THACKER, R.W.*; ERWIN, P.M.; BEVIS, K.P.; PAUL, V.J.; Univ. of Alabama at Birmingham; Univ. of Alabama at Birmingham; Univ. of Alabama at Birmingham; Smithsonian Marine Station, Fort Pierce, FL: Effects of Reduced Light Availability on a Sponge-Cyanobacterial Symbiosis

The tropical marine sponge Dysidea herbacea contains large populations of the symbiotic, blue-green alga Oscillatoria spongeliae. These cyanobacteria may benefit their host sponges by fixing carbon or nitrogen that is subsequently acquired by the host sponge. To determine whether cyanobacteria significantly contribute to sponge growth and maintenance through photosynthesis, we manipulated light availability in the field at a depth of 30 feet. Sponges were held in plastic cages to remove the effects of fish predation. Light cages were constructed with transparent tops, while shaded cages had opaque tops. Sponges were distributed among the treatments in a randomized block design, blocked by source sponge colony. After 16 days, sponges in the light cages lost 3.8% of their wet mass, an amount that was not significantly different from zero. Sponges in the shaded cages lost 28.5% of their wet mass, a significantly larger loss than observed in the light cages (ANOVA, P = 0.002), despite a significant amount of variation among source sponge colonies (P = 0.01). These results suggest that D. herbacea may require light-harvesting symbionts for healthy growth and maintenance. Other groups of sponges have been reported to contain cyanobacterial symbionts that do not make a significant contribution to their host’s carbon budget. We hypothesize that the extreme physiological dependence of D. herbacea on cyanobacterial photosynthesis may drive the high degree of host specificity observed between O. spongeliae symbionts and Dysidea spp.