The metamorphosis from egg to froglet or toadlet usually takes about 12 weeks for common toads and frogs (Beebee 1987). The frogspawn is often attached to weeds or laid in a series of clumps close together, which may help absorb and retain heat (Mattison 2011). The strings of toadspawn are also wrapped around water plants (Mattison 2011)..
When the tadpoles first emerge, the head and tail are not much differentiated. There are small gills on the outside that are then absorbed. After about a week, the head is visibly different from the tail and begins to be more oval than round in shape.
The tiny tadpoles cluster round the spawn for a week or more before launching out into the pond. In the spawn is the nourishment they need including melanin, a pigment which helps them absorb warmth from the sun (Beebee 1987). There is safety in numbers and they cluster in the centre of the spawn so the remains of the spawn create a barrier (as well as the surface weed) between them and the predators.
When the tadpoles leave the safety of the spawn (which is now fairly ragged) they gravitate to the shallows or the edges of a pond at first in a mass but then strike off on their own and are more difficult to see. They hang motionless on bits of weed. If they are not attached to weed or are not wiggling, they sink to the bottom. Predators include dragonfly larvae, beetles, goldfish, newts, water boatmen – though some of these prefer the frog tadpoles to the toad taddies. A dragonfly larva will surface like a crocodile and grab the wriggling tadpole. Their attack is probably triggered by a tadpole’s movement since they seem to ignore the motionless ones. Losses are high.
Some tadpoles may never grow up. In very cold ponds, they stay as tadpoles instead of metmorphosising into frogs. Or they will overwinter as tadpoles and turn into frogs in the second year.
Beebee, T., (1985), Frogs & Toads, Stowmarket, UK, Whittet
Mattison, c.,(2011), Frogs and Toads, London, UK, Natural History Museum.