What characteristic do reptiles and fish share?

Reptiles and fish share many characteristics, but one of the most notable is that they are both ectothermic. This means they rely on external sources of heat to regulate their body temperature, unlike mammals and birds which are endothermic and generate their own heat.

Here are some other characteristics they share:

* Vertebrates: Both are members of the phylum Chordata, meaning they have a backbone.

* Aquatic ancestors: Both groups evolved from aquatic ancestors, although reptiles later adapted to land.

* Similar respiratory systems: Fish use gills to breathe underwater, while reptiles have lungs for breathing air. However, some reptiles, like sea turtles, can hold their breath for long periods.

* Scales: Both reptiles and fish have scales, although they are made of different materials. Fish scales are typically bony, while reptile scales are made of keratin.

* Lay eggs: Both reptiles and fish lay eggs. However, some fish give birth to live young.

Overall, while reptiles and fish have evolved along different paths, they share a common ancestry and some key biological features.