In Situ Determination of the Effect of Symbiotic Algae on the Growth of the Fresh Water Sponge Spongilla Lacustris

The freshwater sponge, Spongilla lacustris, which usually occurs in an endosymbiosis with zoochlorellae, can be rendered aposymbiotic by providing darkened conditions. Utilizing this situation, we have determined, in situ, the contribution of the algal symbiont to the growth of this association. Pigment and ultrastructure analyses demonstrated that sponges maintained under darkened conditions were effectively aposymbiotic. Growth of sponges under normal conditions was significantly greater than for aposymbiotic organisms. In two separate experiments, aposymbiotic sponges grew to only 20% and 49% of the size of normal sponges. Clearance rate determinations revealed no difference in the feeding activities of normal aposymbiotic sponges on Aerobacter aerogenes or Chlamydomonas reinhardti. The contribution from symbionts appears important to the high growth rate of this sponge and its successful utilization of soft—bottom habitats. This work represents the first in situ determination of the contribution by an algal in an endosymbiosis with an invertebrate.