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Biology of Haplosporidia


Haplosporidia is a group of protists (single-celled eukaryotes) that parasitize several kinds of marine invertebrates, including oysters, mussels, clams and crabs. They belong in the eukaryotic supergroup Rhizaria (Cercozoa), but no other details are currently known. It is a relatively small group, with only a handful of known genera, although more species could remain undetected.

Most aspects of the biology of haplosporidia are poorly known, including their life cycle. Haplosporidian cells transmit from host to host as spores, these are very small (~2 microns) and are surrounded by a hard shell that resembles a little (brown) jug with a hinged lid. When a spore reaches the soft tissue of its host, the lid opens up and the content is released, which acquires amoeboid shape. The cell then goes into active mode and move their way though connective tissue. They then go through a plasmodial stage (typically in gill tissue) before going into sporogenesis until the sporocysts release the newly formed spored. This is a summarized and generalized life cycle, but it varies from species to species.

Infection with haplosporidia can kill the host, but even when it doesn’t it causes severe commercial damage as infection affects the flavour of shellfish. In Canada, one of the most problematic haplosporidian diseases is known as MSX (Multinucleate Sphere X disease, Haplosporidiosis, Delaware Bay disease, Haplosporidiosis of Pacific oysters). The species involved is Haplosporidium (=Minchinianelsoni and the preferred hosts are the oysters Crassostrea virginica and Crassostrea gigas. The consequences of this pathogen can be devastating: Mortalities can reach 90% to 95% of the oysters in a cohort within 2 to 3 years.


Our research on Haplosporidia has two objectives

  • Shed light on the biology of haplosporidians in order to provide tools to develop methods of control and better ways to carry out epidemiological monitoring.
  • Understand the origin, evolution and consequences of parasitism.

We have recently started a collaboration with the Association of Aquaculture of Nova Scotia aiming to sequence and analyze the genome of the MSX parasite (H. nelsoni) from the Bras D’Or Lakes region in Cape Breton, Nova Scotia. Oysters have been grown in the Bras D’Or lakes for millennia and represented an important economic force for the local communities until 2002, when an MSX outbreak severely impacted the oyster populations.

With funding from NSERC (ENGAGE grant), we produced transcriptomic and genomic sequence data from H. nelsoni, which is currently being assembled in the lab for annotation and analysis.