Succession of macroborer communities
Experimental studies have clearly demonstrated that a distinct succession of macroborers occurs, initially certain
species of polychaetes (Hutchings and Murray, 1982)
with some recruitment occurring regardless of when the
substrate becomes available. A few sipunculans begin to
colonize after 6 months (Davies and Hutchings, 1983;
Hutchings and Peyrot-Clausade, 2002) and later on
bivalve molluscs and sponges are found (Kiene and
Hutchings, 1994a, b; Pari et al., 2002; Osorno et al.,
2005). No studies either on the Great Barrier Reef or in
French Polynesia have ever found sipunculans, molluscs,
or sponges until newly available substrates have been
exposed for at least 6 months, and in many cases sponges
did not appear until after 4 years of exposure (Kiene and
Hutchings, 1994b; Pari et al., 2002). Experimental studies
have shown significant variations in recruitment patterns
across a reef and within geographical areas (Kiene and
Hutchings, 1994a, b; Osorno et al., 2005; Pari et al.,
2002). However, these studies have usually been carried
out over 2–5 years, with only one study extending for 7
and 9 years (Kiene and Hutchings, 1994a). They contrast
with the “mature” communities which have well developed sponge borings, numerous large bivalves, with polychaetes restricted to large eunicids and sabellids, and these
have developed over decades, rather than the few years
over which experimental studies have been conducted. It
is these “mature” communities which have been typically
described in the literature (Neumann, 1966; Hein and
Risk, 1975; Hudson, 1977; MacGeachy, 1977; Risk and
MacGeachy, 1987; Davies 1983).
All these macroborers which have characteristic shaped
burrows must maintain an opening to the outside to obtain
a continual supply of fresh oxygenated water, food supply
and for the release of excretory and reproductive products,
but they cannot leave the substrate as they are effectively
entombed within it. Examination of many cut surfaces
clearly shows that burrows of different organisms rarely
if ever touch or coalesce and presumably boring creates
vibrations to which other borers are sensitive and allows
them to take the necessary avoiding action.
Grazers
Echinoids
In the Indo-Pacific the main grazing species belong to the
following genera, Diadema (Figure 1f ), Stomopneustes,
Echinothrix, Echinostrephus (Figure 1g), and Echinometra,
with D.savigny and Echinometra mathaei (Figure 1d) often
being the two most conspicuous species. In the Caribbean,
the two most commons species are Diadema antillarum
and Echinometra viridis, so while these genera are also present in the Indo-Pacific, different species are present.
Method of grazing
Echinoids use their Aristotle’s lantern specialized plates of
the mouth (Figure 1e) to grind the coral substrate into
a paste which is then swallowed and the contents of the
ruptured algal cells are then absorbed by the gut. Such
grazing occurs mainly at night with the echinoids leaving
their crevices and roaming over the reef and fresh feeding
scars can be seen in the morning (Figure 1g). Faecal pellets produced, consist almost entirely of ground up calcium carbonate and these are deposited on the substrate
or on the lagoon floor.
Rates of grazing
Rates of bioerosion by E. mathaei have been estimated at
6.9 Æ 2.2 kg CaCO 3 m
À2 year
À1 in French Polynesia at
Faaa, Tahiti, a very degraded reef where overfishing has
occurred, lower rates were found at nearby Moorea of
4.3 Æ 3.6 kg CaCO 3 m
À2 year
À1 (Figure 4). Rates of
8.3 kg CaCO 3 m
À2 year
À1 were estimated for the reef flat
at Reunion, Indian Ocean (Peyrot-Clausade et al., 2000)
and similar rates were recorded at Diani reef in Keyna
(McClanahan and Muthiga, 1988), both of these reefs have
also been subjected to overfishing. In the Caribbean on
degraded reefs, rates of grazing by echinoids can exceed
22 kg CaCO 3 m
À2 year
À1 (Glynn, 1988; Reaka-Kudla et al.,
1996) where the dominant species is Eucidaris thouarsii. On
the Great Barrier Reef, Australia, densities of grazing echinoids are very low (Sammarco, 1985) and this may be in part
due to healthy fish populations which predate on juvenile
echinoids. While echinoids have typically been regarded as
grazers feeding on endolithic algae, some species of the family Echinometridae are active borers especially in high
energy situations (Asgaard and Bromley, 2008). For example, Echinometra lucunte which occurs in the Caribbean
and the Atlantic produces cup-shaped burrows as juveniles
and it is suggested that these burrows enable the echinoid
to catch drift algae with their spines, as well as grazing on
the turf and endolithic algae on the walls of the burrow.
Adults tend to occupy elongated grooves and presumably
both types of burrows provide shelter from wave action
and the species tends to stay within the confines of their burrow. A similar behaviour is exhibited by Echinometra
mathaei in the Western Pacific especially in the high-energy
environments outside barrier reefs, although they do leave
their burrow at night especially in a lagoonal situation
(Peyrot-Clausade et al., 2000). Details about the other
echinometrid echinoids are given by Asgaard and Bromley
(2008) together with some excellent illustrations of the various species and their burrows and they summarize all the
information that is available on each species and provide
rates of grazing where known.
Echinoids in the fossil record
Similar genera of grazing echinoids have been recorded
from fossil reefs although as Greenstein (1993) explains
they are not well conserved as their fragile skeletons do
not preserve well so that abundances may be severely
underestimated.
