Bivalve larvae and newly set spat can be affected by diseases which may cause severe mortalities. Unfortunately, there has been little research on the treatment and prevention of bivalve diseases, but a few general concepts seem to be true.
Disease in bivalve larvae, as in other cultured aquatic organism, results from stress. Larvae may be stressed by poor water quality, underfeeding, overfeeding, crowding and temperature extremes, bacterial toxins or by algal metabolites.
Most mortalities occurring in the hatchery and grow out phases are associated with high bacterial counts. Whether or not the bacteria are opportunistic saprophytes (bacteria whose numbers increase because bivalves are dying from other causes) or pathogenic is not important.
The first line of defence against disease is to ensure high water quality. Proper feeding and rearing density are easily controlled by the hatchery operator. Care should be taken to see that only algal cultures in good condition are introduced into the culture vessels. Proper facility design and location can avoid temperature problems.
Figure 14. Land-based nurseries for rearing of juvenile bivalve spat. (Source: CNEXO, 1983).
Figure 15. Passive flow (A) and active flow (B) up-flow systems. (Source: Manzi, 1985).
Diseases of larvae are generally quite dramatic. An infected group of larvae can be reduced to a few survivors in just a few days. Worse still, the infection may spread throughout the culture facility, greatly reducing the output.
Approximately once a day, and certainly no less than once every other day, the larvae should be inspected with a microscope. Active, swimming, well developed veligers with good clean velum are healthy larvae. Good color and fast growth rate also indicate healthy larvae.
Sick larvae exhibit slow growth, pale colouration, weak or no swimming activity (and finally sink to the bottom), often have debris attached to the velum and almost always have a high number of protozoans swimming around them. Protozoans feed on bacteria and are often the best indicators of bacteria infestations. At times the protozoans will even be inside the shells of infected larvae.
Larvae grown at low temperatures (10–20 °C) sometimes have fungal infections. These infections may appear as finger-like projections in the tissue, or the thread-like fungal mycelium may protrude from the shell.
Immediate action should be taken if the larvae display symptoms of disease. Sick larvae should be discarded, the containers and sieves cleaned and a new batch started. If a decision is made to save the larvae (usually because the infection appears to be mild), the following steps should be taken.
Drain the culture and size the larvae through several sieves. Larvae on the smallest size sieve should be discarded as they are slow growing probably because sick, moribund or even dead.
Rinse the larvae to be retained with generous amounts of filtered seawater. Fill a carefully cleaned container with new filtered seawater. If a short wave length ultraviolet light is available, the new filtered seawater should be UV-treated prior to adding the larvae. The used of antibiotics should be done only if necessary, mainly because of costs and inherent dangers of using medications.
Ultraviolet light can be used to sterilize seawater in a hatchery. Ultraviolet light will greatly reduce the number of bacteria, but will also kill most planktonic organisms including bivalve larvae and phytoplankton. Larvae cannot be exposed to ultraviolet light.
Ultraviolet light is a good bactericide, with some limitations. It penetrates water only a few millimeters, and is less effective in water containing suspended sediment. Water should be filtered or centrifuged to reduce the suspended particulate matter.
Fungi are also a common disease in bivalve hatcheries. Fungus may be brought in by untreated water or outdoor algal cultures. Ultraviolet has been found very effective in the control of fungus, whether applied to filtered seawater or algal cultures.
Fungal infections can also be treated by increasing the temperature of the culture, since most fungicides are lethal to larvae. Temperatures as high as 29–34 °C for 12–24 hours may be used to treat fungal infections.
The use of antibiotics in a large hatchery is uneconomical and a diseased batch is usually discarded. However, antibiotics are useful in smaller facilities or in a research environment.
Among those which have been tested and found to be effective are sulmet (sulfamerazine), Combistrep, Chlortetracycline HCl and aureomycin. Larvae of different species show varying degrees of sensitivity to a given antibiotic. The table below lists some commonly used antibiotics, the species they have been tested on, their effective concentrations, and toxicity, where known.
