Protecting tadpoles from frost

There was an air frost last night and I decided that the more portable of my tadpole intensive care units would best be brought in from the cold.

In a lean-to it stayed warm all night and the tadploes were actively feeding at 6am. this morning.
They are cold-blooded so their activity levels (and growth rate) depend on the water temperature.
The water temperature in the bucket was 12C.

The water temperature in the square container (an old freezer tray) which was left outside all night was near freezing at 2C. only.  No wonder that the tadpoles are nowhere to be seen.  They will be huddling in the "warmer" water at the bottom.  If I remember correctly there is an inversive at 4C when water at that temperature sinks to the bottom of a freezing pond.
By comparison, in the pond where these tadpoles came from, their development is much less advanced than those in my hotter containers.