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Work Package 6 & Work Package 7 - Bivalve Disease and Bivalve Parasites

 Disease in the Pacific oyster Crassostrea gigas and the European flat oyster Ostrea edulis

  •  Dr. Sharon Lynch (University College Cork)

As part of the research carried out at University College Cork (UCC), Dr. Sharon Lynch is investigating the role of ostreid herpes virus (OsHV-1) in summer mortalities observed in the Pacific oyster, Crassostrea gigas, in the Irish Sea. This is a continuation of work carried out in a previous INTERREG project by Bangor University and UCC. A new "user friendly" and cost effective conventional polymerase chain reaction (cPCR), using several new primers, has been designed to amplify a smaller OsHV-1 product. Samples have been screened and a positive signal for OsHV-1 has been detected in C. gigas at two Irish sites. 

   
Figure 1. Crassostrea gigas                                       Figure 2.   Ostrea edulis

The European flat oyster, Ostrea edulis and C. gigas are being screened for haplosporidians, Bonamia ostreae, Haplosporidium nelsoni and Haplosporidium amoricanum. Historical samples (heart smears and tissue sections) of C. gigas are being screened, from the shellfish health unit archive, to determine if B. ostreae can be detected in this oyster species and if it may have played a role as a carrier for B. ostreae in the past. To date, 19 samples (n=570) have been screened. As no tissue was stored for molecular analysis, deparaffinisation of paraffin-embedded tissue (Shi et al., 2002) and DNA extraction of archival tissue using protein precipitate and cell lysis (QiagenTM) has been established. The DNA of individuals with suspicious microcells present in the heart smears and tissue sections are being screened by cPCR for the different haplosporidian species.

The population genetics of B. ostreae is also being investigated using available markers. A comparison has been carried out between B. ostreae populations based on geographical distribution within Ireland, carrier/reservoir species, Irish archival material and from European sites where B. ostreae is endemic (Holland, Scotland and Spain). To date no genetic variation has been detected; however, new markers which may be more suitable for the study of population genetics will be developed.

 

  Bivalve Disease Models

  • Mr. Eddie O'Grady (University College Cork)
  • Dr. Dmitrii Rachinskii (University College Cork)

The aim of the work being carried out is to use mathematical techniques and software development to model the experimental work carried out by the researchers in the school of Biology, UCC. The system under consideration thus far examines effects (mutual and individual) of parasites in two shellfish types, namely, the common cockle (Cerastoderma edule) and the peppery furrow shell (Scrobicularia plana). One of the parasites under consideration is a type of digenetic trematode (Myogymnophallus minutus). These flukes mature and emigrate from S. plana, whereupon they enter the second host, the cockle. The cockle is essentially the platform for the flukes to enter their final host, as marine birds prey on cockles in large numbers. However, within the cockle there is another stage to the process. It is here that another hyperparasite (Unikaryon legeri), can infect the relatively much larger flukes. The hyperparasite acts as a regulator in that it prevents the fluke population from growing without bound to reach its carrying capacity within the cockle. Therefore, we are interested in the hyperparasites ability to improve cockle health.

 We aim to determine the nature of the interaction between the hyperparasite and the fluke: namely whether it is a predator-prey or purely parasitic interaction. We also hope to determine the overall effect that the hyperparasite plays in the system at large. The model can incorporate features which may be relevant in future such as climate change. This can influence water temperatures, thereby leading to variations in the numbers of parasites, parasite reproduction times and/or effect bird migratory patterns, which affect the shellfish and indirectly, the parasite populations.

We employ methods of modelling population dynamics using a system of Ordinary Differential Equations (ODE's) based on the literature review. This provides a solid foundation on which to build upon when more realistic phenomena need to be investigated and when experimental data is available for fitting.

We have looked at standard population modelling techniques within the subsystem of a single cockle to examine the fieldwork observation that until the average number of flukes per cockle is 185, parasites are not present in the system. We have reached interesting conclusions that a threshold population exists, but due to a delayed exchange of stability, it is not necessarily at the predefined value of 185. However, due to the study of the equations used, it may be possible to take experimental data including minimum and maximum populations, etc. that will help us to both estimate interaction strengths between flukes and parasites, and to better define the system so that the delayed exchange of stability is in line with observation. The figure below shows two possible outcomes for the subsystem: a non-zero parasite equilibrium (left) and a zero-parasite stable equilibrium (right).

It was found by modelling reasonable biological assumptions that the threshold phenomenon could be explained in mathematical terms, removing the need for a biological/chemical explanation. We have discovered more interesting mathematical results in that our dynamical system exhibits both delayed and immediate stability exchange. This differs quite markedly from the seminal papers of Anderson and May in the 1970's who brought this type of research to the fore.

It is hoped that further development of the model can allow us to incorporate the existing experimental data and provide suggestions for areas of new research. It is hoped to widen the scope of the modelling techniques used as the project progresses to study the mathematical results as well as the impact it will provide on the overall project.

Work is also being done on more theoretical models. In particular, we research systems of Delayed Differential Equations (DDE's). These have the ability to model scenarios where, for instance, a predator takes a certain time interval to mature before being capable of killing its prey. We have found already some interesting mathematical results here, comparable to models describing laser dynamics. We will continue this research alongside the population models.

Further work in these areas will take us to the spread of disease in other shellfish, in particular, the problem of herpes in Pacific Oysters.

  

The effects of climate change on soft sediment bivalves and their pathogens.

  • Ms. Emer Morgan (University College Cork)

This work is being carried out in UCC, under the guidance of Sarah Culloty. I am primarily investigating the common cockle, Cerastoderma edule plus associated species such as the blue mussel Mytilus edulis, the Baltic tellin Macoma balthica and other bivalves. Information is being gathered concentrating on population dynamics, biology and health status of the cockles and associated fauna are being screened as potential reservoirs of disease. A field study was performed from March 2010 to June 2011, two sites 215 km apart were chosen: Bannow Bay, Co. Wexford and Flaxfort Strand, Co. Cork. Temperature loggers were deployed at both sites and on each sampling day 30 moribund and 30 buried cockles were collected. Additionally 30 mussels and any other bivalves/polychaetes encountered were taken also. For each animal collected length, width, height, and a series of weights are taken, before processing for histology, transmission electron microscopy and PCR. The number of age bands on each cockle is also counted. A range of parasites and pathological conditions are being identified using histology such as trematodes and disseminated neoplasia in cockles. Trematodes, ciliates and various gregarines have been recorded in associated fauna. Future work includes lab trials plus an investigation into the population genetics of one of the most common trematode cockles Meiogymnophallus minutus.


Figure 1. Common cockle, Cerastoderma edule, Mytilus edulis, Macoma balthica (left to right).
 

    
Figure 2. Flaxfort Strand, Co. Cork                                              Figure 3. Bannow Bay, Co. Wexford
 

 
Figure 4. Cerastoderma edule distribution

 

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