Trophic segregation in a burrow: the stable carbon and nitrogen isotope ratios of the burrowing shrimp Upogebia major and its commensal bivalve Cryptomya busoensis

: Burrows produced by marine invertebrates often harbor other small commensal invertebrates. The mud shrimp Upogebia is known to coexist with the myid bivalve Cryptomya in a burrow produced by the shrimp. Both species are filter-feeders, and thus interspecific competition or trophic niche segregation may occur in the burrow. Samples for carbon and nitrogen stable isotope analysis were collected from a tidal flat near the tidal inlet of Akkeshi Lake, Hokkaido, northern Japan in April 2013. In addition, stratified benthos sampling was conducted on the tidal flat in August 2018, to clarify the interspecific relationship between U. major and C. busoensis in the burrow. The stratified benthos sampling showed the vertical distribution of these species, and indicated that both species filter water from the same part of the burrow for feeding. The stable carbon and nitrogen isotope analysis showed that important food sources for both U. major and C. busoensis are marine phytoplankton and microphytobenthos. In addition, C. busoensis is likely to consume terrestrial organic matter whereas U. major is unable to utilize it. The partial trophic segregation between the species increases the potential benthic filtering because it allows the Upogebia burrow complex to con sume a wide variety of organic matter, and it might reduce interspecific competition between the filter-feeding host and its commensal species. These results demonstrate how ecologically similar macrobenthos can coexist in a burrow.