Results of a survey on A health survey of 1,200 greenshell (Perna health in in 2000 canaliculus), 1,050 Pacific ( gigas), 300 rock oysters (), 252 The New Zealand aquaculture industry is based largely on bivalve ( novaezelandiae) and 150 molluscs, particularly greenshell mussels () and (), revealed one serious Pacific oysters (Crassostrea gigas). There are also export markets for infection, the presence of Perkinsus olseni in clams. live clams (Austrovenus stutchburyi, also known as cockles), and Mussels and oysters harboured trivial infections and scallops (Pecten novaezelandiae) are prospective candidates for aquaculture. were in good health.

The shellfish industry has been under passive disease surveillance remaining meat was incubated in Ray’s fluid thioglycollate medium since 1986 when epizootics caused by the parasite Bonamia first (RFTM) in the dark at room temperature (16°-18°C) for five days, occurred in flat oysters (Ostrea chilensis)(1)(2), virtually destroying the and then Lugol’s iodine was used to stain Perkinsus colonies(16). Foveaux Strait fishery(3). In that time more than14,000 flat Survey results oysters, 4,000 Pacific oysters, 2,000 greenshell mussels and scallops, and 1,000 clams have been examined for parasites and disease. Greenshell mussels These studies have revealed herpesviruses in larval Pacific(4) and The results of examination of mussels from five farms in the flat(5) oysters, viruses associated with digestive tubule epithelial Marlborough Sounds and three farms on the Coromandel degeneration in mussels(6) and scallops(7), as well as some trivial Peninsula are shown in Table 1. infections(8).After Bonamia sp, the only other potentially serious Table 1: Parasites and lesions observed pathogen that has been discovered is Perkinsus olseni, identified in in greenshell mussels (Perna canaliculus) clams from the Mangamangaroa Stream near Auckland in May Coromandel Marlborough Sounds n = 450 n = 750 2000 (Hine PM, unpublished observations). Sloughing of the digestive epithelium <1% A two-year study of commercial shellfish farms is being conducted Gill ciliates 2% <1% and the results of its first year are reported here. The study is part of Apicomplexan/coccidian 5% the active surveillance required by the European Community, Pseudomyicola spinosus 1% following guidelines set down by the Office International des Lichomolgus uncus <1% 1% Paravortex sp 1% Epizooties (OIE)(9). It has also served to clarify the status in New Haemocytosis <1% Zealand of three OIE-listed diseases: winter mortality, P olseni and Sloughing of the epithelium of digestive tubules, involving Bonamia. A study in the early 1970s(10) reported signs resembling large numbers of rounded epithelial cells shed into the lumen, has those of winter mortality, or mikrocytosis, in native rock oysters been associated with putative epithelial viruses(6). The virus-like (Saccostrea glomerata). Winter mortality is known to occur only in particles are probably widespread in mussels but at such low levels Sydney rock oysters(11), which until recently were classified as that epithelial sloughing does not usually occur. Saccostrea commercialis but are now recognised as S glomerata(12). Native rock oysters from Northland were examined in this study. Light infections with gill ciliates were seen rarely in Coromandel samples from the Mangamangaroa Stream were included to mussels, and even less so in Marlborough Sounds mussels, and were determine the incidence of P olseni. As members of the family always without associated pathological changes.

(13)(14) Arcidae are also particularly susceptible to Perkinsus ,50 Coccidia-like apicomplexans occurred in the connective tissue of Barbatia novaezealandiae from around Rangitoto Island, Auckland the digestive gland, between the digestive tubules and gonad Harbour, were also examined. Bonamia has been found only in flat follicles, in mussels from two Marlborough Sounds farms. The stage oysters found around the and in Wellington Harbour, observed was 8-10 mm long and 2.5-3.5 mm wide, slipper-shaped, and these are genetically different from North Island flat oyster with a central nucleus, and posterior amylopectin granules. It was (15) stocks . Although it was intended to examine the latter, impossible to determine if they were sporozoites or merozoites, but insufficient stocks could be located. the tight bunching of up to ten organisms suggested they might Survey methods have been sporozoites within sporocysts in an oocyst. In the mussel that was most heavily infected they were associated with focal Sample sizes and locations surveyed are presented in Tables 1-5. destruction of the connective tissue (Leydig) cells. However, they Each bivalve was measured along the axis at right angles to the appeared to be minor pathogens, as they are in other bivalves(17)(18). hinge, opened, and a steak cut through the centre was fixed in 10% filtered (0.22 µm) seawater formalin. Haematoxylin and eosin Two species of copepod were encountered: Lichomolgus uncus on stained sections were prepared by routine methods. Samples of the palps of the mussel, and Pseudomyicola spinosus in the gut, Barbatia novaezealandiae were processed as above and the neither associated with damage to epithelia. Lichomolgus uncus was

