Investigating an emerging shellfish disease: is Marteilia a threat to cockle health in the UK?

Hypothesis: The emerging pathogen Marteilia cocosarum is widespread in the UK and plays a role in mass mortality events affecting UK cockle populations.

Objectives:
1. To investigate whether M. cocosarum prevalence and/or pathology is associated with mass mortality in UK cockle fisheries.
2. To investigate the geographic extent of the spread of M. cocosarum within the UK.
3. To develop a protocol for screening live cockles for M. cocosarum and, if this is successful, to conduct laboratory studies on the progress of M. cocosarum infection and its implications for cockle health.

Justification:
In Spain, collapses of several common cockle (Cerastoderma edule) fisheries have been attributed to the paramyxid parasite Marteilia cochillia (Carrasco et al. 2013). Recent research conducted at Aberystwyth University in collaboration with CEFAS detected a similar paramyxid parasite infecting cockles with high prevalence (10-60%) at nine out of ten locations in Wales. Phylogenetic analysis indicates that this parasite is closely related to M. cochillia but forms a distinct species, provisionally named Marteilia cocosarum (manuscript in preparation). Pathology associated with M. cocosarum differs from that of M. cochillia in that it is located mainly within the gill rather than the digestive gland. Preliminary surveys of cockles at four sites in England and four sites in Ireland did not detect M. cocosarum, suggesting that its range may be geographically limited in the UK.

Preliminary surveys have revealed that M. cocosarum is present in the Burry Inlet, a major fishery where recurrent mass mortalities of cockles occur. Following initial engagement with the cockle industry and regulatory bodies, we will conduct a longitudinal study in which cockles are sampled from the burry inlet every month for three years. The cockles will be subjected to molecular and histopathological screening for M. cocosarum using standard protocols that have been developed by Aberystwyth University and CEFAS to investigate whether mass mortality events coincide with peak M. cocosarum prevalence and/or pathology. We will also investigate the current geographical distribution of M. cocosarum by screening cockles sampled from sites throughout the UK and will use this information to predict potential routes of transmission from affected areas to major commercial fisheries.

Laboratory infections of cockles with M. cocosarum are not currently possible because Martelia species have complex life cycles and the intermediate host of M. cocosarum is unknown. In the closely related species M. cochillia, large numbers of spores are released into the water by infected cockles (Darriba et al. 2020). Using currently available ribosomal DNA markers and new markers developed from Marteilia spore DNA, we will attempt to develop a diagnostic test for M. cocosarum in live cockles by maintaining them in individual aquaria and screening water sampled from the aquaria for M. cocosarum environmental DNA (eDNA) by qPCR. If qPCR of water provides a reliable method of diagnosing infection in live cockles then this method will be used to identify infected cockles and to study disease progression in the laboratory.

Carrasco, N. et al. (2013) Marteilia cochillia sp. nov., a new Marteilia species affecting the edible cockle Cerastoderma edule in European waters. Aquaculture 412:223-30.

Darriba, S. and Carballal, M. J. (2020) Marteilia cochillia is released into seawater via cockle Cerastoderma edule faeces. Journal of Invertebrate Pathology 107364.