[Coral-List] Diadema and sea level rise

[Eugene Shinn]

  Dear readers, Here are some observations that may relate to the 
Diadema situation. When I began spearfishing in the Florida Keys in the 
late 1950s (I was a biology  student at University of Miami) One of my 
prime targets was Hog Fish, sold in markets as Hog Snapper. All the 
large Hog Fish had a dozen or more purple dots on their forehead and 
around mouth area. When I gutted them before selling to markets and 
restaurants their most common stomach content were bits of urchin tests. 
One day I came upon a Hog fish carefully removing the spines from a 
Diadema. I just watched and it became clear what caused the purple dots 
on the fishes head area. When almost all the spines had been removed the 
fish, known for its large expandable mouth swallowed the test in one 
gulp. After the Caribbean-wide Diadema die-off in 1983, the purple dots 
on the fish disappeared. It became obvious why the dots were missing. 
Their favorite meals were gone. Curiously I have never seen this noted 
in any publications.
       Another observation that Harold Hudson and I noted back then was 
that the largest fattest Hog fish were located off the upper Keys, 
especially off Elliot Key and Key largo. These were the areas of most 
abundant live coral and lush Turtle grass. He had noted that off the 
middle keys in the vicinity of Grassy Key, Long Key, and opposite the 7 
mile bridge area, the Hog Fish were generally smaller and thinner and 
few had the purple dots.  The areas off the middle keys also had less 
live coral, large sandy areas and noticeably less Turtle grass. and also 
fewer Diadema. Farther southward, including off Key West, the Hog fish 
were a little larger but seldom as large as those off the upper Keys. In 
later years working with Robert N. Ginsburg we attributed the lack of 
lush reefs and sparse Turtle grass off the middle keys to runoff from 
Florida Bay (especially off 7 mile bridge area) during low tide. We also 
noted that the water depth in the middle and lower keys is deeper than 
off the upper keys. We were not the first to notice this. During the 
Holocene sea level rise the middle and lower keys were the first to to 
be affected by the more saline bay water than areas off Key largo were 
there are fewer tidal channels. We determined that outflow of bay water 
retarded or prevented development of coral growth. We published these 
observation several times in the past and attributed the deeper depth to 
the south to subsidence. Subsidence is suggested by several 
observations. The Pleistocene oolite limestone under Miami is 20 or more 
feet above sea level while the same age oolite under Key West is only 
about 10 ft above sea level. There are other evidences of differential 
subsidence toward the south as well. Evidences of subsidence and 
distribution of coral reefs are detailed in,  "Geology of the Florida 
Keys" 2018, Shinn and Lidz, University press of Florida. The book is 
available on Amazon and at Fossil Reef State Park on Windley Key. 
Whether the observations have significance to the recent Diadema die-off 
I do not know. Gene