AIN 06.2001

Soft Shell Clam NeoplasiasBackgroundSoft shell clams have been a staple of AtlanticCanadian seafood harvest since the turn of thecentury. In PEI alone they generated a landedvalue of nearly $1.5M in 2000. Soft shell clamssupport both commercial and recreationalfisheries. Fluctuations in natural recruitment andunexplained crashes in productive beds led tocloser investigation of health profiles. Thisparalleled efforts to develop a cultured clamindustry which resulted in close monitoring ofclam health. In 1999 mass mortalities in DarnleyBasin were associated with abnormally high levelsof a disease condition of clam blood, known ashaemic neoplasia. The role of this disease in thecrash of Darnley Basin beds is under study.What is Neoplasia?Neoplasia is a condition commonly referred to as‘cancer’ which means that cells lose their abilityto function normally and may begin to multiplyout of control. In clams, this condition has beenfound to affect blood (haemic) or the gonads(gonadal). Gonadal neoplasia affects clams inMaine, but is quite rare in Atlantic Canada. Until1999, haemic neoplasia was also believed to berare. The reason for the change in numbers ofclams affected is under investigation.What Causes Neoplasia?As with cancers that affect man, land-animals andplants, multiple causes can play a role intriggering the condition. These types of diseaseshave been related to viral infections and extremeenvironmental changes such as water temperature.Pesticides have also been implicated. There is noclear common factor between clam beds which areaffected by low levels of the disease (Bay ofFundy and Gulf of St. Lawrence, as well as the

eastern United States).Does Neoplasia Affect Consumers?There is no known link between human cancersand consumption of affected clams. Clams withadvanced neoplasia are usually weak (‘gapers’)and likely to be rejected during processing.Can you tell if Clams have Neoplasia?There are no visible clues to tell affected andunaffected clams apart. Scientists need to look atblood samples or pieces of tissue under amicroscope to see the neoplasia. However, clamf ishers andaquaculturistscan play animportant rolein detecting andreporting bedswith high levelsof weak ordying clams.Even beds fullof empty intactshells (as opposed to crushed or drilled) may bean important clue to tracking the cycle of thisdisease. Such findings should be reported toprovincial or federal fisheries departments.What does Neoplasia do to the Clam?Depending on the tissue (blood or gonads) andlevel of neoplasia changes, the clam may rejectthe affected cells and recover, or the neoplasiawill take over the whole body and the clam willdie. Neoplasia that affects the gonads has beenestimated to significantly reduce spawning levels.To date, there is no evidence of this in Canadianclams, since only low levels have been found.Haemic neoplasia, however, has been found inlow (small patches of affected cells) to high

Histological section that illustrates haemic neoplasia insoft shell clams. Neoplastic hemocytes are characterisedby enlarged diffuse nucleii (arrows).

Histological section showing a soft shell clam infectedwith haemic neoplasia. Note the infiltration of neoplasticblood cells in the ovary, digestive gland and gill (dark bluecells).

Histological section showing a healthy soft shell clam.

(throughout the body) levels. When nearly allblood cells are transformed, the clam cannot feed,dispose of waste products, fight infections orexchange gases. It, therefore, weakens and dies.Are any other Shellfish Affected?Similar neoplasia conditions affect mussels,oysters and hard-shell clams (quahaugs), but thereis no evidence that the condition can be spreadfrom one species to another. Interestingly, noneoplasias have been found, to date, in crabs orlobsters.

What Research is being done on ClamNeoplasias?An intense survey of clam beds was conductedthroughout PEI in 1999. This showed thecondition to be common in PEI clams. This surveyis being expanded in 2001 to include NB, NS, NFand Quebec clam populations. Unaffected clamsare being studied for their reaction to exposure toaffected clams and/or tissue samples. This work isbeing conducted at the UPEI-AVC incollaboration with the Department of Fisheries,Aquaculture and Environment and the PEIAquaculture Alliance. This will answer thequestion of whether or not direct infection ofclam-to-clam is possible, or whether otherenvironmental factors are the trigger. Detailedexaminations of environmental parametersassociated with affected and unaffected clam bedsboth within PEI and elsewhere are beingconducted. These will include evaluation of levelsof neoplasia affecting the clams, along withestimates of levels of mortality, so the exact effectof neoplasia on clam bed ‘crashes’can be betterunderstood. ConclusionsCommercial and recreational harvesting of clamshas been plagued by unexplained clam bed‘disappearances’. Only recently have we been ableto closely monitor clam health. This opportunityhas revealed that there is a lot more we need toknow about this staple of Atlantic chowders. Italso underlines the fact that we cannotunderestimate the complexity of diseases affectingaquatic organisms. Quick answers to such diseasesare not possible. Neither is direct intervention.Hopefully answers to the above research willfurnish the clues we need to better assess theimpact of neoplasia on clam health and reduce it,as necessary.

This information sheet was made possible by the followingpartners:� University of Prince Edward Island (AVC)� PEI Department of Fisheries, Aquaculture and

Environment� PEI Aquaculture Alliance� PEI Aquaculture and Fisheries Research Initiative � Department of Fisheries and Oceans (Gulf Fisheries

Centre)

For further information contact:Dr. Sharon McGladderyDFO - Gulf Fisheries CentrePO Box 5030Moncton, NB E1C 9B6506-851-2018/mcgladderys@dfo-mpo.gc.ca