Saccostrea gigas, the Japanese oyster, is ideally suited for cultivation in sea water ponds or in the water from seawater ponds. Major advantages of this oyster are:
- Gigas does well in water with high concentrations of phytoplankton, the normal situation in marine ponds which are growing fish or prawns and which therefore have high nutrient loadings.
- Spat (the juveniles) of gigas are readily available from hatcheries all over the world and are also not too difficult to reproduce in a modest mariculture hatchery.
- Gigas has a ready market and is a fast grower. In phytoplankton rich water it becomes very fat (lots of glycogen stored) and as a result is very tasty.
- Gigas can survive a wide range of temperatures from near freezing up to 30 degrees although it grows fastest and fattest at around 20 degrees.
Table of contents |
2 Cleaning the Trough 3 Self Cleaning Oysters 4 Shell Worm |
Crassostrea produce large amounts of faeces and pseudofaeces when they are in plankton rich water so every few days they will need to be agitated to remove this material. Otherwise they will smother. Further along in this article is a way of growing the oysters so that they are pretty well immune from this build up of sediment.
Experimental Cultivation of gigas
Before investing in pilot studies or commercial growing, financial backers will want to see that oyster actually do grow in the water of seawater ponds. A simple way to demonstrate this is to construct some simple baskets of stiff plastic mesh (netlon for instance). Put in a few tens of small, young gigas and suspend the baskets from the monks (water outlet structures) of the pond. Use mesh of at least 10mm opening so that it doesn't plug up too quickly with marine fouling organisms. A simple way of doing this is to cut two disks of, say, 30cm diameter one for the top and one for the bottom and a strip of, say 10cm wide for the side. Sew up with plastic cord and leave a place where the baskets can be opened to add and remove oysters as necessary. Attach three cords equally spaced around the upper rim, tie them together and attach a cord long enough to suspend baskets in the pond.Oyster Morphology
If you cup your left hand and cover the cup with your right hand which you keep flat, this represents the shells of an oyster. The hinge is at the part of your hand nearest to the wrist and the opening at the ends of your fingers. Imagine your body shrinking until it will fit inside your hands in the same orientation as you are sitting now. The cupped left hand is the left shell of the oyster but since this is the shell that is attached to the substrate (often intertidal rocks in the case of Saccostrea sp.) it is commonly referred to as the lower shell while the flat right shell is commonly called the upper shell. a membrane or skin attaches along the oysters back and lines the shells. This membrane is called the mantle. Oyster Feeding
Oysters feed by means of their gills. They close the gap between the shells with their mantle, leaving an opening where your thumb and little finger are in the above paragraph. Cillia on the gills beat, creating a flow of water in one opening and out the other. The mucus on the gills captures particles and the action of cillia move the mucus with its edible particles along pathways on the gills towards the mouth. Inedible particles are sent along other pathways away from the mouth. From time to time, the oyster closes quickly, blowing out the unwanted particles which are stuck together with mucus. This expelled material is referred to as pseudofaeces. As it is now in larger particles than when it was individual cells of phytoplankton, it sinks faster and forms an organic layer along with ordinary faeces on the bottom of its growing vessel (pond, trough, tank etc.)More Efficient Use of Phytoplankton
Oysters also produce pseudofaeces when exposed to edible particles in high concentrations. This, of course, wastes food that could have been used to grow more oysters. It also produces more grunge which rapidly turns anaerobic and must be cleared more often from the growing vessel. Since marine fish and prawns are fed with large quantities of prepared feed, in sunny areas, where much of this type of cultivation occurs, very rich blooms of phytoplankton develop in the ponds. Any system which presents this food at the correct concentration to the oysters will increase the quantity of oysters that can be grown from a given area of sea water pond. A way which has been used successfully is to build a trough (1m x 1m for instance) which is a little lower than the pond. The water leaving the pond is run through the trough. Instead of letting all the water in one end and out the other, the water is introduced all along the trough. This keeps renewing the phytoplankton as the oysters remove it and ensures that all the oysters are growing in an algae soup which is more dilute in phytoplankton than the water in the pond. Cleaning the Trough
Oysters produce very large amounts of faeces and if in rich water, large amounts of pseudofaeces. After a time this material builds up on the bottom of the trough, becomes anaerobic and begins to produce hydrogen sulphide an ammonia. It is wise to fit the oyster trough with monk boards or some similar system so that the water can be let down completely and the trough flushed out. At the same time the oysters can be washed off although this becomes much less critical with the following growing method.