Little Bugs and the Great Coral Crunching Sponge

Abstract

A 4.6-fold increase in the bimass of Cliona delitrix infesting Montastrea cavernosa substrate occurred in a portion of the Grand Cayman fringing reef affected by the discharge of untreated fecal sewage.It is suggested that the 6.3-fold increase in bacteria biomass (both coliforms and natural marine bacterioplankton) is linked to the sponge proliferation at the polluted site. Since demosponges normally obtain much of their nutritional needs from URPOC and only 11 from bacteria, the significance of the elevated bacteria count may be limited to its importance as a flag as an indicator of untreated sewage effluent. At the polluted study site, Montastrea cavernosa exhibited a 451 reduction in the amount of substrate occupied by living polyps.The loss of this respiring coral biomass is probably not compensated for by the biomass increases of C. delitrix and of the microflora inhabiting the dead substrate.The increased C. delitrix biomass reflects a. similar increase in the amount of M. cavernosa skeleton that has been eroded and reduced to silt-sized sediment. Thus, the discharge of untreated sewage into the reef environment can have a profound effect upon the trophic distribution of reef fauna, leading to a disturbance of the precarious balance between carbonate production and destruction on the reef.