Induction of Metamorphosis of Larvae of the Green Sea Urchin, Strongylocentrotus droebachiensis, by Coralline Red Algae

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Abstract
The coralline red algae, Lithothamnion glaciale, Phymatolithon laevigatum, P. rugulosum, and Corallina officinalis, induced >85% of laboratory-reared larvae of Strongylocentrotus droebachiensis to metamorphose. Larvae must contact live L. glaciale or its spores for metamorphosis to occur; the inducer is not sensed in the water column. However, aqueous extracts of L. glaciale can induce metamorphosis, suggesting that the inducing factor is chemical. Neither ashed nor boiled L. glaciale induces metamorphosis, indicating that the factor is heat-labile and that thigmotaxis, per se, is not important in the response. The amino-acid, γ-aminobutyric acid (GABA), which induces settlement of other marine invertebrate larvae, also induces significant rates of metamorphosis of S. droebachiensis at concentrations ≥ 10-4 M. A reduction (with antibiotics) in the number of live bacteria on the surface of L. glaciale does not affect the rate of metamorphosis of larvae.
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