Intraspecific variation in stable isotope signatures indicates no small-scale feeding interference between a horse mussel and an ascidian

Interspecific feeding interference between neighboring suspension-feeders is an important but understudied structuring process in marine benthic communities. The comparison of delta C-13 and delta N-15 stable isotope ratios (SIR) between species is widely used to identify the dietary overlap and competition for food. Results from this approach are sometimes substantially biased by the variation of isotope fractionation rates among species and tissues. We suggest that the difference in SIR within a single species in the presence and absence of a potential competitor provides stronger evidence of feeding interference. In the White Sea, shallow subtidal solitary ascidians Styela rustica L. frequently develop in clumps on horse mussels Modiolus modiolus (L.). Horse mussels without ascidians and ascidian clumps attached to gravel are also common. We analyzed SIR in muscular tissues of neighboring (within 0.25 m(2)) M. modiolus with and without ascidians attached and in S. rustica from nearby (within 0.25 m(2)) clumps on gravel and mussels. Species (M. modiolus or S. rustica), tissue (foot and posterior adductor muscles in M. modiolus) and site (8.1 km apart) factors had strong effects on isotope ratios, whilst the presence of a potential competitor had no effect on SIR in either species. We conclude that the diets of M. modiolus and S. rustica are not affected by co-occurrence and do not overlap much, giving no evidence of competition. Further research should take in account the substantial difference in SIR between the 2 M. modiolus muscular tissues studied: -21.582 +/- 0.048 (delta C-13) and 6.551 +/- 0.063 (delta N-15) in foot muscle versus -20.970 +/- 0.063 (delta C-13) and 7.806 +/- 0.074 (delta N-15) in adductor muscle.