The adaptive significance of sequential hermaphroditism, typically found in tropical fish and marine invertebrates, has been successfully explained by the size-advantage principle (SAP) and notable exception has not been reported. Here we focus on a novel type of protandrous sequential hermaphroditism in an intertidal holothurian as a case not amenable to SAP. This species typically participates as a male in the first three spawning events, and then spawn eggs at the fourth, which is followed by the second sex change to release sperm again. By incorporating published empirical findings into evolutionary invasion analyses, we successfully specified the theoretical conditions that a population containing the reversible sex change is evolutionarily stable. Our scheme is a natural extension of the classical Fisher's principle of sex allocation, and is also capable of consistently explaining a strongly biased operational sex ratio found in sequential hermaphrodites.