page 3 Surveillance 29(1) 2002 first described by Jones(10) from mussels in Wellington Harbour Rock oysters between 1973 and1974, with a prevalence of infection of 0-32%. The results of examination of rock oysters from Northland and the Over the same period the prevalence of infection in mussels from Coromandel Peninsula are presented in Table 3. Crail Bay in the Marlborough Sounds was up to 10.4%(10).The The sloughing of diverticular epithelial cells resembled the sloughing prevalence of P spinosus recorded in the present study is lower than reported to be associated with the presence of virus-like particles in the 2-4% infection reported in mussels from Wellington Harbour(10), mussels(6) and scallops(7), but appears to be a natural process. but the prevalence is normally highest in summer, and the samples examined here were collected at the end of the southern autumn in Table 3: Parasites and lesions observed in rock oysters (Saccostrea glomerata) June. Pseudomyicola spinosus is distributed worldwide and normally Northland Coromandel (19) causes no harm to host mussels , although occasionally it may n = 150 n = 150 leave the gut and penetrate the digestive gland(20). Sloughing of the digestive epithelium 3% 1% Paravortex is a turbellarian that may be free-living but is sometimes Adductor bacterial necrosis 1% found in close association with bivalves; it occasionally enters the gut. R-LOs digestive tubule 1% Ciliates on gills 6% 3% Haemocytes had infiltrated the connective tissue in the digestive Ciliates in gut 11% 8% gland of five mussels from the Marlborough Sounds, but the reason Pseudomyicola 5% for this was not apparent. Haemocytosis is a non-specific response Haemocytosis 6% and is not necessarily caused by underlying infection but can be Gut lesion 1% caused by other factors such as absorption of unspawned gametes. One from Northland had a necrotic lesion in the adductor muscle associated with short Gram-negative rod-like Overall, mussels in the samples examined appeared to be in good bacteria, but otherwise appeared healthy. health, and this is reflected in the low mortality rates on mussel farms. Rickettsiales-like or Chlamydiales-like inclusions (here referred to as Pacific oysters R-LOs) occurred in the epithelial cells of the digestive tubules of The results of examination of 1,050 Pacific oysters are shown in one rock oyster from the Coromandel Peninsula, and ciliates were Table 2. common on the gills and in the gut of oysters from both regions, There were no apparent pathological changes associated with the but without associated pathology. epithelial rickettsial inclusions, the ciliates or P spinosus infection. Pseudomyicola spinosus was observed in the gut and digestive ducts The overall prevalence of P spinosus (~1%) was low compared with without associated changes in gut epithelium. As noted in a (21) the average one third (16-54%) prevalence reported by Dinamani , previous study(26), there was no evidence of associated lesions, and it but this may be because he examined the pallial cavity fluid where appears that copepods are largely free-living in the mantle cavity the copepod is more frequently encountered. and only occasionally enter the oyster host(21). The prevalence of P Table 2: Parasites and lesions observed in Pacific spinosus infection in this study (5%) is much lower than the 54% oysters (Crassostrea gigas) reported by Jones(10), again explained by the fact that prevalence in Northland Coromandel Marlborough Sounds n = 450 n = 450 n = 150 this study is based on examination of sections, whereas Jones Rickettsia in gills <1% dissected each animal. Rickettsia in digestive tubules 1% The haemocytoses observed were associated with absorption of Ciliates on gills <1% 2% <1% gametes and, despite thorough examination, no microcells Ciliates in gut 3% 1% <1% resembling Mikrocytos roughleyi, the aetiological agent of winter Ciliates on mantle <1% <1% Pseudomyicola <1% <1% mortality, were seen. Spionid polychaetes 63% 41% 39% In one oyster the rear gut lacked an epithelium in an area in which Spionid polychaetes, or mudworms, occurred in the shells of oysters the connective tissue below the epithelial basement membrane was in all three regions, with infestation rates declining southwards. heavily infiltrated with haemocytes. There was no apparent cause Identification of the spionids responsible was not attempted but for the infiltration and the basement membrane was intact. previous studies report four species (Polydora websteri, Polydora Scallops hoplura, Boccardia [Paraboccardia] acus, and Boccardia [Boccardia] The results of examination of scallops from the Coromandel chilensis) infesting Northland Pacific oysters, with P websteri the Peninsula and Marlborough Sounds are shown in Table 4. most common(22). Six species (P websteri, P hoplura, Boccardia knoxi, B acus, B atokouica, and B chilensis) infest Pacific oysters in the The typical rounding and sloughing of digestive tubule epithelial Marlborough Sounds, with B knoxi the most common(23)(24). Statistical cells, associated with the presence of picorna-like viruses (P-LVs)(7), analysis has shown that heavy B knoxi infestations may reduce the occurred in scallops from both sites but was more prevalent in those condition of cultured oysters in the Marlborough Sounds(25). from around the Coromandel Peninsula. However, P-LVs appear to

