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A. Fouling Organisms

1. Predators

There are many species of oyster predators such as Sparus swinhonis, Octopus vulgaris and Asteria amurensis, etc. Carnivorous gastropods such as the following drill into the oysters: Purpura clavigora, Purpura bronni, Ocenebra japonicum, Goratostoma founari, and Rapana thomasiana, etc. Drill damage usually occurs from April to November. Greater damages have been observed when water temperature goes up. R. thomasiana causes drill damage all year round even if the water temperature is below 10 degrees Celsius. The damage by Rapana thomasiana had been the most serious (Table 6-1). In a ten-day test, R. thomasiana, P. clavigora and O. japonicum drilled 12, 5 and 2 oysters, respectively.

Predators and their egg sacs should be removed by hand-picking at low tide.

Starfish should be thoroughly removed from the oyster growing area because their revival ability is strong and they feed on oysters all year round.

2. Sessile fouling organisms

There are many species of sessile fouling organisms. Among these are mussels, Hydroides norvegica, sea squirts, flat worms, fungi, diatoms, sponges, seaweeds, hydroids, annelids, and other crustaceans (Figure 6-1). Some are microscopic in size.

Most predators attach themselves to the growing facilities or to oyster shells; others like Polydora, Protella and Caprella species, are food competitors.

Table 6-1. Number of oysters preyed upon during a given time period.
DescriptionThais clavigeraThais bronniChicoreus asionusCeratostoma burnettiRapana venosaStarfish
Number of oyster fed by predator18.114.322.424.461.7208.9
Period (days)307292235236350365
Figure 6-1

Figure 6-1. Typical fouling organisms in oyster culture grounds.
A, Codium sp.; B, Porphyra sp.; C, Ulva sp.; D, flat worm; E, sponge; F, sea anemone; G, Bagula sp.; H, barnacle; I, M. edulis; J. H. norvegica; K, Ciona intestinalis; L, Styela sp.; M, Didemnun noseleyi.

Ecology and life cycle of important sessile fouling organisms

Mytilus edulis (mussel). Though this is native to North Europe, it is now widely distributed over the world. It is prolific. The mussels compete for attachment space and food, and their growth and reproduction may be superior to those of the oyster, This species matures in about one year, and one-year-old spats have the ability to spawn all year round.

Hydroides norvegica. It is one of the Polydora species and lives in its own calcious tube. This species feeds on microscopic food. The larvae develops into the trocophore stage 24 hours after hatching, freely float for a week, and develop into the nectrochaetal stage, when it settles down to attach to some solid substrate. Settlements are abundant in places where water current is slower than 1.8 knots. This species is not resistant to fresh water with salinity below 15%.

Halocynthia spp. Halocynthia spp. are of two types, solitary and colonial. Among them, the Ciona intestinalis does great damage to oyster, competing for space and food. It spawns from April to December, the main season being June to September. C. intestinalis larvae freely float for a month and then cling to some substrate with their adhesive ciliated feed. Immediately after settlement, their anatomy changes drastically.

Balanus sp. The shell of Balanus sp. consists of solid compounds of calcium. If spat collectors are set out too early, great numbers of Balanus spp. with habits similar to those of oysters may be attached.

Balanus has the same life cycle in larval stages as that of shrimps and crabs. It undergoes eight stages of metamorphosis before attaching to solid substrates. The larvae develop from nauplius stage to cypris stage with transparent shell. Cypris stage larvae have a unique organ which is called a cement line between ear and gullet. The larvae secrete a mucous glue and attach themselves firmly to a solid substrate.

2. Appearance of Fouling Organisms and Fluctuation of their Biomass

The sessile fouling organisms that infest cultured oyster include Hydroides spp., Bugula neritina, Ascidians, sponges, Balanus spp., and Mytilus spp. Most of these organisms attach during the period May to September in warm water season, but the period and depth of their attachment are different according to the species.

Figure 6-2

Figure 6-2. Settlement frequency (%) of major fouling organisms in Hansan-Goje Bay in relation to the settlement of the Pacific oyster (C. gigas).

The quantity of attached fouling organisms in oyster farms located in Hansan and Goje Bay differs by species. For example, Ascidians largely settle in May, B. neritina in June to September, massel and sponges in June, and barnacles in September (Figure 6-2).

3. Control of Fouling Organisms

Oyster farmers need to eradicate fouling organisms from oyster spats being harmful and from growing oysters before setting the oyster facilities. Some effective control methods are as follows:

Hot water treatment. Heat up sea water to 55–60 degrees Celsius in a great can or oven on a barge. Soak the strings of oyster in the heated sea water for 10 to 15 seconds. This method is effective in eradicating Mytilus spp., Balanus spp., and Ascidians (Table 6-2).

Freshwater treatment. The method makes use of the osmotic pressure of organisms; in effect, the lesser density fresh water “drains out” the “thicker” body fluid of the organism, thus killing it. Soak the strings of oyster in stream water or in a tank of fresh water. Soaking should be at least for 50 hours in 15 to 20 degrees Celsius of water temperature, or 30 hours in 20– 25 degrees Celsius. Hydroides is easy to eradicate by this method (Table 6-3).

