[Coral-List] Diadema and sea level rise

[Douglas Fenner]

Gene,
       Thanks for this, fascinating observations, who knew that
hogfish ate *Diadema
*urchins?  I didn't.

       Just to make it explicit, in case anyone has not made the
connection, the purple spots are likely the tips of the *Diadema* urchins'
needle-sharp spines.  Many people, myself included, have had the same
experience with their hands and/or feet.  You can't touch the tips of the
spines without getting them stuck in you, and the tips then break off.  You
can remove parts of the broken-off spines that are sticking in you, but the
tips break off and remain in you, as little dark spots.  In time (maybe a
week) they dissolve in you and disappear.  I never got infected from them.
A rather effective defense, one would think, similar to cactus spines, an
example of convergent evolution.  I presume that the same thing was
happening with the hogfish.  The spines are single crystals of calcite,
with tiny reverse barbs, and a little bit of weak venom.  The complexity of
the construction of *Diadema* spines can be seen in a scanning electron
microscope photo of a broken *Diadema* spine on page 413 in Pechenik (1985,
and probably subsequent editions), I can see the reverse barbs.
     For those in the Indo-Pacific who are not familiar with what is called
a hogfish in the Atlantic, they are a wrasse.  *Lachnolaimus maximus*.
Lieske and Myers say "Prefers open areas with abundant growth of
gorgonians.  Roots in sand for molluscs, crabs, and sea urchins."

Cheers, Doug

Pechenik, J. A. 1985. Biology of the Invertebrates.  Prindle, Weber &
Schmidt, Boson.  513 pp.

On Mon, May 15, 2023 at 1:49 AM Eugene Shinn via Coral-List <
coral-list at coral.aoml.noaa.gov> wrote:

>   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
>
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