Search

To investigate how the interactions between the closely related sponge species Aplysina cauliformis and Aplysina fulva and their symbiotic microbial communities vary under changing environmental conditions, we conducted a manipulative shading experiment with treat- ments spanning a gradient of 6 irradiances. In A. cauliformis, there was a tight coupling of sym- biont and host metabolism across treatments, and changes in growth rate were correlated more with shifts in symbiont δ 13 C and δ 15 N values than with shade treatment. In contrast, symbiont and host C metabolism were weakly coupled in A. fulva, and the growth of this species was not corre- lated with shifts in symbiont δ 13 C and δ 15 N values. In addition, although photosymbiont meta - bolism was an important driver of shifts in holobiont C and N metabolism of both host species, host and photosymbiont C metabolism were only correlated in A. cauliformis. Thus, although both species host stable, abundant, and similar photosymbiont communities, each host forms a unique mutualism with its symbionts. These 2 host species may be on different evolutionary trajectories, potentially allowing each to exploit novel niche space in coral reef ecosystems. This study pro- vides data in support of the hypothesis that these symbioses represent a dynamic balance of costs and benefits and provides evidence that, because these costs and benefits are highly species - specific, not all species will respond similarly to environmental fluctuations.

Coral reef sponges frequently harbor symbiotic cyanobacteria that can potentially ben- efit their hosts by providing supplemental nutrition. Although nutritional mutualisms have been doc- umented for Indo-Pacific sponges, previous investigations suggest that Caribbean sponges obtain no nutritional benefits from their symbionts. We conducted field-based shading experiments to deter- mine the effects of reduced irradiance on symbiont abundance and host growth in the Caribbean coral reef sponges Aplysina fulva and Neopetrosia subtriangularis, both of which host the unicellular cyanobacterium Synechococcus spongiarum. For both host species, 6 wk of shading significantly reduced chlorophyll a concentrations to less than half of those in control sponges. Shaded A. fulva individuals exhibited less than half the growth of control sponges. For N. subtriangularis, no signifi- cant differences in growth rates were observed between shaded and control sponges. Measurements of gross photosynthesis to respiration (P:R) ratios predicted that both host sponges were heterotrophic at low irradiances ( 500 µmol m -2 s -1 ), with measurements obtained at high irradiances likely to be more indicative of natural conditions. Phylo- genetic analyses of cyanobacterial internal transcribed spacer (ITS) ribosomal RNA gene sequences revealed 3 genetically distinct clades of S. spongiarum (designated A, B and C). Clade A symbionts were isolated exclusively from A. fulva, Clade B symbionts from both host sponge species and Clade C symbionts exclusively from N. subtriangularis. These findings demonstrate that Caribbean sponges associate with diverse clades of cyanobacterial symbionts that may confer variable nutritional bene- fits; in some cases, these symbionts appear to substantially enhance host growth rates.