Until recently, people simply grew bivalves. In spite of the example of agriculture where over the past several thousand years selective breeding and genetics have produced plants and animals that are far superior to the original wild plants and animals, little selective breeding has been undertaken in bivalve culture operations. This has been due in large measure to the method of culture. Juveniles for most bivalve culture operations are obtained from natural sets and collected from natural breeding areas. They are planted in areas selected for good growth and then the crop is harvested when it reaches commercial size. Bivalves cultured over an extensive area essentially come from the same source and form one large genetic pool. Seed bivalves, whether produced in hatcheries or from natural sets, are frequently transported over considerable distances and even to different countries so that the same gene pool can extend over a wide geographic range. Any regionally distinctive strains or races that might have developed in the past have quickly disappeared in the general gene pool. Development of genetic strains under such circumstances was difficult if not impossible and attempts made to undertake local breeding work were minor.
Studies of population genetics of some species of bivalves have been made. A focus of these studies has been to determine if different sub-populations, races or strains of these species exist throughout the animal’s distribution. Results indicate that subpopulations of some bivalves do exist within their range and this raised the question of whether juveniles from one sub-population should be transferred into areas with a different sub-population. It also raises the question of whether animals from one subpopulation would perform better if transferred to the area of another sub-population. Population genetic studies have also included assessment of some bivalve populations that over time have become isolated from the parent stock to determine if significant differences now exist in the two populations. A good example is populations of Pacific oysters along the west coast of North America compared to those in Japan where the North American stock originated. Results of these studies indicate that little if any genetic drift has occurred in these widely separated populations.
Our knowledge and interest in the field of bivalve genetics and the potential it has in culture operations has increased greatly in the past twenty years because of two factors; development of hatcheries and the advent of technology in the field of genetics, e.g. electrophoresis used to examine genetic variation. With the development of bivalve hatcheries it has become possible to undertake selective breeding programs to develop strains or races of bivalves. There is considerable interest in developing strains of bivalves that are better suited to particular growout conditions than the original stock. A further impetus for development of bivalve genetic programs has been the production of strains of oysters that are resistant to the devastating diseases that have decimated stocks in North America and Europe.
The field of bivalve genetics is highly complex and technical and a thorough discussion of work being done currently in the field is not appropriate for this publication. The intent here is to briefly mention the scope of work being done and its implications for hatchery production in the future. A list of reading material is given in section 7.3 to provide the reader with further information on the subject.
One area of bivalve genetics that has been investigated and is now widely practiced is polyploidy, particularly the production of animals that are triploids (3n). Although triploid scallops, clams and mussels have been produced, most work has centred on the production of triploid oysters and in particular triploid Pacific oysters.
Interest in developing the technology for the production of triploid oysters on the Pacific coast of North America arose for two reasons. First there was the desire by industry to have an oyster that was of good eating quality throughout the year in order to maintain and extend the marketing season. Gonads of Pacific oysters can occupy up to 50% of the weight of the soft body parts. When glycogen is converted into gametes in the spring the oysters become unpalatable and after spawning the soft parts become emaciated and watery. Both states render the product unsuitable for marketing. Secondly, if spawning could be avoided there was possibility of reducing mortalities due to the so called "summer disease" which is believed to result in part from physiological stress at the time of breeding. If transformation of glycogen into gametes can be prevented by growing triploid oysters, it is conceivable that mortalities could be significantly reduced.
Triploids are produced by preventing the egg undergoing meiosis so that it remains in the diploid (2n) state. When such an egg is fertilized by sperm in the 1n (haploid) stage the result is a triploid (Figure 109).
Bivalve eggs can be prevented from undergoing meiosis to the 1n state by subjecting them to pressure, heat or chemicals. Originally most triploids were produced by treating eggs with a chemical, cytochalasin B. Eggs were stripped from females and fertilized with sperm. Gametes were kept separate until ready to fertilize so that the process could be closely controlled. After the first polar body appeared, the fertilized eggs were treated with cytochalasin B preventing the eggs from undergoing meiosis. Thus, the eggs remained in the 2n state and with the contribution of the male chromosome set, the result was a triploid embryo. The technique was perfected over time so that success rate in producing triploids was about 90%.