Treatment can be administered conveniently while the larvae are concentrated in calibrated containers during a water change. More stubborn infections can be treated with a wide spectrum antibiotic, such as chloramphenicol sodium succinate (trade name, chloromycetin; 10 mg of antibiotic per litre of water containing the larvae). After 1–2 hours, the antibiotic should be washed away by collecting the larvae on an appropriate sieve and rinsing them with filtered seawater prior returning them to a clean tank.
|Antibiotic||Species Tested||Effective Concentration||Toxic Concentration|
|Streptomycin||M. mercenaria||100 ppm|
|Combistrep||M. mercenaria||100 ppm|
|C. virginica||100 ppm|
|Sulmet||M. mercenaria||33 ppm|
|C. virginica||33 ppm|
|Chlortetra-cycline||M. mercenaria||10 ppm|
|Aureomycin||M. mercenaria||3 ppm|
|Chloromycetin||M. mercenaria||20 ppm|
|C. virginica||20 ppm|
|Retarded growth of larvae at all concentrations tested.|
Indiscriminate use of antibiotics or using antibiotics as a disease preventive should be avoided. There is an ever present danger of an antibiotic-resistant bacteria developing in the hatchery. Antibiotics should not be used on eggs or embryos, as they may prevent development.
Other methods presently used to sterilize both seawater and containers are chemical sterilization (followed by neutralization) using acid or sodium hypochlorite. Chemical sterilization is commonly used in phytoplankton culture before inoculation of the sterilized water with algae.
The acid method is as follows: (1) fill a container with seawater and add sufficient hydrochloric acid to lower the pH to 3.9, (2) allow the treated water to stand for 4 hours and (3) add an alkaline compound such as nitrate or sodium bicarbonate to raise the pH to about 6.5. This method works very well for algal culture containers because the 3.9 Ph can be raised to 6.5 with sodium nitrate. This is a fertilizer commonly used in growth media for algae. Then the water can be inoculated with the chosen species of algae. Bacteria-free (axenic) cultures of algae can be maintained if the preceding steps have been done properly.
Large volumes of seawater and their containers can be sterilized using liquid home laundry bleach (sodium hypochlorite) such as Cloroxγ. This method is as follows: (1) add about 0.5 ml bleach per litre of seawater to be sterilized, (2) use disposable ATI Chlorine Water Chexγ to test the water to verify that the residual is about 10 ppm (indicated by a color change), add more sodium hypochlorite if necessary, (3) add stoppers and hoses, etc. to the container and draw water through them so that entire unit is wet and exposed to the hypochlorite solution, (4) tip or invert containers to make sure all surfaces are contacted by the solution so that they will be sterilized, (5) draw some chlorinated seawater into the air line, (6) after 2–4 hours, dechlorinate using 0.1–0.15 ml of normal sodium thiosulfate solution per litre of chlorinated seawater, (7) test with ATI Chlorine Water Chexγ to make sure the chlorine is totally inactivated, indicated by no color change (if not, add more sodium thiosulfate and test again) and (8) aerate the chlorinated/dechlorinated seawater before use.
A routine washing and drying of all equipment is often the best means of preventing infection and insuring successful culture of larvae. Between uses, wash all equipment with freshwater and a biodegradable detergent. Rinse the equipment thoroughly in cold freshwater, followed with a hot freshwater rinse and allow the items to dry completely. Sieves and hoses, if left damp, harbors bacteria. Be sure they are stored so that they will drain and dry completely. Proper air circulation will aid drying. Filters make excellent substrates for bacterial cultures; therefore, special care must be taken to either wash, sterilize or dispose of filters. Generally, if an item is washed with freshwater and then dried, bacteria will not develop on its surface.
Brushes or mops used for cleaning larval containers should not be used to clean anything else. Larval containers should not be used for anything else. When disease strikes a hatchery, it is necessary to close for a thorough cleaning of all the equipment. Be sure to include pumps, pipes and containers.
It is essential to clean the water system regularly with hypochlorite solution. Either a liquid solution of sodium hypochlorite can be used. The hypochlorite solution is flushed through the system and allowed to stand overnight. Before using the system, be sure all the hypochlorite has been flushed out. This is best determined with a chlorine test kit.