What is an example of a structural adaptation?

A tortoise's hard shell is an excellent example of a structural adaptation. This feature serves multiple critical functions for the tortoise's survival. Firstly, the hard shell provides protection from predators, acting as a defensive barrier that allows the tortoise to retreat into it when threatened. Secondly, the shell contributes to the tortoise’s ability to survive in harsh environmental conditions, as it can help to retain moisture and protect against extreme temperatures. Structural adaptations, like the hard shell, involve physical characteristics that enhance the organism's ability to survive and reproduce in its specific environment, making this answer a strong choice when identifying structural adaptations.

Other options, while they demonstrate adaptation, refer to different types. For instance, migration is a behavioral adaptation, the growth of plants towards sunlight is an example of a physiological response known as phototropism, and fish using gill filaments reflects a functional adaptation that involves physiological processes for respiration rather than a structural characteristic.