Haemocyte motility: a marker of inflammation in Mytilus sp.

Bivalve immunity relies exclusively on innate cellular and humoral mechanisms, during which cells named haemocytes maraud across tissues to survey the organism and cope with invaders through migration towards infected site. Immune response is therefore governed by haemocyte motility. This review focuses on the different types of haemocyte movement in Mytilus sp. to address their role in immunity, from random patrolling of organs to directed pathogen elimination. By forming cell clusters or aggregates of different sizes, haemocyte displacements define inflammation per se in mussels. Although described for many years, motility can now be quantified by advanced microscopy techniques that give access to cell velocity values, allowing us to quantify inflammation. As various biotic and abiotic factors have been found to modulate haemocyte velocity, this parameter can be considered a marker to assess the inflammation level, paving the way for future developments in determining the immune status of mussels.