There are two problems in producing triploids by this method. The first is that it does not produce 100% triploids. The second is that the chemical cytochalasin B is carcinogenic and although it is only used in the fertilization of the animals and hence poses little possibility of carrying over a toxic effect, there has been concern of repercussions from the public. The chemical method to produce triploid oysters is no longer generally used in hatcheries.
Figure 109: Representation of the process of triploidy induction.
The method now used by some hatcheries is heat shock. Fertilized eggs, normally held at 25°C, are suddenly subjected to a temperature of 32°C for two minutes and then are returned to 25°C. The temperature shock is applied after the emission of the first polar body, about twenty minutes after fertilization. Again this method has been perfected and the success rate in producing triploids is about the same as with the chemical method, i.e. averaging about 90%.
Both the chemical and heat shock methods are effective but the main disadvantage of both is that 100% triploids are rarely, if ever, achieved. A method was needed that could consistently produce 100% triploids with each breeding.
Research in both Europe and the United States has led to development of methods to produce tetraploid (4n) oysters. To date only male tetraploids have been produced and the method is proprietary so few details of the methods can be given. Arrangements can however be made with companies that produce tetraploids to obtain them for use in hatcheries as broodstock. When mated with diploid oysters they always produce triploids. The method is effective and will probably be employed widely by the hatchery and growout industry as tetraploids become more readily available.
On the Pacific coast of the United States a major portion of the production of juvenile Pacific oysters in hatcheries is now triploids.
Results of polyploidy work have been significant and work in the field will continue, but the real advantage to hatcheries will be in other fields of genetics, e.g. quantitative genetics, which includes selective breeding, and molecular genetics, focussing on the actual genotype of the individual animal. Most people in the industry are expressing interest in the potential arising from selective breeding programmes. The possibility exists to develop disease resistant strains and bivalves which grow faster, produce more meat per animal and are able to grow quickly at higher or lower temperatures. It should now be possible in aquaculture to approach the example of agriculture where it is estimated there has been a 30% increase in the efficiency in producing protein since 1900 from genetic improvements alone.
Research work in bivalve genetics is being carried out at several institutions in different parts of the world. Most studies involve oysters since this is the animal of most concern to the industry, but research is also being undertaken with other bivalve species. These studies not only focus on producing improved strains of bivalves but they are also concerned with the preservation of the gene pool of original natural populations in the event that such stocks are required for future work.
The goal of much of the research is to improve both the yield per recruit and survival including resistance to disease. There have already been encouraging results. Improvements in the live weight of mass selected Sydney rock oysters, Saccostrea commercialis, have been 4% and 18% after one and two generations of selection compared to non-selected controls. A 16% to 39% increase in growth rate was found after one generation of mass selection in the eastern oyster, C. virginica and a 21% to 42% increase in growth rate of the European flat oyster, O. edulis, compared with non-selected controls. Similarly an increase of about 10% was found in live weight of Pacific oysters, C. gigas, after one generation in selected lines compared with nonselected controls. Increases in the resistance of eastern oysters to MSX (Haplosporidium nelsoni) have also been reported through selection.
Selected brood-lines of some species of oysters are now established in some countries in the world and work continues to improve them. It is not unrealistic to believe that further selection with these lines will lead to even greater improvements and that eventually the selected stocks will become generally available to hatcheries for use in producing seed stock. One institution on the west coast of the USA is now actively seeking input from industry as to what characteristics industry wishes to have in oysters so that they can begin to incorporate them in specific brood-lines. The possibility of producing a brand name oyster is now not beyond the realms of possibility.
