Bufo bufo tadpoles are dark, gregarious and often conspicuous in open water, a combination that would be reckless in a fish pond without a defence. They carry bufadienolide compounds in the skin that most fish find distasteful, and predatory fish typically reject them after a first mouthful. Common frog tadpoles lack comparable protection and are readily eaten.
The consequence is a division of breeding habitat. Common toads use larger, deeper and more permanent waters, including those holding fish, while common frogs favour shallow, often temporary pools that fish cannot occupy. The toads' clutches are laid as long strings wound through submerged vegetation rather than as clumps.
The defence is partial rather than absolute. Newts, dragonfly larvae and some invertebrate predators still take toad tadpoles, and a few specialist fish will eat them. Adult toads are separately defended by parotoid glands, and the persistent claim that handling a toad causes warts in humans has no basis: warts are caused by human papillomaviruses.