Final answer:
The hook in this passage is the statement about unique and diverse adaptations serving many different purposes.
Step-by-step explanation:
The hook in this passage is: These adaptations are all unique and serve many different purposes. A hook is a line or phrase at the beginning of a passage that is meant to grab the reader's attention and engage them. In this case, the statement about unique and diverse adaptations serves as a hook because it introduces an intriguing aspect of deep-sea creatures and entices the reader to learn more about them.