Anamniotes vs. Amniote — What's the Difference?

Anamniotes are a group of animals, including fish and amphibians, that lay eggs without an amniotic sac, making them primarily suited for aquatic environments. This characteristic means their eggs are more susceptible to drying out, limiting their habitats. On the other hand, amniotes, comprising reptiles, birds, and mammals, possess an amniotic egg, which includes a protective layer that prevents water loss, facilitating a broader range of habitats, including terrestrial environments.

While anamniotes are ectothermic, relying on external sources to regulate their body temperature, amniotes include both ectothermic (reptiles) and endothermic (birds and mammals) species, allowing them some to maintain a constant body temperature irrespective of the environment. This thermal regulation contributes to the amniotes' ability to inhabit diverse and fluctuating climates.

The skin of anamniotes is often permeable to water and gases, a trait that necessitates a moist environment for survival, as seen in amphibians. Conversely, amniotes have developed keratinized skin or feathers in birds, scales in reptiles, and fur in mammals, offering protection against dehydration and facilitating life in dry environments.

In terms of reproduction, anamniotes typically require water bodies for the process, as their eggs and sperm must meet in an aquatic medium. Amniotes have evolved internal fertilization mechanisms, significantly broadening their potential habitats by eliminating the dependency on water for reproduction.