• References

    Carrasco N et al. (2013). Marteilia cochillia sp. nov., a new Marteilia species affecting the edible cockle Cerastoderma edule in European waters. Aquaculture. 412-413, 223-230.

    Grizel H et al. (1974). Epizooty of the common oyster Ostrea edulis. Part 1. Study of the agent of digestive gland disease in Ostrea edulis (Linne). Sci. Peche 240, 7-30.

    Herrbach B. (1971). Sur une affection parasitaire de la glande digestive de l'huître plate, Ostrea edulis Linné. Revue des Travaux de l'Institut des Pêches Maritimes 35, 79-87.

    Skujina I et al. (2022). Discovery of the parasite Marteilia cocosarum sp. nov. In common cockle (Cerastoderma edule) fisheries in Wales, UK and its comparison with Marteilia cochilliaJ Invertebr Pathol 192: 107786.

    Villalba A et al. (2014). Cockle Cerastoderma edule fishery collapse in the Ría de Arousa (Galicia, NW Spain) associated with the protistan parasite Marteilia cochillia. Dis Aquat Organ 109, 55-80.

The Marteilia Manifestation

02 March, 2023
The Marteilia Manifestation

2022 Bio-Rad Science Writing Competition Joint 2nd Place

Rowan impressed the judges with his expressive writing style and clear enthusiasm for his PhD topic. He eloquently told the story of the spread of the Marteilia parasite in cockles in a clear and interesting manner.

Rowan Cox is in the second year of his PhD at Aberystwyth University (Wales, UK). He specializes in parasites, specifically those of the genus Marteilia, which affect shellfish around the world. His work focuses on a newly discovered species, Marteilia cocosarum, which poses a threat to cockle fisheries.

We are delighted to publish his entry below:

Marteilia parasites are plaguing productivity in shellfish beds across the world, and they continue to spread.

Often, it’s the smallest things that cause the biggest problems. European fisheries have learned this the hard way through the rapid decline of commercial cockle stocks. The culprit? Marteilia, a genus of single-celled parasites that starve their hosts as they multiply. The emergence of a new species, Marteilia cocosarum, has prompted fresh fears that the worst could be yet to come.

A Horrible History

To explain why, let me take you back to 1967, when France was a prominent supplier of European Flat Oysters (Ostrea edulis). Their stocks were exported as food and even sent to other countries for kickstarting new shellfish beds. However, this is the year that these fisheries collapsed, with mortalities reaching as high as 90% (Herrbach 1971).

The resulting socioeconomic turmoil demanded that the cause was investigated and identified quickly. This led to the first description of a Marteilia parasite, Marteilia refringens (Grizel et al. 1974). With new knowledge of this parasite, steps could be taken to mitigate its impact. Old fisheries could be repaired, and surviving ones protected. Unfortunately, the diagnosis came too late. The French now knew that they had inadvertently exported infected oysters around Europe…

Let’s jump to 2008. Marteilia outbreaks have been a common occurrence over the last few decades and several new species have been identified. This diversification of Marteilia enabled it to infect a greater variety of hosts, and even spread as far as Australia. Until now, the common cockle (Cerastoderma edule) remained relatively untouched. That was about to change, as Marteilia cochillia entered the fray.

A mass mortality event struck the Ebro Delta in Eastern Spain. As the population of cockles was crashing, a Marteilia species was found to be prevalent in 40% of the cockles, enough to cause the high mortalities. Molecular work revealed that it was genetically distinct from M. refringens, and transmission electron microscopy (TEM) observations confirmed that it had a different cellular structure (Carrasco et al. 2013).

This first instance of M. cochillia was not the last. Things escalated in the Ría de Arousa, an estuary on the Galician coast (NW Spain). It was once home to some of the most productive cockle beds in Spain, but the parasite reached 100% prevalence, which the fisheries could not survive (Villalba et al. 2014).

A Fearful Future

The reason for the history lesson is because a new parasite, M. cocosarum, has recently been identified in Wales, UK. Its closest relative is M. cochillia, so the numerous cockle fisheries here could be hit hard, just like in Spain. This is where I enter the story. I’m trying to learn as much as I can about this new parasite, to determine how worried we should be.

Most of what we already know about M. cocosarum is documented by Skujina et al. (2022), who first found it to be close to, but genetically distinct from, M. cochillia. Both species share a similar ultrastructure. Marteilia spp. are single-celled, and they’re characterized by cell-in-cell development. The outermost cell is called the primary cell. The primary cell produces secondary cells internally. These secondary cells are where spore production occurs. To make this Inception-like development more confusing, the spores are tricellular. This means there are spores within spores within spores.

It's okay if that was tough to follow, you don’t need to know the ins and outs of these parasites’ development. What’s important is that species often vary based on the number of secondary cells they produce, and M. cocosarum produces roughly the same amount as M. cochillia. This could make it equally as threatening.

A glimmer of hope is that so far, there’s no evidence that M. cocosarum can actually complete its lifecycle in cockles. Typically, a Marteilia parasite enters shellfish through their gills and uses the hemolymph to travel to the digestive gland. Here it produces lots of spores before finally rupturing to release them into the environment. M. cocosarum has only been found in the gills, surrounded by high levels of inflammation. This may mean that cockles have a strong immune response, one capable of stopping the parasite in its tracks.

If this is the case, then we can rest easy. To find out, I am conducting a longitudinal study to uncover M. cocosarum’s lifecycle. By collecting cockles every month, I am working out how prevalence changes throughout the year. I screen each individual, searching for Marteilia DNA. In addition, I am preserving sections of each cockle, which I can later use for histological analyses. These will provide microscopic photographs of where M. cocosarum is located and how developed the cell is. I can then piece the images together like a comic book and watch the parasite’s life from beginning to end.

Conclusion

It’s difficult to raise awareness about cockles. They don’t attract much interest. They’re not bright and colorful like the struggling coral reefs (another important issue), and most people I meet don’t even like the taste of them (including me). This doesn’t mean they’re not important. They supply vital services to coastal ecosystems, not to mention their commercial value to countries with a coastline.

The spread of Marteilia has left a storm in its wake; it’s crucial that we keep learning more and more about these parasites so we can change the forecast.

 

Bio-Rad's Science Writing Competition Results

We were delighted to receive entries from PhD/Grad Students from all around the world. The judges were impressed by the high standard of submitted articles across a vast range of topics. Rowan is the winner of a commemorative certificate and a copy of ’The Scientist’s Guide to Writing: How to Write More Easily and Effectively throughout Your Scientific Career’.

 

References

Carrasco N et al. (2013). Marteilia cochillia sp. nov., a new Marteilia species affecting the edible cockle Cerastoderma edule in European waters. Aquaculture. 412-413, 223-230.

Grizel H et al. (1974). Epizooty of the common oyster Ostrea edulis. Part 1. Study of the agent of digestive gland disease in Ostrea edulis (Linne). Sci. Peche 240, 7-30.

Herrbach B. (1971). Sur une affection parasitaire de la glande digestive de l'huître plate, Ostrea edulis Linné. Revue des Travaux de l'Institut des Pêches Maritimes 35, 79-87.

Skujina I et al. (2022). Discovery of the parasite Marteilia cocosarum sp. nov. In common cockle (Cerastoderma edule) fisheries in Wales, UK and its comparison with Marteilia cochilliaJ Invertebr Pathol 192: 107786.

Villalba A et al. (2014). Cockle Cerastoderma edule fishery collapse in the Ría de Arousa (Galicia, NW Spain) associated with the protistan parasite Marteilia cochillia. Dis Aquat Organ 109, 55-80.

 

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