The Great Barrier Reef
80
and create burrows for a suite of non boring organisms or
nestlers to colonise. In contrast, the sipunculans, sponges,
bivalves and some of the larger polychaete borers are
long-lived and once they have created their burrow they
are entombed in the substratum and may then live for
several years (Fig. 8.3A, B). These organisms bore by either physically eroding the substratum or chemically dissolving it, or by a combination of these methods.
Rates of boring decrease once the borers are established, and subsequent rates are just sufficient to allow
those organisms to grow. This is particularly true for
sponge colonies. Borers must obviously retain a link to
the outside of the substratum in order to obtain their
food, for respiration and for discharging their gametes.
Once established in the substratum they are effectively
entombed, often in flask-shaped burrows (Fig. 8.3B).
They cannot leave their habitat, although some species
of molluscs, primarily Conus spp., search out for particular boring species of polychaetes and sipunculans
and insert their proboscis into the burrow and then
proceed to suck out the worm.
Experimental studies have shown that recruitment of
boring organisms is seasonal with maximum recruitment
occurring during the summer months; however, some recruitment of borers occurs throughout the year. This
means that within weeks of substratum becoming available it is already being colonised by borers. Recruitment
also varies between years and this is presumably related
to the availability of larvae, the supply of which will be
influenced by weather patterns at the time of spawning.
Net rates of bioerosion (losses due to grazing and boring
plus gains from accretion from coralline algae and encrusting organisms, plus physical and chemical erosion)
vary between sites on an individual reef as well as between reefs (Fig. 8.4), and differences occur between
oceans. Factors such as water quality and sediment load
influence not only the rates, but the agents responsible for
grazing and boring.
On the Great Barrier Reef, various groups of scarids
(parrotfish) are important grazers. Scarids can be divided into three distinct functional groups depending
on the osteology and muscle development on the oral
BOX 8.2 HOW ARE RATES OF BIOEROSION MEASURED?
While a piece of dead coral substratum can be cut open and the amount of calcium
carbonate that has been removed calculated, and borers identified, this does not give
you any idea of erosion rates. A better way to study bioerosion is to use experimental
blocks of freshly killed coral that show no sign of boring and to lay them out on the reef
(Fig. 8.5C) for fixed time periods. As the original dimensions and density of the blocks
are known, losses and gains can be calculated and assigned to the various organisms.
Obviously, replicates need to be collected for each time period in order to determine the
variation within a site before considering variation between sites. Thin sectioning of the
blocks allows the distribution and density of the various microborers to be determined.
Part of the block can be dissolved in order to extract the borers that can then be counted
and identified to species level. While net rates of grazing can be determined from changes
to the dimensions of the block (See Fig. 8.5D), potential grazers in the region need to be
identified, their densities calculated and the amount of calcium carbonate in their faecal
pellets measured. In the case of scarids, the depth and dimensions of the feeding scars
(Fig. 8.3A) can be measured and for echinoids, their population density can be measured
and faecal pellets collected over a 24 hr period to estimate the amount of calcium carbonate they contain in order to calculate rates of grazing. Knowing rates of calcification in
the area, a balance sheet of losses and gains for the area can be constructed. However, it
must remembered that these rates may vary considerably within a reef, so numerous
replicates must be used in order to gain reliable data.
80
and create burrows for a suite of non boring organisms or
nestlers to colonise. In contrast, the sipunculans, sponges,
bivalves and some of the larger polychaete borers are
long-lived and once they have created their burrow they
are entombed in the substratum and may then live for
several years (Fig. 8.3A, B). These organisms bore by either physically eroding the substratum or chemically dissolving it, or by a combination of these methods.
Rates of boring decrease once the borers are established, and subsequent rates are just sufficient to allow
those organisms to grow. This is particularly true for
sponge colonies. Borers must obviously retain a link to
the outside of the substratum in order to obtain their
food, for respiration and for discharging their gametes.
Once established in the substratum they are effectively
entombed, often in flask-shaped burrows (Fig. 8.3B).
They cannot leave their habitat, although some species
of molluscs, primarily Conus spp., search out for particular boring species of polychaetes and sipunculans
and insert their proboscis into the burrow and then
proceed to suck out the worm.
Experimental studies have shown that recruitment of
boring organisms is seasonal with maximum recruitment
occurring during the summer months; however, some recruitment of borers occurs throughout the year. This
means that within weeks of substratum becoming available it is already being colonised by borers. Recruitment
also varies between years and this is presumably related
to the availability of larvae, the supply of which will be
influenced by weather patterns at the time of spawning.
Net rates of bioerosion (losses due to grazing and boring
plus gains from accretion from coralline algae and encrusting organisms, plus physical and chemical erosion)
vary between sites on an individual reef as well as between reefs (Fig. 8.4), and differences occur between
oceans. Factors such as water quality and sediment load
influence not only the rates, but the agents responsible for
grazing and boring.
On the Great Barrier Reef, various groups of scarids
(parrotfish) are important grazers. Scarids can be divided into three distinct functional groups depending
on the osteology and muscle development on the oral
BOX 8.2 HOW ARE RATES OF BIOEROSION MEASURED?
While a piece of dead coral substratum can be cut open and the amount of calcium
carbonate that has been removed calculated, and borers identified, this does not give
you any idea of erosion rates. A better way to study bioerosion is to use experimental
blocks of freshly killed coral that show no sign of boring and to lay them out on the reef
(Fig. 8.5C) for fixed time periods. As the original dimensions and density of the blocks
are known, losses and gains can be calculated and assigned to the various organisms.
Obviously, replicates need to be collected for each time period in order to determine the
variation within a site before considering variation between sites. Thin sectioning of the
blocks allows the distribution and density of the various microborers to be determined.
Part of the block can be dissolved in order to extract the borers that can then be counted
and identified to species level. While net rates of grazing can be determined from changes
to the dimensions of the block (See Fig. 8.5D), potential grazers in the region need to be
identified, their densities calculated and the amount of calcium carbonate in their faecal
pellets measured. In the case of scarids, the depth and dimensions of the feeding scars
(Fig. 8.3A) can be measured and for echinoids, their population density can be measured
and faecal pellets collected over a 24 hr period to estimate the amount of calcium carbonate they contain in order to calculate rates of grazing. Knowing rates of calcification in
the area, a balance sheet of losses and gains for the area can be constructed. However, it
must remembered that these rates may vary considerably within a reef, so numerous
replicates must be used in order to gain reliable data.