page 4 Surveillance 29(1) 2002 occur in all scallops from around New Zealand, with the degree of inner core, occurred in the kidneys of a few Coromandel scallops sloughing related to the intensity of P-LV infection (Hine PM, and only one from the Marlborough Sounds. They resemble unpublished observations). Apparently identical P-LVs also occur in granules reported from the kidneys of other bivalves(37), which are the digestive tubule epithelial cells of scallops (Pecten alba) in composed of deposited metals in a calcium/phosphorus matrix. Victoria, Australia, and in scallops () in Scotland Such granules may contain manganese, zinc, and magnesium, with (Hine PM, unpublished observations), and are probably ubiquitous concentric rings composed of these three elements(38). in Pecten spp. Scallops are easily stressed by handling and transport, Clams and this may result in increased virogenesis and epithelial Table 5 shows the parasites and lesions seen in clams. sloughing. Table 4: Parasites and lesions observed in scallops R-LOs occurred in the connective tissue of the digestive gland and (Pecten novaezelandiae) mantle, and in the gills, where the typical basophilic inclusions were Coromandel Marlborough Sounds n = 125 n = 127 often large (>250 µm diameter; Figure 1). Sloughing of the digestive epithelium 14% 2% Table 5: Parasites and lesions observed in clams Branchial R-LOs 81% 98% (Austrovenus stutchburyi) Trichodina 1% Kaipara Harbour Mangamangaroa Stream Nematopsis 10% n = 75 n = 75 Pseudoklossia 2% 20% R-LOs gills 12% 8% Encysted cestodes 4% R-LOs connective tissue 11% Paravortex 8% 16% R-LOs mantle 8% Copepod 1% Pseudoklossia sp Kidney 1% Inclusions in kidneys 4% 1% Perkinsus olseni histology 44% Perkinsus olseni RFTM 69% The most prevalent infections in scallops were R-LO infections of Perkinsus olseni total 80% the gill and mantle epithelium. Serial sectioning would probably Trematode foot 35% 28% reveal 100% prevalence in scallops from both the Marlborough Trematode gonad 3% 3% Sounds and Coromandel Peninsula. Infection was more common in Trematode gills 1% 1% Trematode kidney 3% 4% the gills where large areas of epithelium contained the characteristic Cestode 4% 1% deeply basophilic bodies. Interestingly, while rickettsial infections Pseudomyicola 1% 1% occur widely and commonly in bivalves and usually do not cause Haemocytosis 4% pathology, and scallops do not suffer from many diseases compared Pseudoklossia macrogametes and microgametes occurred in the with other bivalves, scallops have frequently been reported to suffer kidney of one clam from the Mangamangaroa Stream, but intensity from rickettsial diseases(27)(28)(29)(30)(31)(32)(33), sometimes in association of infection was light compared with some published reports(39), and with mass mortalities(30)(32). Wild scallops around New Zealand there were no apparent changes in kidney histology. experience high annual mortality rates (23-39%)(34), and only the branchial R-LO infections appear sufficiently severe to be a possible cause of mortality.

