February 10, 2011

Frog spawn

Photo courtesy Ray Tomes
The first reports of frogspawn are coming in to the Phenology UK website; as is usual with spring records they are from Cornwall, Devon and South Wales. Across the country, records show that frogs are spawning an average of ten days earlier now than in the middle of the last century.

Each batch of spawn numbers 23000 eggs, of which only five or six adult frogs will survive to return and breed again the following year. The eggs are black, helping to absorb available light; this boosts the embryo's temperature and speeds development. The eggs will be frogs in 612 weeks, depending on the weather and available sunlight; spawn rises on sunny days, and in cold weather may sink in order to protect it from freezing.

Once the tadpoles (or 'pollywogs') have swum free from the eggs they will eat algae until ready to undergo their main metamorphosis. During this process they will lose their gill pouch, their jaw will grow and teeth will appear, their gut will shorten to that of a carnivore's, their eyes will move up and grow eyelids, their skin will thicken and they will develop an eardrum and some bones. This amazing process takes only 24 hours.

At the end of their metamorphosis they will need to breathe air and will be seen near the surface of the water. They still have a tail, but not for long; it is finally absorbed, and its material used to grow the frog's legs.