Fish are ____ meaning their body temperature matches the environment.

• A. Ectotherms
• B. Homeotherms
• C. Endotherms
• D. Poikilotherms

Answer: (D)

The concept being tested is how an animal’s body temperature relates to its surroundings. Fish do not actively generate and maintain heat to stay warm; their body temperature largely follows the temperature of the water around them. This pattern—variable body temperature that changes with the environment—defines poikilothermy.

Endotherms keep a constant internal temperature by metabolic heat production, and homeotherms are those that maintain that steady temperature regardless of outside conditions. Ectotherms is a broader term describing reliance on external heat sources, but it doesn’t by itself emphasize the temperature variability with the environment as directly as poikilothermy does.

So, fish are poikilotherms because their body temperature tracks the ambient water temperature, with only a few exceptions where certain species exhibit regional endothermy.