Molluscan microbiomes: current research focus, knowledge gaps, and future directions

Microbiome research has increased exponentially in recent years. This is not different for molluscs. We now know that molluscan-associated microbes are involved in the most diverse roles, from aiding digestion to the production of light. However, impacts of the microbiome on molluscan ecology and evolution are still poorly understood. Here, we present a systematic review of the literature on molluscan microbiomes, aiming to portray the current state of the field and with no pretension to summarise all the knowledge on molluscan microbiomes. Based on our review, we identified knowledge gaps and opportunities for future research. We have found that gastropods and bivalves are the most researched classes, and the gut is the most used tissue for microbiome assessments in molluscs. While studies focusing on microbiome characterisations are extremely common, a widespread concern among malacologists is microbiome responses to anthropogenic pressure. We propose that diversifying the molluscan groups assessed and tissues used will be important to address gaps in knowledge. At the same time, partnership with the aquaculture sector and greater use of the ‘omics toolbox’ are promising avenues for a more thorough understanding of molluscs as holobionts.