Basic Anatomy of an Oyster

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  108 Basic Anatomy of an Oyster Basic anatomy of an oyster. rectum pericardial cavity anus denticle stomach digestive gland mouth mantle gill adductor muscle  109 SECTION 3 - MOLLUSCAN DISEASES Basic Anatomy of an Oyster SECTION 3 - MOLLUSCAN DISEASES M.1 GENERAL TECHNIQUES M.1.1 Gross Observations M.1.1.1 Behaviour  M.1.1.2 Shell Surface Observations M.1.1.3 Inner Shell Observations M.1.1.4 Soft-Tissue Surfaces M.1.2 Environmental Parameters M.1.3 General Procedures M.1.3.1 Pre-Collection Preparation M.1.3.2 Background Information M.1.3.3 Sample Collection for Health Surveillance M.1.3.4 Sample Collection for Disease Diagnosis M.1.3.5 Live Specimen Collection for Shipping M.1.3.6 Preservation of Tissue Samples M.1.3.7 Shipping Preserved Samples M.1.4 Record Keeping M.1.4.1 Gross Observations M.1.4.2 Environmental Observations M.1.4.3 Stocking Records M.1.5 ReferencesDISEASES OF MOLLUSCS M.2 Bonamiosis (   Bonamia sp., B. ostreae  ) M.3 Marteiliosis (   Marteilia refringens, M. sydneyi   ) M.4 Mikrocytosis (   Mikrocytos mackini, M. roughleyi   ) M.5 Perkinsosis (   Perkinsus marinus, P. olseni   ) M.6 Haplosporidiosis (   Haplosporidium costale, H. nelsoni   ) M.7 Marteilioidosis (   Marteilioides chungmuensis, M. branchialis  ) M.8 Iridovirosis (  Oyster Velar Virus Disease  ) ANNEXES M.AI OIE Reference Laboratories forMolluscan Diseases M.AII List of Regional Resource Experts for MolluscanDiseases inAsia-Pacific M.AIII List of Useful Diagnostic Guides/Manuals toMolluscan Health 108110111111111111114114116116116116116116117118118118119119119121125129133138144147149150152  110 M.1 GENERAL TECHNIQUES (SE McGladdery) Fig.M.1.1.1. Gaping hard shell clam, Mercenaria mercenaria , despite air exposureand mechnical tapping.(MG Bondad-Reantaso) Fig.M.1.1.2a. Mollusc encrustment (arrows) ofwinged oyster, Pteria penguin, Guian PearlFarm, Eastern Samar, Philippines (1996).(D Ladra) Fig.M.1.1.2b. Pteria penguin cultured at GuianPearl Farm, Eastern Samar, Philippines with ex-tensive shell damage due to clionid (boring)sponge (1992).(MG Bondad- Reantaso) Fig.M.1.1.2c,d. Pteria penguin shell with densemulti-taxa fouling, Guian Pearl Farm, EasternPhilippines (1996).(SE McGladdery) Fig.M.1.1.2e. Polydora sp. tunnels and shelldamage at hinge of American oyster, Crassostrea virginica , plus barnacle encrustingof other shell surfaces.(MG Bondad-Reantaso) Fig.M.1.1.2f. Winged oyster, Pteria penguin ,shell with clionid sponge damage. Guian PearlFarm, Eastern, Philippines (1996). cd >  111 M.1 General Techniques General molluscan health advice and othervaluable information are available from the OIEReference Laboratories, Regional ResourceExperts in the Asia-Pacific, FAO and NACA. A list is provided in Annexes M.A1 and M.AII,and up-to-date contact information may beobtained from the NACA Secretariat inBangkok (e-mail: ). Otheruseful guides to diagnostic procedures whichprovide valuable references for molluscan dis-eases are listed in Annex M.AIII. M.1.1 Gross Observations M.1.1.1 Behaviour  (Level I) It is difficult to observe behavioural changes inmolluscs in open-water, however, close atten-tion can be made of behaviour of both broodstockand larvae in hatcheries. Since disease situa-tions can erupt very quickly under hatchery con-ditions, regular and close monitoring is worthLevel I efforts (see Iridovirus - M.8).Feeding behaviour of larval molluscs is also agood indicator of general health. Food accumu-lation in larval tanks should be noted and samplesof larvae examined, live, under a dissecting mi-croscope for saprobiotic fungi and protists (  e.g. ciliates) and/or bacterial swarms. Pre-settlementstages may settle to the bottom prematurely orshow passive circulation with the water flow cur-rents in the holding tanks.Juvenile and adult molluscs may also cease feed-ing, and this should be cause for concern undernormal holding conditions. If feeding does notresume and molluscs show signs of weakening(days to weeks depending on water temperature)samples should be collected for laboratory ex-amination. Signs of weakening include gaping (   i.e. bivalve shells do not close when the mollusc istouched or removed from the water) (Fig.M.1.1.1), accumulation of sand and debris inthe mantle and on the gills, mantle retractionaway from the edge of the shell, and decreasedmovement in mobile species (  e.g. scallop swim-ming, clam burrowing, abalone grazing, etc.  ).Open-water mortalities that assume levels ofconcern to the grower should be monitored todetermine if there are any patterns to the losses.Sporadic moralities following periods of intensehandling should be monitored with minimal addi-tional handling if at all possible. If the mortalitiespersist, or increase, samples should be collectedfor laboratory analysis. Mortalities that appear tohave a uniform distribution should be examinedimmediately and environmental factors pre- andpost-mortality recorded. Mortalities that appearto spread from one area to another suggest thepresence of an infectious disease agent andshould be sampled immediately. Affected animalsshould be kept as far away as possible from un-affected animals until the cause of the mortali-ties can be determined. M.1.1.2 Shell Surface Observations (Level I) Fouling organisms (barnacles, limpets, sponges,polychaete worms, bivalve larvae, tunicates,bryozoans, etc.  ) are common colonists of mol-lusc shell surfaces and do not normally presenta threat to the health of the mollusc (Fig.M.1.1.2a,b). Suspension and shallow water cul-ture, however, can increase exposure to foulingand shells may become covered by other ani-mals and plants (Fig. M.1.1.2c,d). This can af-fect health directly by impeding shell opening andclosing (smothering) or indirectly through com-petition for food resources. Both circumstancescan weaken the mollusc so cleaning may be re-quired. Such defouling should be undertaken asrapidly as possible, to minimise the period of re-moval from the water, during cooler periods ofthe day. Rapid cleaning is usually achieved us-ing high pressure water or mechanical scapers.Defouled molluscs should be returned to cleanwater. Fouling organisms should not be discardedin the same area as the molluscs, since this willaccelerate recolonisation. Signs of weakeningthat persist or increase after cleaning, should beinvestigated further by laboratory examination.Shell damage by boring organisms, such assponges and polychaete worms (Fig. M.1.1.2e,f)is normal in open-water growing conditions. Although usually benign, under certain conditions(especially in older molluscs) shells may be ren-dered brittle or even become perforated. Suchdamage can weaken the mollusc and render itsusceptible to pathogen infections.Shell deformities (shape, holes in the surface),fragility, breakage or repair should be noted, butare not usually indicative of a disease condi-tion(Fig.M.1.1.2g, h). Abnormal colouration andsmell, however, may indicate a possible soft-tis-sue infection which may require laboratory ex-amination. M.1.1.3 Inner Shell Observations  (Level I) The presence of fouling organisms (barnacles,sponges, polychaete worms, etc.  ) on the innershell surface is a clear indication of a weak/ 
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