An interesting development in oyster breeding occurred in a programme on the Pacific coast of the United States. The Kumomoto oyster, Crassostrea sikamea, was virtually exterminated in its original location in southern Japan. Populations of this species were imported to the west coast of the United States but their gene pool had become contaminated with the Pacific oyster, C. gigas. Breeding work at a hatchery facility has enabled production of Kumomoto oyster stocks that breed true and can be used for culture in the USA. They could also be used to re-introduce the species to Japan.
Research in the field of molecular genetics and in modifying specific genes is in its infancy with bivalves. It is a more controversial field compared with selective breeding but advances made in molecular genetics in agriculture are impressive and similar results with bivalves could lead to important advances in production. Research on genetically modified bivalves is being undertaken at several institutions in the world but it will probably be many years before results in this field are considered for application in commercial bivalve hatcheries.
Most research in bivalve genetics is currently being undertaken at university or government facilities. The research is expensive, requires highly trained personnel, considerable space for holding selected lines and may take many years to yield results. Genetic programs must be carefully planned and proper protocols observed or serious problems can arise. Sufficient broodstock must be used in breeding otherwise problems with inbreeding depression may occur. Before any breeding work is undertaken in the field of genetic improvement, goals must be set and mating schemes and proper broodstock selected. Most commercial hatcheries don’t have the time or resources to undertake such long-term programmes, however, they can be active participants.
Improved strains could be developed at commercial hatcheries jointly with research institutes, which could then be mass produced for sale to growers. Certainly, in planning the construction of a hatchery, the need for facilities to carry out genetic work should be kept in mind and incorporated in building plans. With the ability to ship eyed larvae successfully over great distances, larvae of improved strains could be transported anywhere in the world for remote setting and subsequent growout.
The role of genetics in bivalve culture is in its infancy and undoubtedly will become more important to culture operations in the future. Bivalves with faster growth rates, resistance to disease, variously coloured soft parts, oysters with deeper cups, etc. will become a reality in the near future. It will no longer be common practise to simply culture a species of bivalve. Carefully selected strains or breeds will be farmed to produce a specific product to be marketed as a particular brand. The field of bivalve genetics probably offers the best scope to increase production in culture operations throughout the world and every opportunity should be given to encourage research and development in this exciting field.
The increasing demand for seafood, including bivalves, will undoubtedly continue in the future and production will need to be increased to meet this demand. Supply is unlikely to increase significantly from traditional bivalve fisheries since most natural stocks are being harvested at or near maximum rates. Any significant increases in supply will likely come from aquaculture. Indeed, the present goal of many culture operations is to restore populations to levels that existed prior to over-harvesting. Future culture operations will need to be as efficient as possible, not only for reasons of economical viability, but to make optimum use of production areas that will come under ever increasing pressure from human activities and may even be reduced as human populations continue to increase.
Any future increases in bivalve production will require an increase in seed supply that must be reliable, plentiful and inexpensive. Collection of juveniles from natural sets will continue to be important but such areas are limited. Major increases in seed supply will be from hatcheries. There are added advantages in producing seed in hatcheries over the collection of natural sets including reliability, the capability to supply to meet demand and the ability to provide seed of selected strains, along with seed of exotic species.
Continuing research and development will improve hatchery technology and make them more efficient and hence more profitable. There are many areas where research is needed and some have already been mentioned in the text.
Improvements in nutrition are needed to produce healthy larvae that will metamorphose into healthy spat and can be grown quickly and economically to market size. Producing algae to feed the larvae and juveniles is a major cost in operating a hatchery. This expense could be greatly reduced if artificial diets of equal nutritional value to the best algal species could be formulated. Studies have been made but to date, although progress has been made, a satisfactory product is not available for sale. One of the obstacles is the size of market for such products which, at this time, is not large enough to attract investment in development by the major feed manufacturers.
