Symbiotic associations can vary along a continuum from parasitism to mutualism and the nature of these relationships is often context-dependent. Consequently, studying symbiotic relationships requires quantifying costs and benefits to each partner under different circumstances. Marine sponges host a diverse array of mesofaunal symbionts including polychaetes, crustaceans, and brittle stars. This system is profitable for examining symbiotic relationships because 1) sponge hosts can be subdivided to control for genotype and size and 2) mesofaunal associates can be added or removed. Sponge associates that feed on their host frequently have morphological features adapted for shearing or shredding sponge tissue, and sponges respond to the affront by repairing damaged tissue, increasing production of spicules, and thickening skeletal fibers. While sponge hosts provide a clear benefit to their symbionts in the form of food and shelter, the effect of the symbionts on the sponge host is relatively unknown because of a lack of manipulative experiments. To assess the costs of polychaete feeding on sponge growth and skeletal structure, I set up a high-density and low-density polychaete treatment and controlled for host genotype. I measured and photographed sponges weekly for 3 weeks and collected tissue samples for histology and electron microscopy.
Results/Conclusions
Sponges in the low-density polychaete treatment grew more than sponges in the high-density polychaete treatment. 53% of the sponges in the low-density polychaete treatment attached to the CPVC pipes while only 35% of the sponges in the high-density polychaete treatment attached to the CPVC pipes. These results suggest that polychaete feeding negatively affects a sponge hosts’ ability to regenerate and grow. During the experiment, I was surprised to discover that polychaete density decreased when brittle stars were present. Following this observation, I determined that each brittle star ingests on average 1-2 polychaetes per day. Because brittle stars influence the abundance of polychaetes, brittle stars may mitigate the costs of polychaete feeding on the sponge host. Brittle star abundance naturally varies between habitats (mangroves and seagrass) and across geographic locations (Florida Keys & Panama) which helps explain spatial variation in species interactions and fitness. These results underscore the context-dependent nature of species interactions where the magnitude or sign of the effect on fitness changes as a function of the abiotic or biotic context. Furthermore, this study highlights the importance of examining variation in species interactions rather than focusing on mean outcomes to elucidate patterns of variation in nature.