Chaotropic ions and multivalent ions activate sperm in the viviparous fish guppy Poecilia reticulata

The control system of sperm motility in the fishes with internal fertilization still remains to be studied. In the present study, we examined the sperm activation in the viviparous fish guppy Poecilia reticulata as a function of solute composition. The degree of sperm activation by inorganic and organic solutes correlated well with the lyotropic or Hofmeister series. The solutes that induced sperm activation corresponded to those that have been termed "chaotropes" or macromolecule-destabilizing solutes. In addition to this Hofmeister rule, multivalent cations and anions had valency-dependent activating effects at low concentrations. We investigated the cell signaling triggered by chaotropes and divalent cations and found an intracellular cAMP concentration increase upon the activation of sperm motility. From the present findings, we propose a novel regulation mechanism of sperm motility in the viviparous fish.