Shipworms are wood-feeding bivalves that are proposed to harbor cellulolytic bacteria in their digestive system like those observed in the gut of xylophagous animals. However, diversity and roles of intestinal bacteria of shipworms have not been adequately addressed. Here, we isolated 197 bacterial strains associated with the shipworm Teredo navalis and 47 bacterial strains from wood particles around shipworms. We assayed cellulose, xylan and alginate degrading activities of each isolate and found that 34 isolates from the shipworm and 7 isolates from the wood particle showed cellulose-degrading activity. Based on partial 16S rRNA gene sequences of cellulose-degrading isolates, the isolates were affiliated with 10 genera. Some cellulolytic bacterial isolates associated with the shipworm exhibited alkali-tolerant/alkaliphilic and/or moderate/extreme halophilic property, suggesting that shipworms are rich reservoirs of novel cellulolytic bacteria in extreme environments.

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