Basic Anatomy of an Oyster digestive gland denticle mouth stomach rectum pericardial cavity mantle anus gill adductor muscle Basic anatomy of an oyster. 108 SECTION 3 - MOLLUSCAN DISEASES Basic Anatomy of an Oyster 108 SECTION 3 - MOLLUSCAN DISEASES M.1 GENERAL TECHNIQUES 110 M.1.1 Gross Observations 111 M.1.1.1 Behaviour 111 M.1.1.2 Shell Surface Observations 111 M.1.1.3 Inner Shell Observations 111 M.1.1.4 Soft-Tissue Surfaces 114 M.1.2 Environmental Parameters 114 M.1.3 General Procedures 116 M.1.3.1 Pre-Collection Preparation 116 M.1.3.2 Background Information 116 M.1.3.3 Sample Collection for Health Surveillance 116 M.1.3.4 Sample Collection for Disease Diagnosis 116 M.1.3.5 Live Specimen Collection for Shipping 116 M.1.3.6 Preservation of Tissue Samples 117 M.1.3.7 Shipping Preserved Samples 118 M.1.4 Record Keeping 118 M.1.4.1 Gross Observations 118 M.1.4.2 Environmental Observations 119 M.1.4.3 Stocking Records 119 M.1.5 References 119 DISEASES OF MOLLUSCS M.2 Bonamiosis (Bonamia sp., B. ostreae) 121 M.3 Marteiliosis (Marteilia refringens, M. sydneyi) 125 M.4 Mikrocytosis (Mikrocytos mackini, M. roughleyi) 129 M.5 Perkinsosis (Perkinsus marinus, P. olseni) 133 M.6 Haplosporidiosis (Haplosporidium costale, 138 H. nelsoni) M.7 Marteilioidosis (Marteilioides chungmuensis, M. 144 branchialis) M.8 Iridovirosis (Oyster Velar Virus Disease) 147 ANNEXES M.AI OIE Reference Laboratories for 149 Molluscan Diseases M.AII List of Regional Resource Experts for Molluscan 150 Diseases in Asia-Pacific M.AIII List of Useful Diagnostic Guides/Manuals to 152 Molluscan Health 109 M.1 GENERAL TECHNIQUES (SE McGladdery) (MG Bondad- Reantaso) c Fig.M.1.1.1. Gaping hard shell clam, Mercenaria mercenaria, despite air exposure and mechnical tapping. (MG Bondad-Reantaso) d Fig.M.1.1.2c,d. Pteria penguin shell with dense multi-taxa fouling, Guian Pearl Farm, Eastern Philippines (1996). (SE McGladdery) Fig.M.1.1.2a. Mollusc encrustment (arrows) of winged oyster, Pteria penguin, Guian Pearl Farm, Eastern Samar, Philippines (1996). (D Ladra) Fig.M.1.1.2e. Polydora sp. tunnels and shell damage at hinge of American oyster, Crassostrea virginica, plus barnacle encrusting of other shell surfaces. (MG Bondad-Reantaso) Fig.M.1.1.2b. Pteria penguin cultured at Guian Pearl Farm, Eastern Samar, Philippines with ex- tensive shell damage due to clionid (boring) sponge (1992). > Fig.M.1.1.2f. Winged oyster, Pteria penguin, shell with clionid sponge damage. Guian Pearl Farm, Eastern, Philippines (1996). 110 M.1 General Techniques immediately and environmental factors pre- and General molluscan health advice and other post-mortality recorded. Mortalities that appear valuable information are available from the OIE to spread from one area to another suggest the Reference Laboratories, Regional Resource presence of an infectious disease agent and Experts in the Asia-Pacific, FAO and NACA. should be sampled immediately. Affected animals A list is provided in Annexes M.A1 and M.AII, should be kept as far away as possible from un- and up-to-date contact information may be affected animals until the cause of the mortali- obtained from the NACA Secretariat in ties can be determined. Bangkok (e-mail: [email protected]). Other useful guides to diagnostic procedures which M.1.1.2 Shell Surface Observations provide valuable references for molluscan dis- (Level I) eases are listed in Annex M.AIII. Fouling organisms (barnacles, limpets, sponges, Gross Observations M.1.1 polychaete worms, bivalve larvae, tunicates, bryozoans, etc.) are common colonists of mol- M.1.1.1 Behaviour (Level I) lusc shell surfaces and do not normally present a threat to the health of the mollusc (Fig. It is difficult to observe behavioural changes in M.1.1.2a,b). Suspension and shallow water cul- molluscs in open-water, however, close atten- ture, however, can increase exposure to fouling tion can be made of behaviour of both broodstock and shells may become covered by other ani- and larvae in hatcheries. Since disease situa- mals and plants (Fig. M.1.1.2c,d). This can af- tions can erupt very quickly under hatchery con- fect health directly by impeding shell opening and ditions, regular and close monitoring is worth closing (smothering) or indirectly through com- Level I efforts (see Iridovirus - M.8). petition for food resources. Both circumstances can weaken the mollusc so cleaning may be re- Feeding behaviour of larval molluscs is also a quired. Such defouling should be undertaken as good indicator of general health. Food accumu- rapidly as possible, to minimise the period of re- lation in larval tanks should be noted and samples moval from the water, during cooler periods of of larvae examined, live, under a dissecting mi- the day. Rapid cleaning is usually achieved us- croscope for saprobiotic fungi and protists (e.g. ing high pressure water or mechanical scapers. ciliates) and/or bacterial