Major problems in pearl oysters farming are caused by biofouling organisms which settle and grow on the oyster shells, by boring organisms which riddle through the shells making them weak and friable, and by predators which feed on the oysters. Singly or in combination, these organisms can cause heavy mortality to the farm stock through physiological stress and diseases. The removal of foulers, borers and predators is a labour intensive activity. The seasonal variations of the dominant fouling organisms and predators have to be carefully investigated, and suitable techniques for their control should be adopted on a periodical basis.
The cirriped Balanus amphitrite variegatus is one of the major fouling organism. Two peaks of heavy settlement of these organism, particularly in the culture sites, have been recorded in India: one from mid-June to August and the other from September to November. In the farm at Veppalodai a minimum of 2,500 barnacles (July) to a maximum of 3,460 barnacles (June) have been recorded on an area of 25 square cm during the first peak, while 1,290 (September) to 2,700 (November) in the second peak. The settlement was considerably less from January to May. Heavy settlement of barnacles cause physical obstruction to the opening and closing of the oyster valves. When the barnacles were dense they completely cover the entire surface of shell valves (Plate VI A and B). In addition to this, during the removal of barnacles, the shell margins were also damaged resulting in the recession of shell growth.
Ascidia depressiuscula, Dicarpa sp., compound ascidians belonging to the genus Diplosoma and, species of Botrilloides have been recorded throughout the year. Ascidians are particularly found in large numbers during the period from October to December.
PLATE VI. (A) Fouling organisms on adult ovsters and rearing cage and (B) Oysters heavily encrusted with barnacles.
Species of Membranipora, Thalamoporella and Lagenipora represent the group almost throughout the year. The peak period of occurrence is usually between November and December. Other species, such as Watersipora and Bugula are more commonly found during February and June.
Among the fouling molluscs, Avicula vexillum and spat of Crassostrea sp. are numerous on the farm during April to June. The spatfall of Avicula vexillum can be so numerous in the rearing cages, that the pearl oyster spat cannot be easily separated without causing damage to or killing the spat. Modiolus metcalfei is another common fouling mollusc, usually occurring in July. The spatfall of Pinctada sp. has been recorded in the farm during May-July and November-January. The settlement of these organisms can significantly affect the culture of the mother oysters as well as the production of the cultured pearls.
The profuse growth of sponges such as Callyspongia fibrosa and Haliclona exigua may result in the complete covering of an individual oyster or a cluster of oysters. The occurrence frequency of these sponges is usually low at the present Indian farm site and the damage caused to the oysters is negligible.
Besides the above mentioned groups, the fouling community may be composed of a large number of other organisms such as amphipods, hydroids and algae typically occurring in June, October and December. Major hydroids belong to genera such as Campanularia, Sertularia, Abeitinaria, Lytocarpus, Diphasia and Thuiaria. Commonly occurring algae are Gracilaria, Codium, Boergesenia and Ceramium. Other organisms such as anthozoans, juveniles of Panulirus sp., crabs, tubicolous polychaetes, Pycnogonids, polyclad worms, crinoids, alcyonarians, opisthobranchs, blennid fishes and Pinna spat may occur on the oysters and rearing facilities in certain months of the year.
Boring organisms comprising polychaetes, sponges, molluscs and isopods may cause considerable damage to the pearl oyster shell.
Polychaetes belonging to the families Sillinidae, Nereidae, Spionidae, Terebellidae and Cerratulidae have been found to bore pearl oyster shells. Among them the spionids Polydora ciliata and P. flava and the cirratulid Cirratulus cirratus are the most common borers. Polydora spp. typically caused simple and compound blisters on the inner side of the oyster shells. In a few cases, the blisters erupted as tumour-like protrusions, mostly near the adductor impression. Blister formation by boring polychaetes in oysters of 40 mm in length and less is usually less than in large oyster specimens. The cirratulid Cirratulus cirratus is found in furrows between the layers of periostracum of the pearl oyster shell. As a result, the furrow eventually becomes deeper and wider causing the peeling of the periostracal layer thus weakening the shell.
Major boring sponges are Cliona celata, C. vastifica and C. margaritifera. In several Indian farms over 20 % of the oysters have been found to be infected by such sponge. These borers initially attach themselves near the umbo region of the shell and later spread over the surface of the two valves. Oysters affected by these sponges have to secrete more nacre in order to seal off the perforations. With heavy infestation the oyster shell usually becomes extremely fragile and susceptible to further infestation and damage (Plate VI C).
The pholadid bivalve Martesia sp., the mytilid Lithophaga sp. and, the isopod Sphaeroma sp. are occasionally found in the culture farms. Martesia sp. has been found to make a significant number of holes on the oyster shells.
Besides fouling and boring, predation is another menace encountered in pearl culture farms as well as in natural pearl oyster beds. Predators in wild beds are mainly benthic fish which feed on young oysters below one year of age, while rays, octopods and starfish feed on adult oysters. Recently, Cymatium cingulatum (Plate VI D) and Murex virgeneus have been found to be serious predators in natural oyster beds. A study on the feeding rate of Cymatium cingulatum in the laboratory showed that 20 oysters were consumed by two Cymatium 26.0 mm in size within a period of 37 days; 20 oysters in 20 days by two Cymatium of 40.5 mm and 20 oysters in 19 days by two Cymatium of 61.8 mm. Two specimens of M. virgeneus, 54.0 mm in size consumed 20 oysters in 49 days in a laboratory experiment. These gastropods have been shown to survive starvation for 57 to 125 days.
Usually in the culture sites crabs are the worst predators. These crustaceans enter the spat rearing cages during their larval phase and, as they grow, they crush and feed on the pearl oyster spat. Charybdis lucifera, Atergatis integerrisimus, Leptodius exaratus, Neptunus spp. and Thalamita spp. are some of the crabs commonly found inside the pearl oyster cages in the Indian oyster farms.
PLATE VI. Cont'd. (C) Damage caused by a boring sponge and (D) Cymatium cingulatum, a major pearl oyster predator.
Three species of parasitic trematodes have been isolated from the foot, mantle, gill, liver and gonads of pearl oysters. Their monthly percentage of infection has been recorded to vary from 3–20 % in a single farm site.
The most effective method of controlling fouling growth is by cleaning the oysters, cages and farm materials regularly. Suspending the oyster cages at depths below 5 m during the peak barnacle settlement period usually reduces the degree of settlement of this organism. In addition, periodical exposure of the oysters to sun light for a few hours results in the killing of the larvae of most undesirable settlers. Fresh water, brine and chemical treatment are also found to be effective. Finally, the peak spawning and settlement season of major fouling organisms can be also avoided by timing the introduction of the new spat stocks in the farms.
The boring polychaetes are easily killed by immersing the oysters in freshwater for about 6 hours. The oyster shell valves infested with boring organisms can also be brushed with 1 % formalin, dipped in freshwater and returned to the sea. The above treatment is found to be effective against sponges and Martesia sp. and partly against Polydora sp.. At a concentration of 78 %, brine has been shown to kill all polychaete species within 8 hours.
Periodic monitoring of the culture facilities and manual removal of the predators is the only way of containing predation on the oysters. Oyster spat can be additionally protected from fish by covering the rearing cages with old fish net.
