In addition to the somewhat dry Zone System reference I gave previously, another, better illustrated reference is Luminous Landscape's A Simplified Zone System.
When you go to that page and scroll down halfway to The Heart of The Matter: Determining Exposure the photo of the mountains, you'll see where you can meter that scene in three locations.
In the case of the frogfish, I would spotmeter it with your D80 and place it in Zone III as a starting point.
To learn how to use the spot metering function on your D80, see this D80 diagram and this D80 cheat sheet.
Note: The "Simplified Zone System" pages that have the info on your D80 also have a technique that uses five zones two stops apart, not 9 zones each one stop apart for the "real" Zone System Ansel Adams used (the Luminous Landscape tutorial). It's OK to use this for color negative film due to the extended latitude; but I've found it to be dodgy for chrome films & digital, because 2 EV (2 stop) increments can quickly lead to blown highlights/blown pixels.
Hope this helps the next time you coax that frogfish out of his hole!
...And, I might add, for shots of this type, exposure
One of the biggest "issues" with digital photography is that, between in-camera automation and Photoshop skills, even a below-average photographer can still turn out a decent photo... And with little-to-no schooling, either.
That is, until they get into an unusual situation they can't handle; or conversely getting the most out of a given image capture, either onto film or digitally.
As it turns out, for shots like Spen's frogfish, Ansel Adams figured out what to do eight decades ago with The Zone System. Understanding how this "intermediate level" technique works on film will lead you to the adaptations for the digital camera/Photoshop realm.
In the case of the frogfish photos, everything Spen needs to do involves luminosity... In other words, the issues he is having would be the same as if for a B&W photo (i.e. color is not an issue here).
.. me too.
