First report of microbial symbionts in the digestive system of shipworms; wood boring mollusks

Shipworms, historically known for their destruction of wooden ships and coastal structures, play a key role in carbon and nutrient cycling in aquatic ecosystems. Despite extensive research, the exact mechanisms underlying their ability to digest wood have remained elusive, particularly regarding the degradation of lignin, a major component of wood that resists breakdown. Here we employ a combination of microbial culture, metagenomics, and FISH-probe microscopy analyses to explore previously overlooked aspects of their digestive system. Specifically, we identify the presence of bacterial symbionts within the typhlosole, a specialized structure within the main wood digestive organ (cecum), challenging the long-held belief that shipworm foreguts are nearly sterile environments. The discovery of Alteromonas species in bacterial clusters within the typhlosole suggests a symbiotic relationship that may play a crucial role in the digestion of woody substrates, offering a potential explanation for the shipworm's ability to degrade lignocellulose without ligninases from the host or gill symbionts. These findings provide insight into the digestive physiology and host defense of shipworms, but also suggests that the typhlosole in other related organisms might also harbor symbionts. This study also opens new insights into research into the ecological and biotechnological applications of shipworms and their gut symbionts, enhancing our understanding of lignocellulose degradation in marine environments.