Table 6-2. Comparison of lethality (%) between oyster (C. gigas) and mussel (M. edulis) under different heat treatment.
Treated time 

Water temperature

( ) : Shell height in centimeter.

Table 6-3. Lethality of H. norvegica in relation to fresh water treatment.
Soaking time (min.)Lethality (%)

B. Diseases

The more oysters are cultured, the more disease problems encountered. It is difficult to find out the cause of various diseases affecting oyster farms. Researchers have not clearly pinpointed the causes. In 1960, a considerable number of oysters died of an unknown disease in Chesapeake Bay, USA. It took almost ten years for researchers to confirm the pathogens, a parasite known as Michinia nelsoni and a mold known as Labyrinthomyxa marina.

The classification of oyster diseases, as reported by researchers, is as follows:

1. Viral diseases

Herpes virus diseases. The digestive organ of oyster infected with this virus usually changes into white colour and the oyster dies of the herpes virus. The shape of the virus is hexagon and its diameter ranges from 70 to 90 mm.

In an experiment, a great number of oysters reared in water temperature of 12 to 18 degrees Celsius died when transferred to water of 28–30 degrees Celsius.

The following methods are used to prevent virus diseases:

  1. Water temperature of oyster growing area must not rise above 27 degrees Celsius.

  2. Infected oysters should be removed from the oyster farm.

2. Bacterial diseases

Some marine microorganisms have also been known to cause oyster diseases. Researchers have carried out experiments which showed microorganisms to be a major cause of high shellfish mortalities.

In 1967, American conchologist Colwell succeeded in making pure isolation of Pseudomonas enalia which is believed to have caused mass mortality in young oysters. In 1977, Sindermann reported that Vibrio anguillarium and V. angullarum-like species were the causes of severe mortality in young oyster.

In general, a deterioration of environmental conditions in oyster grounds makes it easier for these disease-causing microorgansms to infect oysters,.

Figure 6-3

Fig. 6-3. Bacterial Foci (Focal Necrosis) from Pacific oyster.

Figure 6-4

Fig. 6-4. Bacillary necrosis of oyster larvae, showing typical bacterial swarming.

In 1953, when a large number of cultured oysters died at Hiroshima Bay, gram-negative bacteria showed an incidence of 22 percent; this incidence was the highest among. all microorganisms found in the dead oysters. It was presumed to be Achromobacter sp. although the organism was not clearly identified (Figures 6-3, 6-4).

3. Protozoa

Sporozoa. Among oyster diseases, the damage caused by sporozoa and Minchinia nelsoni are very serious. From 1957 to 1960, 95 percent of all oysters in the U.S. east coast died from Minchinia nelsoni infection. The pathogen was not identified until about four years after the incident. Researchers named it as MSX (multinucleate sphere unknown) due to the presence of many nuclei.

Infected oysters in an oyster farm were easily picked out at that time. It was also easy to determine whether they were infected through microscopic test.

In oyster farms in the U.S.A., Minchinia nelsoni frequently appear at low water salinity conditions of around 15% and in sheltered waters of high temperatures. On the other hand, Minchinia costalis is frequently observed in high salinity waters.

A sporozoan similar to this organism is occasionally observed in Korea.

Egg disease. The egg disease occurs only in Crassostrea gigas, its causal organism having been thought to be an amoeba (Chun, 1979). Recently however, the pathogen has been identified as Marteilioides chungmuensis (Comps et al, 1986) of Phylum Ascetospora. In Korea, this disease chiefly appears from August to November.

The parasites attach to the eyes of the oyster. The adult is 10 to 12 um in length (Figure 6-5).

Figure 6-5

Figure 6-5. Infected ovum of Pacific oyster (C. gigas) with Marteilioides chungmuensis (p = parasite).

C. Eutrophication of Oyster Farm

The productivity of oyster varies with the location and conditions of the ongrowing ground.

If the oysters are cultured fairly densely, productivity of the oyster ground could decrease and certain environmental factors that favour growth and fattening may disappear. Oyster ground deterioration is mainly caused by the waste products from the dense oysters and other fouling organisms. Spoilage caused by fouling organisms also affect the water quality of an oyster growing area. Toxic gases such as ammonia and hydrogen sulfide are produced by microbial decomposition of fouling organisms. Deterioration of oyster grounds may be more serious during summer months.

D. Red Tide

Red Tide is caused by a sudden increase in the population of certain planktonic microalgae. These organisms exhaust the dissolved oxygen so that the red tide is extremely harmful to fish-growing areas. Recently, adjacent sea waters have been polluted by industries on the southern seacoast of Korea. Water pollution may also lead to the development of red tide. As a matter of fact, red tide has occurred often in the Chinhae-Masan Bay in the southern part of Korea.

The planktons which are related with red tide are Chaetoceros, Gymnodinium, Gonyaulax, Ceratium, Peridinium, Prorocentrumn, Noctiluca, ciliate, etc. (Figure 6-6).

Among these, the phytoplankton Gymnidium is poisonous to other living organisms, including oyster by releasing toxins. At present, there are no known methods to adequately protect fishing and culture grounds from red tide. Measures should be tried in order to lessen the damage from red tide, but unfortunately, the best that can be done at present is to prevent water pollution.

Figure 6-6

Figure 6-6. The causative organisms of red tide.

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