Delayed metamorphosis usually occurs when scallop Pecten maximus (L.) larvae are reared in still water. Metamorphosis rates may be increased by adding aqueous extracts of certain marine red algae, including one [Delesseria sanguinea (Linne) Lamouroux (Delesseriaceae, Rhodomelaceae)] from which various chemical inducers have been isolated. Of these, jacaranone (a simple quinol) is by far the most active component; other components (p-OH phenylacetic acid methyl ester, p-OH benzaldehyde, p-OH benzyl alcohol) are only weak inducers. All are catabolic derivatives of tyrosine. Homogentisic acid (biosynthetically related to jacaranone) and some catecholamines (L-DOPA, epinephrine) can also induce metamorphosis of P. maximus larvae. All these compounds can be transformed into quinones by oxidation. This result, together with literature data, strongly suggests that the true inducer(s) present in the environment might possess a quinone or quinone-like structure, as is the case for jacaranone.