It is difficult for terrestrial vertebrates to invade the sea, and little is known about the transitional evolutionary processes that produce secondarily marine animals. The utilization of marine resources in the intertidal zone is likely to be an important first step for invasion. An example of this step is marine scavenging by the Florida cottonmouth snakes (Agkistrodon piscivorus conanti) that inhabit Gulf Coast islands. These snakes principally consume dead fish that are dropped from colonial nesting bird rookeries, but they also scavenge beaches for intertidal carrion, consuming dead fish and marine plants, and occasionally enter seawater. Thus, allochthonous marine productivity supports the insular cottonmouth population through two pathways, and one of these pathways connects the snakes directly to the sea. The trophic ecology and behaviors of this unusual snake population suggest a requisite evolutionary scenario for the successful transition of vertebrates from a terrestrial to a marine existence.