Haematological deterioration of Hematodinium-infected decapod crustaceans

Parasitic dinoflagellates, namely Hematodinium spp., infect a growing number of decapod crustacean species worldwide. These parasites represent a longstanding concern for fisheries in Europe and North America, and an emerging concern for aqua/polyculture systems in Asia. Known as bitter/pink/milky crab disease or post-moult syndrome, Hematodinium spp. infection can be fatal, yet there are no treatments or disease management strategies. We interrogated the available literature to enhance knowledge of Hematodinium-crustacean pathosystems, specifically haemolymph condition during parasitaemia. In this context, we sought to determine if there were invariant biomarkers (biochemical, cellular) in the haemolymph. Using meta-analytic approaches, we scrutinised published data and gathered 191 effect sizes from 17 original studies (out of >1790) that met strict inclusion criteria covering established haematological properties like phenoloxidase activity, and ran a series of generalised linear mixed models. Additional models were constructed to consider the putative links between environmental variables (water temperature, salinity), host traits (sex, size), and parasite burden. Overall, depleted haemocyte numbers (e.g., hyaline cells) and protein levels (e.g., haemocyanin) coincided with patent Hematodinium presence in crabs and langoustine. Crustaceans were more likely to have severe burdens of Hematodinium when external salinity levels exceeded 30 psu, and potentially immune-compromised ≥20°C. Hematodinium-driven hypoproteinemia and hematocytopenia were more pronounced in wild-caught animals than those infected in laboratory trials, thereby emphasizing the need to secure data in natural settings. This is the first meta-analytic study to present clear evidence in support of broad haematological deterioration in crustaceans parasitised by Hematodinium spp., and environmental factors linked to immunopathology.