of the intertidal zone. A claire can also be situated on land - sometimes even inside a building, usually connected to the sea in one way or another, possibly with pipes, valves, and pumps. Claires can also describe a series of simple saline water pools, the result of soil excavation in suitable ground, sometimes connected to man-made canals which can be opened or closed. A dike or wall of some sort is usually built around the perimeter (piled up soil, boards, concrete...) to contain or at least slow the run-off of the water. Many claires are also made up entirely of concrete.
The principal purpose of almost all claires is to improve plain "ocean oysters" in one way or another, possibly even in a number of ways. The utilization of a claire can thus be quite simplistic or, as in the case of the Marennes-Oléron region, be elevated to a form of art in the cultivation process.
"Clair" simply means "clear",
and the most rudimentary application of a claire is the purification
of an oyster. Oysters are exposed to clean water for a period
of time, during which they naturally purge any kind of impurities
(mud, sand...). Some modern claires serve as quarantine areas
of sorts, where the oysters filter water which has been treated
to be void of any marine bacteria or other potentially harmful
agent. A claire can also serve as an easily accessible holding
area for oysters ready for market, as claires are largely or
even completely unaffected by the rhythm of the tides. Almost
invariably the water level in claires can be more or less controlled.
By lowering the water level and exposing the oysters to air,
oysters can literally be trained to stay closed for long periods
of time. Oysters, whose adductor muscles have been conditioned
over time in this manner will prove superior when the time comes
to be shipped long distances or to remain fresh in a vendor's
seafood case for an extended amount of time.
In some areas, where temperature extremes are seasonally prevelant, certain types of "claires" can also help oysters survive. Thanks to a truly fantastic design by nature, the formidable shell fortress makes every culinary oyster species remarkably resilient to temperature extremes - to a point. Some oyster species are tougher in this respect than others. For instance, the Eastern oyster (Crassostrea virginica) and Pacific oyster (Crassostrea gigas) are proven survivalists of the first order. The European oyster (Ostrea edulis) and Olympia oyster (Ostrea lurida or Ostrea Conchaphila) are more sensitive. However, even a gladiator like the Pacific oyster can only take that much. During extreme cold spells, it will hunker down and reduce its metabolic rate to almost to zero. During extreme heat, it will bank on its prominently cupped shell design, capable of holding much fluid to help maintain a tolerable temperature and moisture level. At a certain point, however, either extreme will get too much for an oyster to bear. Oystermen somberly refer to this point as "winter kills" or "summer kills".
Some oystermen in cold areas such as Germany and Scotland have resorted to bringing all their Pacific oysters "in from the cold" during the winter months. They are then deposited in somewhat warmer water pools inside buildings. Likewise, in the late 1800s, American oystermen on the West Coast, built diked compounds in the intertidal zone of South Puget Sound (around Totten Inlet). These "claires" of sorts, helped their Olympia oysters survive the extended exposure to winter chills, compounded by the seasonally low tides during the night. Maintaining an insulating layer of water above the oysters made all the difference between life and death of the Olympia oysters. There are some striking similarities between these "claires" of Puget Sound and the claires of Marennes-Oléron - although they serve different purposes.
In Marennes-Oléron, however, quite the opposite climatic problem exists. These French oystermen struggle with (occasionally massive) "summer kills". While the hot summer sun delights tourists taking a pleasant dip in the cool Atlantic Ocean, countless oysters in thousands of mesh bags are baking on cultivation racks exposed to the blazing sun, for hours on end, at low tide. These oysters are fighting for their lives. To compound matters, Pacific oysters spawn in the summer months. After spawning, they are weakened considerably and thus particularly vulnerable. The oysters in the claires, covered by an insulating sheet of water, are provided with better protection from the blazing sun. Nonetheless, the claires of Marennes-Oléron solve very little when massive summer kills occasionally strike the oyster population of this region. The principle purpose of claires around Marennes-Oléron is thus not to help oysters survive the occasional ravages of summer heat, but rather to fatten them naturally and greatly improving their taste.