Induction of metamorphosis in larvae of a sponge

Elevated exogenous KCl and CsCl are effective artificial metamorphic triggers for many, but not all, invertebrate larvae. Neither taxonomic nor functional patterns in their effectiveness, however, are apparent. In our experiments, elevated exogenous KCI promoted metamorphosis of larvae of the demosponge Aplysilla sp., but only when applied simultaneously with a biofilmed artificial substratum. Although the presence of this artificial substratum alone significantly enhanced settlement over seawater controls, its effectiveness was further augmented significantly in the presence of elevated KCl, indicating a synergistic interaction. Exposure of larvae to a 2-min pulse of 30 mM KCl promoted metamorphosis as effectively as continuous exposure to 30 mM KCl. Moreover, 30 mM CsCl proved an effective trigger of metamorphosis both in the absence and presence of a biofilmed substratum. As with KCl, the CsCl data indicate the presence of a synergistic interaction between CsCl and the biofilmed substratum. This report provides preliminary insights into the induction of settlement and metamorphosis of sponges, metazoans which lack established sensory and neuronal elements, and it permits comparison with responses of eumetazoans.