Environment, rather thanHematodiniumparasitization, determines collateral disease contraction in a crustacean host

Abstract

Host, pathogen, and environment are determinants of the disease triangle, the latter being a key driver of disease outcomes and persistence within a community. The dinoflagellate genus Hematodinium is detrimental to crustaceans globally – considered to suppress the innate defences of hosts, making them more susceptible to co-infections. Evidence supporting immune-suppression is largely anecdotal and sourced from diffuse accounts of compromised decapods. We used a population of shore crabs (Carcinus maenas), where Hematodinium sp. is endemic, to determine the extent of collateral infections across two distinct environments (open water, semi-closed dock). Using a multi-resource approach (PCR, histology, haematology, population genetics, eDNA), we identified 162 Hematodinium- positive crabs and size/sex-matched these to 162 Hematodinium-free crabs out of 1,191 analysed. Crabs were interrogated for additional disease-causing agents; haplosporidians, microsporidians, mikrocytids, Vibrio spp., fungi, Sacculina, trematodes, and haemolymph bacterial loads. We found no significant differences in occurrence, severity or composition of collateral infections between Hematodinium-positive and Hematodinium-free crabs at either site, but crucially, we recorded site-restricted blends of pathogens. We found no gross signs of host cell immune reactivity toward Hematodinium in the presence or absence of other pathogens. We contend Hematodinium sp. is an immune-evader rather than immune-suppressor, which suggests an evolutionary drive toward latency in this environmentally plastic host.

Invasive slipper limpets Crepidula fornicata act like a sink, rather than source, of Vibrio spp.

“…Negative controls consisted of DEPC-treated Molecular Biology Grade Water (Sigma Aldrich) in the absence of DNA template to avoid false positives due to contamination. Positive controls consisted of 1 μL DNA purified from the haemolymph of an infected donor crab (Carcinus maenas; Davies et al, 2021). PCR products were visualised using 2 % (w/v) agarose/TBE gels stained with 3 μL Greensafe premium nucleic acid stain (NZYTech, Lisboa, Portugal).…”

The immunobiology of Crepidula fornicata and its potential to act as a harbourer of infectious disease

“…Using PCR-based methods and Sanger sequencing revealed the Milford Haven and Swansea Bay populations of Crepidula fornicata to be free of these target pathogens (i.e. Interestingly, a study carried out by Davies et al (2021) in Swansea Bay found populations of shore crabs Carcinus maenas, as well as seawater eDNA samples, to be positive for the same parasite taxa screened for in this chapter. This indicates that although these pathogens are present in the environment and other invertebrates, Crepidula fornicata does not appear to be susceptible to them.…”

“…3%) it is likely that C. fornicata has become an accidental host in the two study locations and that the principal primary intermediate hosts are probably various native littorinids. Despite shellfish, such as crabs, in the locality being known to be infected with pathogens such as haplosporidians, and microsporidians (Davies et al, 2020(Davies et al, , 2021, C. fornicata appears to lack general vulnerability to these either as main or accidental hosts. This ability to not succumb to disease could be an important reason for its success as an invader.…”