Mud shrimp burrows as dynamic traps and processors of tidal-flat materials

Bacterial abundance, electron transport system activity (ETSA) and organic matter content (total organic carbon [TOC], total nitrogen [TN] and chl a) on the burrow wall of the mud shrimp Upogebia major were determined and compared with those in surrounding non-burrow sediments and on the tidal-flat surface. The values of each parameter in burrow sediment tended to decrease outwardly from the wall, while bacteria abundance was highest in the subsurface of the burrow wall. In summer, the abundance of bacteria on the burrow wall was double that in the non-burrow sediments. In winter, both bacterial abundance and ETSA were at the same level as those in non-burrow sediments. The levels of TOC and TN on the burrow wall were more than 3 times higher than those in non-burrow sediments, regardless of the season. However, there was no significant difference in chl a content between burrow wall and non-burrow sediments. These results suggest that fresh organic matter in the burrow wall is supplied from the tidal-flat surface, making the burrow environment a suitable niche for microbial populations in the sediment. The mud shrimp burrow functions as a trap for organic matter and thus, helps prevent the outflow of carbon and nitrogen from the tidal flat.