No pathological changes were seen in association with either the spores of the gregarine Nematopsis encysted in the connective tissue, or the gametogenic stages of the gregarine Pseudoklossia in the kidneys, in agreement with many previous studies(35).

Encysted cestodes had hymenolepid-like scolices, and the definitive hosts may have been aquatic .

The turbellarian Paravortex sp occurred among the gills, mantle folds and in the main gut of some scallops, with up to five Paravortex per section. This flatworm is regarded as a commensal of scallops, although Woods and Hayden found higher prevalences in dead and dying scallops(36), suggesting they may also act as scavengers.

The identity of the copepod found between the gills of one Coromandel scallop is unknown.

Refractile granules 8-10 µm in diameter, often composed of an outer ring-like portion separated by concentric circles from an Figure 1: R-LO inclusions in the gills of a clam

page 5 Surveillance 29(1) 2002 Perkinsus olseni infected 60 of the 75 clams (80%) examined from Therefore P olseni may occur from the Pacific across mainland the Mangamangaroa Stream (Figure 2), of which 25 (33%) were Australia, but not , and through Southeast Asia. With such diagnosed by histology and RFTM, 27 (36%) by RFTM only a distribution and its apparent affinity for warm water, P olseni (histology false negatives) and 8 (11%) by histology only (RFTM probably occurs naturally in New Zealand and is probably confined false negatives). Altogether 69% of the 75 clams were diagnosed by to the north of the North Island. RFTM, and 44% by histology. Perkinsus olseni was observed in Trematode metacercariae were found in many organs of the clam granuloma-like lesions in the connective tissue of the digestive but particularly the foot. Although they cannot be definitively gland, mantle and gills. Lesions were often large (>600 µm identified from histological sections, they appear to be the diameter), even when only few Perkinsus schizonts were present, echinostomatid trematode Curtuteria australis(51). This primarily similar to the connective tissue lesions seen in carpet shell clams infects the foot of clams, reducing their ability to burrow(52)(53) and (Ruditapes decussatus) (Figure 3 in Navas et al 1992)(40). making them susceptible to predation by avian definitive hosts. Perkinsus olseni was originally reported as the cause of mass Barbatia novaezealandiae mortalities among () during a hot summer Of the 25 B novaezealandiae examined from Rangitoto Island in the in southern Australia(41)(42). Although no further mortalities have Waitemata Harbour, Auckland, 14 (56%) contained schizonts of P been reported from these stocks, continuing infection causes lesions olseni. This infection level is similar to the reported 60% infection Figure 2: Schizonts of of other members of the Arcidae on the Great Barrier Reef(13), and Perkinsus olseni (centre) the 50% infection of Barbatia helblingii on the northwest coast of surrounded by infiltrating haemocytes, in the mantle Australia(14). of a clam Conclusions The results of this study revealed that, with the exception of Perkinsus olseni, only relatively trivial infections occur in New Zealand commercial bivalve shellfish. Many of the infections, such as R-LOs, ciliates, coccidians, encysted trematodes and cestodes, and Pseudomyicola, are common and usually harmless in bivalves around the world(17). Greenshell mussels and Pacific oysters in particular consistently appeared to be in good health. Acknowledgements that make the abalone unmarketable(43). Following the abalone I would like to thank the farmers who contributed bivalves for this mortalities, studies on bivalves on the Great Barrier Reef, Australia, study, NIWA staff who collected samples, Andrea Rudy and Ian Ellis showed that Perkinsus occurs in many bivalve genera and families for processing the samples, and