The axolotls lives its entire life in larval form, never growing up, and is native to one lake in Mexico which is under threat from the expansion of Mexico City.

The creature becomes sexually mature but fails to undergo the metamorphosis which other salamanders do in order to become land-living animals.

"They are missing a vital thyroid hormone that is needed for metamorphosis. Apparently an injection of the right hormone can trigger transformation, but otherwise they stay babies for ever," said displays supervisor Kieran Copeland.

The new arrivals, numbering around 50 and measuring about 10mm, are being cared for in their own nursery tank and can be seen by visitors.

"To successfully breed creatures as rare and endangered as these offers hope for their long-term survival. Even their one surviving wild home, Lake Xochimilco, is under real threat from the growth of Mexico City," said Mr Copeland.