SYMBIOTIC ZOOXANTHELLAE ENHANCE BORING AND GROWTH-RATES OF THE TROPICAL SPONGE ANTHOSIGMELLA VARIANS FORMA VARIANS

Several species of boring sponges harbor symbiotic zooxanthellae, and it is believed that the symbiont enhances boring activity of host spon ges. This hypothesis was tested using manipulative field experiments t o assess the effect of intracellular zooxanthella populations on borin g rates of the tropical sponge Anthosigmella varians forma varians. Po rtions of sponge were attached to 60 calcium carbonate blocks of known weight. Three sets of 10 blocks were grown at high light levels and t hree sets of 10 blocks were grown at low light levels for 105 d in the Florida Keys, Florida, USA. Boring rates, growth rates (lateral growt h and within-substratum tissue penetration), and zooxanthella populati ons were measured at the end of the experiment. Absolute rates of bori ng and growth of A. varians forma varians were significantly greater w hen zooxanthella densities were higher. Boring rate and tissue penetra tion related to final surface area of sponge attachment was also enhan ced when zooxanthella densities were higher, suggesting that the symbi ont plays a physiological role in the decalcification process. This is in contrast to the role that zooxanthellae play in coral hosts. Based on the results of this study, it appears that the presence of zooxant hellar symbionts has important ecological and life-history consequence s for host sponges. Ability to laterally overgrow competitors will be correlated with the size and activity of zooxanthella populations. In addition, the fitness of host sponges will be enhanced by algal symbio nts, since greater penetration within substrata will result in an incr ease in production of tissue that can be converted into storage, feedi ng and reproductive functions.