Dr Ben Diggles for his critical on the reef(13), and that isolates from one infected species readily comments. infect other species that are not closely related(44). The size and appearance of different stages of the same isolate of Perkinsus vary References depending on the host, therefore phenotypic characters are of little (1) Dinamani P, Hine PM, Jones JB. Occurrence and characteristics of the haemocyte parasite Bonamia sp. in the New Zealand Tiostrea (42) use in identifying species . lutaria. Diseases of Aquatic Organisms 3, 37-44, 1987. (2) Hine PM. Ultrastructural observations on the annual infection pattern of Mortalities among carpet shell clams, and to a lesser extent Manila Bonamia sp. in flat oysters Tiostrea chilensis. Diseases of Aquatic Organisms clams (Ruditapes philippinarum) in Spain, were associated with a 11, 163-71, 1991. Perkinsus similar to P olseni, named P atlanticus(45)(46). Molecular (3) Doonan IJ, Cranfield HJ, Michael K. Catastrophic reduction of the oyster, Tiostrea chilensis (: Ostreidae), in Foveaux Strait, New Zealand, due to biological studies on subsequent mortalities among Manila clams in infestation by the protistan Bonamia sp. New Zealand Journal of Marine and Freshwater Research 28, 335-44, 1994. Japan(47) and Korea(48)(49) showed the isolates to have features of P (4) Hine PM, Wesney B, Hay BE. Herpesviruses associated with mortalities among olseni and P atlanticus, and similar results have been obtained in a hatchery-reared larval Pacific oysters Crassostrea gigas. Diseases of Aquatic study of P atlanticus from Spain(50). It is therefore believed that P Organisms 12, 135-42, 1992. (5) Hine PM, Wesney B, Besant P. Replication of herpes-like viruses in larvae of atlanticus in Spain derived from P olseni in clams introduced into the flat oyster Tiostrea chilensis at ambient temperatures. Diseases of Aquatic Spain from Southeast Asia for aquaculture. Whereas P olseni/P Organisms 32, 161-71, 1998. atlanticus may infect many bivalve genera in the tropics(13), in more (6) Jones JB, Scotti PD, Dearing SC, Wesney B. Virus-like particles associated with marine mussel mortalities in New Zealand. Diseases of Aquatic temperate countries such as Korea, Japan and Spain, it infects only Organisms 25, 143-9, 1996. clams (Ruditapes spp) of the family , and the New Zealand (7) Hine PM, Wesney B. Virus-like particles associated with cytopathology in the digestive gland epithelium of scallops Pecten novaezelandiae (Reeve, 1853) clam (A stutchburyi) is also a venerid. A Perkinsus sp infects an and toheroa ventricosum (Gray, 1843). Diseases of Aquatic arcid, Arca ventricosa, in Manihiki lagoon, the Cook Islands, central Organisms 29, 197-204, 1997. Pacific (Hine PM, unpublished observations), and may be P olseni. (8) Hine PM. Health status of commercially important molluscs in New Zealand. Surveillance 24(1), 25-8, 1997.

page 6 Surveillance 29(1) 2002 (9) OIE. International Aquatic Health Code. Office International des novaezelandiae Reeve 1853, in the Marlborough Sounds. PhD thesis. Victoria Epizooties. Paris, p 155, 2001. University of Wellington, Wellington, p 175, 1976. (10) Jones JB. Studies on closely associated with some New Zealand (35) Lauckner G. Diseases of Molluscs: Bivalvia. In: Kinne O (ed) Diseases of Marine marine shellfish. PhD thesis. Victoria University of Wellington, Wellington, 1975. Animals, volume II, Biologische Anstalt Helgoland, Hamburg, Germany, pp 477- (11) Hand RE, Nell JA, Smith IR, Maguire GB. Studies on triploid oysters in Australia. 961, 1983. XI. Survival of diploid and triploid Sydney rock oysters (Saccostrea (36) Woods CMC, Hayden BJ. An observation of the turbellarian Paravortex sp. in the commercialis (Iredale and Roughley) through outbreaks of winter mortality New Zealand scallop Pecten novaezelandiae (Bivalvia: Pectinidae). 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