Larvae of the Australian sea urchin Holopneustes purpurascens are induced to settle and metamorphose (termed settlement herein) by a water-soluble compound produced by the red alga Delisea pulchra, the main host plant of new recruits. The settlement cue for H. purpurascens had previously been identified as a floridoside-isethionic acid complex, and this paper presents new evidence correcting that finding. The actual settlement cue produced by D. pulchra was isolated from the polar extract by cation-exchange chromatography and identified as histamine, using one- and two-dimensional nuclear magnetic resonance spectrometry. The chemical identity of the cue was confirmed by gas chromatography-mass spectrometry (GC-MS) and matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry. Synthetic histamine and histamine at 4.5 microM isolated from D. pulchra both induced rapid settlement in 80%-100% of the larvae of H. purpurascens. Lower concentrations of histamine (0.9-2.3 micro M) induced larval settlement, but this response varied from 0%-90%. The histamine content of two host plants of H. purpurascens--D. pulchra and Ecklonia radiata--and of four other common species was quantified using GC-MS. D. pulchra had the highest histamine content, which is consistent with H. purpurascens recruiting to this species. Histamine was also detected in the seawater surrounding these host algae. This is the first time that a settlement cue has been quantified in the habitat of a marine organism. ; 12 page(s)