swarms. Pre-settlement Defouled molluscs should be returned to clean stages may settle to the bottom prematurely or water. Fouling organisms should not be discarded show passive circulation with the water flow cur- in the same area as the molluscs, since this will rents in the holding tanks. accelerate recolonisation. Signs of weakening that persist or increase after cleaning, should be Juvenile and adult molluscs may also cease feed- investigated further by laboratory examination. ing, and this should be cause for concern under normal holding conditions. If feeding does not Shell damage by boring organisms, such as resume and molluscs show signs of weakening sponges and polychaete worms (Fig. M.1.1.2e, (days to weeks depending on water temperature) f) is normal in open-water growing conditions. samples should be collected for laboratory ex- Although usually benign, under certain conditions amination. Signs of weakening include gaping (i.e. (especially in older molluscs) shells may be ren- bivalve shells do not close when the mollusc is dered brittle or even become perforated. Such touched or removed from the water) (Fig. damage can weaken the mollusc and render it M.1.1.1), accumulation of sand and debris in susceptible to pathogen infections. the mantle and on the gills, mantle retraction away from the edge of the shell, and decreased Shell deformities (shape, holes in the surface), movement in mobile species (e.g. scallop swim- fragility, breakage or repair should be noted, but ming, clam burrowing, abalone grazing, etc.). are not usually indicative of a disease condi- tion (Fig.M.1.1.2g, h). Abnormal colouration and Open-water mortalities that assume levels of smell, however, may indicate a possible soft-tis- concern to the grower should be monitored to sue infection which may require laboratory ex- determine if there are any patterns to the losses. amination. Sporadic moralities following periods of intense handling should be monitored with minimal addi- M.1.1.3 Inner Shell Observations (Level I) tional handling if at all possible. If the mortalities persist, or increase, samples should be collected The presence of fouling organisms (barnacles, for laboratory analysis. Mortalities that appear to sponges, polychaete worms, etc.) on the inner have a uniform distribution should be examined shell surface is a clear indication of a weak/ 111 M.1 General Techniques (MG Bondad-Reantaso) (B Jones) g Fig.M.1.1.3a1. Abalone (Haliotis roei) from a batch killed by polydoriid worms. (D Ladra) h Fig.M.1.1.2g,h. Pinctada maxima, shell with clionid sponge damage due to excavation of tunnels exhalent-inhalent openings (holes) to the surface (arrows). Other holes (small arrows) are also present that may have been caused by polychaetes, gastropod molluscs or other fouling organisms. Guian Pearl Farm, Eastern Philippines (1996). (MG Bondad-Reantaso) b Fig.M.1.1.3a. Winged oyster, Pteria penguin, c shell showing clionid sponge damage through to the inner shell surfaces, Guian Pearl Farm, Fig.M.1.1.3b,c. b. Shells of Pinctada maxima Eastern Philippines (1996). showing a erosion of the nacreous inner sur- faces (arrows), probably related to chronic mantle retraction; c. Inner surface of shell showing complete penetration by boring sponges (thin arrows). 112 M.1 General Techniques (MG Bondad-Reantaso) (MG Bondad-Reantaso) d Fig.M.1.1.3g. Inner shell of winged pearl oys- ter showing: tunnels at edge of the shell (straight thick arrow); light sponge tunnel excavation (transparent arrow); and blisters (small thick arrow) at the adductor muscle attachment site. Guian Pearl Farm, Eastern Philippines (1996). (SE McGladdery) e Fig.M.1.1.3.h. Extensive shell penetration by polychaetes and sponges causing weakening and retraction of soft-tissues away from the shell margin of an American oyster Crassostrea virginica. f Fig.M.1.1.3d,e,f. Pinctada maxima (d), Pteria penguin (e) and edible oyster (Crassostrea sp.) (f) shells showing Polydora-related tunnel dam- age that has led to the formation of mud-filled blisters. 113 M.1 General Techniques sick mollusc (Fig.M.1.1.3a and Fig.M.1.1.3a1). pea crabs in mantle cavity The inner surfaces are usually kept clean parasitic copepods attached to gills through mantle and gill action. Perforation of polychaetes, nematodes and turbellarians in the inner surface can be sealed off by deposi- mantle cavity or on surrounding surfaces tion of additional conchiolin and nacre (Fig. M.1.1.4g) (Fig.M.1.1.3b,c). This may result in the forma- redworm (Mytilicola spp.) usually exposed tion of a mud- or water-filled “blister” only on dissection of the digestive tract (Fig.M.1.1.3d,e,f). Shell coverage can also oc- ciliates (sessile or free-swimming) and other cur over irritants attached to, or lying up against protistans (for larvae only) the inner shell, a process that may result in a bacteria (for larvae only) “blister pearl” (Fig.M.1.1.3g).