What is a characteristic fish have but no other animal does as well?

This is a tricky question! While there are many features unique to fish, it's hard to say one is exclusively theirs, as other animals might possess some similar traits. Here's a breakdown of some common fish features and why they're not entirely unique:

* Gills: While fish are known for their gills, some other aquatic animals like amphibians have larval stages with gills.

* Scales: Some reptiles, like snakes, have scales.

* Fins: While fins are crucial for swimming, other aquatic animals like whales and sea turtles use modified limbs for similar purposes.

* Laying eggs: Many animals, including birds, amphibians, and reptiles, also lay eggs.

A possible answer that comes close:

* Lateral line: This sensory system allows fish to detect vibrations and pressure changes in the water. While some other aquatic animals possess a similar sense, it's much more developed and prominent in fish.

Ultimately, it's difficult to pinpoint a single characteristic that's completely unique to fish. Instead, their combination of features like gills, scales, fins, and the lateral line system make them exceptionally well-adapted for aquatic life.