Answer: What kind of fish?

Short answers:  

1. Fish?  Stoplight Parrotfish (Sparisoma viride). 

2.  Geology?  Parrotfish bite into the coral, scraping off a bit with each bite.  They digest the algae they get in that bite, then the coral bits are excreted into the water, making up a large amount of sand.  In essence, much of the white sand of Caribbean island beaches is made up of fish poop!  (Okay, the excreted, indigestable bits of pulverized coral.)  

Stoplight parrotfish (Sparisoma viride)

My solution:  As you can see from the comments, there are many paths to solving this.  I solved it by searching for: 

     [ Caribbean fish guide ] 

And finding ReefGuide.org which has a very high quality guide to the reef fishes of the world.  From their home page I chose "Caribbean, Bahamas, Florida" and then did a quick scan of the fish families.  It wasn't a "small oval" fish, or any of the other's I clicked, until I found the parrotfish category.  Once you see the parrotfish page, it's pretty clearly that kind of beast.  

ReefGuide.org's parrotfish categorization page

To find the geological effects of this fish... My first search (like many of you) was for 

     [ stoplight parrotfish geology ] 

but the results were too diffuse to be useful.  (Search question:  Why didn't I include the double quotes a la "stoplight parrotfish"?  Because I figured that bigram--stoplight then parrotfish would be pretty common only as a fish reference.  So I didn't bother.)  

So I modified my query to be:  

     [ parrotfish geology ] 

and quickly found many articles about the effect of parrotfish (in general, not just the Stoplight parrotfish) on sand creation in the Caribbean.  

Just to triangulate what I was reading, I did the same search on Scholar to see a few scholarly papers on the topic.  

One paper,  "Rate of bioerosion by Parrotfish in Barbados marine environments" (1) points out that "[stoplight parrotfish] were observed to spend 80% of their time feeding on surfaces covered with filamentous algae and 20% on sand. Only Sparisoma viride consistently leaves well-defined scars on live coral."

But there's a debate among different sources about how MUCH sand a parrotfish produces each year.  If you look at the comments, some say it's 1 ton (2,000 pounds) / year / fish, while others say it's 200 pounds / year / fish.  Since that's SUCH a variance, I'm going to let the crowd work on the rate of sand production / fish for a bit longer.  The mystery remains!  
3.  How MUCH sand does a single parrotfish create in a year?  

As noted in the comments, some male parrot fish keep harems of females nearby. When the dominant male dies, one of the females will change gender and color and become the alpha male for the harem.


There's also a striking difference in appearance between the adults and the juveniles. Here's a photo I took yesterday of a juvenile Stoplight parrotfish. 

Stoplight parrotfish,  juvenile (Sparisoma viride)

Tomorrow I'll comment more on the "sand production" question, but give it a shot. It's a genuine difference of data question.  Since they vary by a factor of 10, one estimate should be clearly very wrong!  

Back to vacation!  Nice job everyone.  

And... just for Remmij, who asked for a picture of the Hilma Hooker, here's one shot of one our buddies entering the wreck through a hatch opening (it was on the top, but the wreck is laid over on its side, so this is now on the side of the wreck...)   

Entering the Hilma Hooker wreck at 80 feet.

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(1) Frydl, Paul, and Colin W. Stearn. "Rate of bioerosion by parrotfish in Barbados reef environments." Journal of Sedimentary Research 48.4 (1978): 1149-1157.