I. The Hatchery and Oyster Propagation (Caution: heavy biology)
Individual bivalves (oysters and clams) being members of the animal kingdom develop as either males or females each summer season. An individual oyster can, however, change from one sex to the other from one year to the next. The female oysters and clams have very small eggs with a diameter of 50 to 90 microns (50 to 90 one thousandth of a millimeter). A medium size female oyster has tens of millions of eggs. A small thimble can hold about one million eggs. The male sperm is rather traditional in size with a length of about ten microns.
Fertilization of the eggs takes place when the female eggs and male sperm are released into the water. The spawning of either sex will stimulate their opposites to release the reproductive products. Very often a general spawning in the bay will result in nearly all the oysters releasing the eggs or sperm. However, in the hatcheries the spawnings are controlled by such means as temperature thus assuring a supply of larvae during the spring and summer season.
Larval development after fertilization proceeds through several stages which are defined on the morphology (shape) of the larvae. The single celled egg starts to divide about two hours after fertilization. Within about six hours the multi celled egg develops cilia (hair-like structures) and is now capable of locomotion within the water column. The larvae at this point are in the trochosphere stage which will last for less than one day. In the second day the multi celled larvae, which still has not increased much in diameter from the egg develops the two very thin shells (termed valves) which characterize all bivalve members of the mollusca. The shell in this early stage of development is D shaped with the straight portion of the D being the hinge line. These are now known as D-hinged larvae. From this stage and until they take up the sedentary life either attached (e.g. oysters) or in the substrate (e.g. clams) these larvae move through the water with the ciliated organ called a velum (Figure 1). This important structure has other functions such as gathering in the small single celled algae as a food source. The overall term for all bivalve larvae due to all having a velum is veliger larvae.
In about the fourth day (development time varies based many factors such as food and temperature) the bivalve larvae takes more of an adult shape with the hinge area becoming more of a rounded location (called an umbo) connecting the two valves (shells). The larvae have now reached the early stage called an umbo larvae. Actually we could call this point of development an early umbo veliger larvae.
Oyster veliger larvae measuring about 0.3 mm
or about 12/1000 inches from umbo to velum
Oyster and clam larvae will continue to grow with this basic morphology until they develop certain additional structures which will be critical to their settling or setting. The oyster veliger larvae show the major anatomical parts of this complex swimming animal. Note the small size of the larvae - about the diameter of the period at the end of the sentence. The clam larvae (below) which measure around 200 microns (0.2 millimeter or around 0.01 inch), have developed their foot and spend various amounts of time crawling on the bottom. Oyster larvae grow larger reaching about 330 microns ( 1/3 millimeter or 1/75th of an inch or 0.013 in). The oyster and clam larvae use their cement gland to attach themselves to a favorable substrate after using their foot to search out this spot. After attaching the larvae get rid of the velum and in the case of oysters also the foot. Most adult clams retain the foot.
One swimming clam larvae on the right shows the velum extended and being used to propel the larvae through the water. Another larvae has the foot extended. Oyster and clam larvae use their foot to crawl over the substrate or object (old shells) before they settle down and attach themselves.
Shellfish larvae grown in the various hatcheries go through the growth stages above. However, in the hatchery the concentrated larvae in large fiberglass tanks must have supplied to them the necessary conditions and nutrients for growth. This is not an easy task as mother nature is often hard to duplicate. One of the more time consuming and often unpredictable requirements for growing larvae is the production of their food.
To this point we really have not disclosed what both the larvae and adult oysters and clams eat. Algae is their major source of nutrition. In the hatchery the algae has to be grown and when out in the marine waters the natural algae are the source of growth. Shellfish larvae need large volumes of single celled algae (phytoplankton). The upper picture shows pure stocks of algae to start cultures with and the lower image is of a few drops of bay water showing the many different types of mainly diatoms present in a volume of a few drops. The scale in the microscope across the field of view is one millimeter.
Larvae at various growth sizes require different sizes of phytoplankton. The small cell size of these marine algae species usually range from 5 to 15 microns. Human red blood cells fall into this size category. Various species of diatoms are one of the most cultured for the purpose of feeding to the growing larvae. The many hatcheries utilize a variety of methods in growing these algal cultures but have in common the need for clean starting cultures containing cells of a specific algal species. Since the algae are plants the requirements for culture are familiar to most. They need their salt water media, nutrients such as nitrogen, phosphorus, potassium, carbon dioxide, trace elements and be pampered in the bacteria free water within the correct temperature range.
The Nursery and First Year of the Small Oyster or Clam >>