For bivalve aquaculture to fully achieve its potential it must follow the methods of agriculture. This will require extensive research programmes for all phases of production. One of the most important fields for future research already discussed in section 7.1 is genetics where perhaps the greatest gain will be from the development of strains and varieties of bivalves that are suited to particular environments. This requires extensive research in the selection of brood-lines. Once stains are established they can only effectively be maintained by breeding them in hatcheries. A major goal for hatcheries will be to improve technology so that seed from such strains can be supplied to growers on demand and as inexpensively as possible.
Some developments in the field of genetics such as the production of triploid oysters have already been of major benefit to the industry, particularly the oyster industry on the west coast of North America. Continued improvements in polyploidy will ensure that a reliable supply of triploid seed of any desired bivalve species is available to industry.
Figure 110: A - a device for exerting pressure on eggs to prevent chromosomal reduction through the suppression of meiosis. B - experiments in the cryopreservation of bivalve gametes and larvae.
Developments in cryopreservation technology for male and female gametes and even larvae will be of great benefit to hatcheries since gametes could be obtained when adults are in prime condition and stored for future use. Space and time needed to condition adults and the requirement to produce large quantities of food to keep adults in prime breeding condition could all be eliminated. Fertilization of thawed gametes could be effected in a short time period whenever desired. Progress has been made in this field but at present it is costly and beyond the scope of hatcheries to utilize the technology in-house (Figure 110B).
Siting of hatcheries will assume greater importance in the future. The advent and success of remote setting methods demonstrates that hatcheries do not need to be situated close to growout operations. With modern trade networks they can be located where ideal conditions exist to rear larvae and juveniles and then be transported over great distances to growout sites with virtually 100% survival. A case in point is provided by the practice of some hatcheries in the State of Washington in the USA. They have transferred part of their hatchery operations to Hawaii where a source of nutrient rich water requiring little if any heating is available year round. The abundant sunshine in Hawaii is used to culture algae. It is cheaper to transport mature larvae and juveniles from Hawaii to Washington State than it is to heat water and grow algae there.
Large hatcheries with highly trained staff can be operated efficiently and produce seed more economically than smaller ones. Economies of scale apply. If hatcheries are equipped with quarantine facilities they can produce seed of any commercially valuable species from any part of the world without major risk of introducing exotics to the local environment. Since larvae are generally cultured in water filtered to 1 µm, which could be treated with UV-light or ozone, the danger of transferring pests, parasites and diseases from one area to another is greatly reduced. This applies to the shipping of eyed larvae compared to shipping juveniles that have been exposed to the open environment in the area of origin.
Large hatcheries could supply metamorphically competent larvae of any bivalve species anywhere it is needed in the world. This is the practice that agriculture has adopted. Seed required in many growing operations is often produced at great distances from where it is eventually planted. Similarly, many juvenile animals are often not produced where they are eventually raised.
It is necessary to get over a parochial attitude in bivalve culture and realize that the industry exists in a global economy. It is no longer essential for every area or even every country to have a bivalve hatchery to supply seed needed to meet local growout requirements. One well-placed, well equipped and well staffed hatchery can supply the seed requirements for many culture operations in many different parts of the world.
A possible major problem for hatcheries will be diseases as it is when any organism is mass cultured intensively. Future research needs to include the development of methods to control diseases in hatcheries so as to minimize instances of mass mortalities caused by either obligate or opportunistic pathogens. Results of genetic research are likely to be of value in selecting strains of bivalves that are more resistant to disease. Research is also required to develop inexpensive and effective treatments should diseases occur in a hatchery situation.
Future bivalve landings will undoubtedly continue to increase to meet the demands of an ever increasing human population. Most of this increase in production will be from culture operations and this will require the availability of large quantities of juveniles (seed) to meet culture demands. While collection of seed from natural sources will remain important, most of the seed needed for increased production will be from hatcheries. This is particularly true as the industry begins to demand strains or races of bivalves that are developed to grow in specific areas. Hatcheries will eventually become the mainstay of seed production for bivalve growout operations. In the future every effort must be made to improve hatchery technology to enable them to supply an abundant, reliable and inexpensive supply of juvenile bivalves for the culture industry.
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