Shipworms, historically known for their destruction of wooden ships and coastal structures, also play a key role in carbon and nutrient cycling in aquatic ecosystems. Despite extensive research, the exact mechanisms underlying their ability to digest wood, and particularly how they digest lignin, have remained elusive. A better understanding of the way that they attack and digest wood will potentially lead to new methods for controlling how they attack wood in the built environment. Here we employ a combination of microbial culture, metagenomic, and FISH-probe microscopy analyses to explore previously overlooked aspects of their digestive system, and discover for the first time microbial symbionts embedded in the gut tissue of the Teredo navalis shipworm. We discovered previously overlooked bacterial symbionts living within the typhlosole, a specialised structure within the main wood digestive organ (cecum) of the T. navalis shipworm, 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 defence of shipworms, but also suggests that the typhlosole in other related organisms might also harbour symbionts. Our findings of bacterial symbionts in the gut of the shipworm suggest that the use of selective biocides in wood that interfere with the Alteromonas species’ ability to aid in the digestion of lignin, could disrupt the ability of shipworms to digest wood. This in-turn would provide an effective control for shipworm damage thus limiting the need for broad-spectrum preservative biocides in wood used in marine applications. 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. Portions of this research have previously been published in the Peer Reviewed Journal, Int. Biodeterioration & Biodegradation, document://doi.org/10.1016/j.ibiod.2024.105816.

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