There was a time when The National Oceanic and Atmospheric Administration was known for more than just mermaid denials. Environmental study and education, resource management, weather reporting and charting all fall under the NOAA’s purview. Their photo library, for another, is an excellent repository of public domain images. And at least one notable recording. (Of mermaids?)
The picture above captures lobate ctenophore Ocyropsis maculata (we had to look it up). We weren’t quite sure how this would fit on the book cover, but it’s a remarkable shot.