Abstract
To investigate the importance of symbiont‐derived nutrition to host sponges, we coupled manipulative shading experiments with stable isotope analyses of isolated symbiont and host cell fractions. Experiments were conducted with four common reef sponges: Aplysina cauliformis , A. fulva , Neopetrosia subtriangularis , and Niphates erecta . The sponge N. erecta lacks photosymbionts, had a higher growth rate under shaded conditions, and displayed no difference in chlorophyll a (Chl a ) concentrations across treatments. Isotope values suggested that this sponge obtains nutrition from particulate organic matter in the water column. In contrast, sponges hosting cyanobacterial symbionts ( Aplysina spp. and Neopetrosia ) had lower growth rates and lower Chl a concentrations under shaded conditions, suggesting that these hosts rely on photosymbiont nutrition. δ 15 N and δ 13 C values of sponge and microbial cell fractions demonstrated that, while both carbon and nitrogen are transferred from symbionts to host cells in A. cauliformis , only carbon is transferred in N. subtriangularis , and only nitrogen is transferred in A. fulva . Under shaded conditions, shifts in symbiont δ 13 C values were coupled to shifts in host δ 13 C values in some, but not all, host species, suggesting that the stability of these interactions varies across host species. Symbiont‐derived nutrients are transferred to the cells of host sponges, and the variability observed among host species indicates that these interactions are more complex than originally hypothesized.