The trophic symbioses of freshwater sponges


The immediate effects that symbionts exert on their host(s) are sometimes obvious, like a parasitoid wasp that erupts morbidly from the exasperated husk of its caterpillar host. However, intimate symbiotic interactions often lead to complex and unexpected long-term outcomes. Though unfamiliar to many, freshwater sponges are often prolific members of benthic freshwater communities world-wide that contribute greatly to benthic productivity; as much as 40% of total production in some systems! Much of their productivity is derived from a facultative endocellular symbiosis with green algae called zoochlorellae. Freshwater sponges are also plagued by specialist spongivorous insects that display morphological, behavioral, and phenological adaptations for consuming poriferan prey. Using natural abundance stable isotope signatures, Dr. Mac Strand and I found that freshwater sponges, like marine corals and sponges, obtain energy from their photosynthetic symbionts. The algae in turn likely obtain inorganic carbon and nitrogen from the waste-products of their host. Moreover, the insects that feed on the sponges appeared to be getting more nutrition from the algae inside the sponges than from the sponges themselves. Thus, the evolution of symbiosis with autotrophic algae provided the sponges a novel carbon source, but it may have also facilitated the evolution of algivorous enemies. Learn more here: