Molecules from cyanobacteria and red algae that induce larval settlement and metamorphosis in the mollusc Haliotis rufescens

Potent inducers of metamorphosis of planktonic larvae of the gastropod mollusc Haliotis rufescens have been found in the following phycobiliprotein-producing cyanobacteria. Synechococcus spp. (one marine and one freshwater strain). Synechocystis spp. (one hypersaline and one freshwater strain) and Spirulina platensis (a freshwater strain). No inducers were detected in the bacterium Escherichia coli. Inducers from one of the cyanobacteria (S. platensis) were partially purified and compared to inducers from the foliose red macroalga Porphyra sp. and the crustose coralline red alga Lithothamnium californicum. In all three species the inducers can be largely separated from the biliproteins, with which they appear to be associated, by high-resolution gel-filtration chromatography. The molecular weights of the relatively small inducing molecules resolved by these procedures from cyanobacteria and red algac are similar, falling in the range of 640 to 1 250 daltons. The amenability of the cyanobacteria to largescale cultivation, and to physiological and genetic manipulation, make them useful for production of metamorphic inducers of marine invertebrate larval metamorphosis, and for further studies of the synthesis, structure and mechanism of action of such inducing molecules.