We have investigated the Antarctic sponge Isodictya erinacea as part of our ongoing study of Antarctic chemical ecology. I. erinacea was found to produce a tryptophan catabolite as its yellow pigment. The pigment, erebusinone, causes significantly reduced molting and proportionally increased mortality at ecologically relevant concentrations when fed to sympatric predatory amphipods. This appears to be the first example of molt inhibition as a mechanism of chemical defense. (C) 2000 Elsevier Science Ltd.