Peduncle of Hydra and the heart of higher organisms share a common ancestral origin

The heart is assumed to have evolved as the organ for pumping blood. Here we report a pumping phenomenon in Hydra, a member of the phylum Cnidaria. We find that the peduncle, lower quarter of the body column, stores most of the gastrovascular fluid when the animal is an elongate form. Upon contraction of the polyp, the peduncle contracts and transfers the fluid into the rest of the cavity. We also find that Hydra RFamide III, a homolog of cardioexcitatory RFamide neuropeptides in higher organisms, elevates this transfer activity. Further, CnNk-2, a homolog of a cardiomuscular tissue marker Nkx-2.5, is expressed in the endodermal tissue of the peduncle. These observations indicate that the transfer of fluid by the peduncle has a similar neurological and genetic basis to the pumping of blood by the heart, suggesting that the Hydra peduncle and the heart of higher organisms share a common ancestral origin.