Hemolymph microbiota and host immunity of crustaceans and mollusks

Crustaceans and mollusks have major economic importance and are also key players in aquatic biogeochemical cycles. However, disease outbreaks, temperature fluctuations, pollutants, and other stressors have severely threatened their global production. Invertebrates generally rely on their innate immune system as the primary defense mechanism, operating at cellular and humoral levels to protect against pathogens. The hemolymph plays a vital role in immune responses, containing microbial communities that interact with the host's immune processes. Significant advances in molecular methods such as metagenomics, metatranscriptomics, metaproteomics, and metabolomics have revealed the presence of a resident hemolymph microbiome and delineated its potentially vital role in immune homeostasis and overall host health. Accordingly, understanding the composition and role of the hemolymph microbiota, alongside innate immune responses, has become a key focus in recent research aimed at unraveling disease resistance mechanisms and supporting sustainable aquaculture practices. Here, we summarize the latest advancements in understanding the host and environmental factors that shape hemolymph microbiota diversity in various crustacean and mollusk species. We also consider the innate immune responses of the hosts, as these modulate interactions between hosts, microbes, and environments. Interactions within the hemolymph microbiome significantly affect host health, providing critical insights for advancing sustainable aquaculture.