Ambient exposure to a short synthetic peptide has enhanced fecundity (number of offspring) in invertebrates and vertebrates, ostensibly by disinhibiting reproduction. In separate experiments nematodes (Caenorhabditis elegans) and guppy fish (Poecilia reticulate) were exposed via their aqueous environment to a dissolved synthetic hexamer peptide, IEPVFT (EPL036), at a concentration of 1 μM. In the case of the worms peptide was added to their aqueous buffer daily throughout the experiment (14 days); in the guppies peptide administration was on the first 15 alternate days in a 50-week experiment. Fecundity rose by 79% among the worms. The number of descendants of the treated guppies was more than four times that of controls by Week 26 (103 vs 25), with double the estimated biomass. It was deduced that treated females bred earlier, at a smaller size, and had larger brood sizes. The total number of fish in the control tank had caught up by termination, but biomass continued to lag the test tank. There were no overt signs of toxicity among either the worms or the fish. Bioinformatics has been unilluminating in explaining these results in terms of mimicry of an endogenous regulator. A mass spectrometric campaign to identify a receptor proved inconclusive. Molecular modelling in silico indicated unexpectedly that the 6mer EPL036 might be acting as an antagonist, to pro-fecundity effect; that is, as an inhibitor of an inhibitor. This suggests that there awaits discovery an evolutionarily conserved reproductive inhibitor and its (anti-fecundity) receptor.