Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 735

of the substrate and scrapers that have a non-excavating bite that removes only
the surface of the substrate ( Fig. 2D ). Although most scarids feed on dead coral
substrates, the large Bolbometopon muricatum is an excavator that feeds heavily
on live coral, especially on species of Acropora , and this can constitute over
50% of its diet on the Great Barrier Reef. Recent studies in the Caribbean also
have suggested that such feeding on live coral is far more widespread than pre-
viously thought ( Rotjan and Lewis, 2005 ).
Echinoids are important grazers of dead coral substrate ( Asgaard andBromley,
2008; Peyrot-Clausade et al., 2000 ) and on reefs that have been impacted by over-
fishingor eutrophication. Dense populations of echinoids candevelop,which leads
to high rates of grazing and substantial loss of reef framework ( Pari et al., 2002 )
( Fig. 2Aand H ). Grazing chitons ( Barbosa et al., 2008 ) and gastropods ( Lamet al.,
2007; Shafir et al., 2008 ) may also be locally abundant and graze on dead coral
substrate ( Fig. 2B ).
All these grazers of both live and dead coral are feeding on the endolithic
algae that live just underneath the surface of the coral substrate, as well as epi-
lithic algae on the surface of dead coral substrate. Specialized mouth parts include
jaws (scarids, Fig. 2C ), the grinding plates of Aristotle's lantern (echinoids), and
the radulae (chitons and gastropods; Fig. 2B ) to scrape off the epilithic algae and
the top layers of the substrate, which contains a network of endolithic algae. Deep
excavating species of scarid fish have well-developed plates within their pharynx
that grind the coral into a fine powder, which helps in breaking down the walls of
the algae releasing their cell contents. Fine sediment is defecated and contributes
to lagoonal sediments. The specialized mouth parts of echinoids, gastropods, and
chitons grind the coral into a fine powder, which releases the algal cells, and it is
presumed that they have the necessary enzymes to break down the plant cell
walls, as experiments show for fish ( Choat et al., 2004 ).
Damage from grazing invertebrate animals is typically shallow and unlikely
to be preserved. Grazing gastropods and chitons leave radular scratches on lime-
stone and skeletal substrates ( Radulichnus ), and regular echinoids can scrape a
set of five radial grooves ( Gnathichnus ) using their Aristotle's lantern.
Trace fossils ascribed to grazing by scarid fish have yet to be named for-
mally, but fossils of parrotfish are well preserved on Cenozoic reefs, the oldest
ones including a species of Bolbometopon , an eroder of Miocene age ( Bellwood
and Schultz, 1991 ).
(E) In-situ dead coral habitat split open to reveal boring bivalves and sipunculans (photo: P. Hutchings).
(F) Experimental study of bioerosion at Osprey Reef, Coral Sea, two replicate grids with newly laid
coral blocks to be exposed for varying periods of time (photo: J. Johnson). (G) Diagrammatic represen-
tationof a coral block illustratinghow the various components of bioerosion (grazing, boring, accretion)
are measured by slicing experimental blocks into a series of sections. Knowing the density of the block,
measurements can be scaled up to rates per square meter and net rates of bioerosion can be calculated
(a, original block; b, accretion; c, block remaining after grazing and boring) (image: K. Attwood).
(H) Experimental blocks after 6 months' exposure at Faaa, Tahiti, illustrating extensive grazing by
Echinometra mathaei (photo: M. Peyrot-Clausade).