ERWIN, P. M.*; THACKER, R. W.; University of Alabama at Birmingham; University of Alabama at Birmingham: Incidence and Importance of Photosynthetic Symbionts in Shallow-Water Sponge Communities
Symbioses between marine sponges and photosynthetic organisms (e.g. cyanobacteria) have been described from coral reef communities; however, the frequency of these associations and the ecological nature of interactions between hosts and symbionts often remain unresolved. In this study, we determined the number of sponge species harboring photosymbionts and assessed their prevalence in the shallow-water reefs of Bocas del Toro, Panama, by conducting line-intercept transects with measurements of chlorophyll a (chl a) concentration. Sponges were dominant members of these benthic communities, second in abundance only to stony corals. Twenty-five of the 67 species investigated (37.3%) exhibited high chlorophyll levels (>150 µg/g) and accounted for 35.6% of the observed sponge community. Association with photosymbionts is a common strategy among coral reef sponges and may represent an important source of primary productivity in oligotrophic coral reef environments. Additionally, two sponge species known to harbor cyanobacterial symbionts (Aplysina fulva, Xestospongia subtriangularis) were experimentally shaded to test the effects of reduced light availability on symbiont load and host growth. Six weeks of shading reduced the abundance of symbionts (>40% reduction in chl a for both species). In A. fulva, control sponges exhibited over twice the growth of shaded sponges (by volume and mass). In X. subtriangularis, no significant difference in growth was observed between control and shaded sponges. These results suggest that the relationship between sponges and their cyanobacterial symbionts varies among host species, with some sponges strongly dependent on symbiont photosynthesis and others unaffected by short-term decreases in symbiont activity.
