MEPS 467:113-120 (2012)  -  DOI: https://doi.org/10.3354/meps09951

ABSTRACT: Interspecific feeding interference between neighboring suspension-feeders is an important but understudied structuring process in marine benthic communities. The comparison of δ¹³C and δ¹⁵N 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 m²) M. modiolus with and without ascidians attached and in S. rustica from nearby (within 0.25 m²) 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 (δ¹³C) and 6.551 ± 0.063 (δ¹⁵N) in foot muscle versus −20.970 ± 0.063 (δ¹³C) and 7.806 ± 0.074 (δ¹⁵N) in adductor muscle.

KEY WORDS: Stable isotope ratios · Bivalves · Ascidians · Trophic interactions · Competition