Molluscs
A high diversity of gastropods and a lower diversity of
chitons occur on reefs, with distinct faunas in the IndoPacific and Atlantic.
146
BIOEROSION
Experimental studies have clearly demonstrated that a distinct succession of macroborers occurs, initially certain
species of polychaetes (Hutchings and Murray, 1982)
with some recruitment occurring regardless of when the
substrate becomes available. A few sipunculans begin to
colonize after 6 months (Davies and Hutchings, 1983;
Hutchings and Peyrot-Clausade, 2002) and later on
bivalve molluscs and sponges are found (Kiene and
Hutchings, 1994a, b; Pari et al., 2002; Osorno et al.,
2005). No studies either on the Great Barrier Reef or in
French Polynesia have ever found sipunculans, molluscs,
or sponges until newly available substrates have been
exposed for at least 6 months, and in many cases sponges
did not appear until after 4 years of exposure (Kiene and
Hutchings, 1994b; Pari et al., 2002). Experimental studies
have shown significant variations in recruitment patterns
across a reef and within geographical areas (Kiene and
Hutchings, 1994a, b; Osorno et al., 2005; Pari et al.,
2002). However, these studies have usually been carried
out over 2–5 years, with only one study extending for 7
and 9 years (Kiene and Hutchings, 1994a). They contrast
with the “mature” communities which have well developed sponge borings, numerous large bivalves, with polychaetes restricted to large eunicids and sabellids, and these
have developed over decades, rather than the few years
over which experimental studies have been conducted. It
is these “mature” communities which have been typically
described in the literature (Neumann, 1966; Hein and
Risk, 1975; Hudson, 1977; MacGeachy, 1977; Risk and
MacGeachy, 1987; Davies 1983).
All these macroborers which have characteristic shaped
burrows must maintain an opening to the outside to obtain
a continual supply of fresh oxygenated water, food supply
and for the release of excretory and reproductive products,
but they cannot leave the substrate as they are effectively
entombed within it. Examination of many cut surfaces
clearly shows that burrows of different organisms rarely
if ever touch or coalesce and presumably boring creates
vibrations to which other borers are sensitive and allows
them to take the necessary avoiding action.
Grazers
Echinoids
In the Indo-Pacific the main grazing species belong to the
following genera, Diadema (Figure 1f ), Stomopneustes,
Echinothrix, Echinostrephus (Figure 1g), and Echinometra,
with D.savigny and Echinometra mathaei (Figure 1d) often
being the two most conspicuous species. In the Caribbean,
the two most commons species are Diadema antillarum
and Echinometra viridis, so while these genera are also present in the Indo-Pacific, different species are present.
Method of grazing
Echinoids use their Aristotle’s lantern specialized plates of
the mouth (Figure 1e) to grind the coral substrate into
a paste which is then swallowed and the contents of the
ruptured algal cells are then absorbed by the gut. Such
grazing occurs mainly at night with the echinoids leaving
their crevices and roaming over the reef and fresh feeding
scars can be seen in the morning (Figure 1g). Faecal pellets produced, consist almost entirely of ground up calcium carbonate and these are deposited on the substrate
or on the lagoon floor.
Rates of grazing
Rates of bioerosion by E. mathaei have been estimated at
6.9 Æ 2.2 kg CaCO 3 m
À2 year
À1 in French Polynesia at
Faaa, Tahiti, a very degraded reef where overfishing has
occurred, lower rates were found at nearby Moorea of
4.3 Æ 3.6 kg CaCO 3 m
À2 year
À1 (Figure 4). Rates of
8.3 kg CaCO 3 m
À2 year
À1 were estimated for the reef flat
at Reunion, Indian Ocean (Peyrot-Clausade et al., 2000)
and similar rates were recorded at Diani reef in Keyna
(McClanahan and Muthiga, 1988), both of these reefs have
also been subjected to overfishing. In the Caribbean on
degraded reefs, rates of grazing by echinoids can exceed
22 kg CaCO 3 m
À2 year
À1 (Glynn, 1988; Reaka-Kudla et al.,
1996) where the dominant species is Eucidaris thouarsii. On
the Great Barrier Reef, Australia, densities of grazing echinoids are very low (Sammarco, 1985) and this may be in part
due to healthy fish populations which predate on juvenile
echinoids. While echinoids have typically been regarded as
grazers feeding on endolithic algae, some species of the family Echinometridae are active borers especially in high
energy situations (Asgaard and Bromley, 2008). For example, Echinometra lucunte which occurs in the Caribbean
and the Atlantic produces cup-shaped burrows as juveniles
and it is suggested that these burrows enable the echinoid
to catch drift algae with their spines, as well as grazing on
the turf and endolithic algae on the walls of the burrow.
Adults tend to occupy elongated grooves and presumably
both types of burrows provide shelter from wave action
and the species tends to stay within the confines of their burrow. A similar behaviour is exhibited by Echinometra
mathaei in the Western Pacific especially in the high-energy
environments outside barrier reefs, although they do leave
their burrow at night especially in a lagoonal situation
(Peyrot-Clausade et al., 2000). Details about the other
echinometrid echinoids are given by Asgaard and Bromley
(2008) together with some excellent illustrations of the various species and their burrows and they summarize all the
information that is available on each species and provide
rates of grazing where known.
Echinoids in the fossil record
Similar genera of grazing echinoids have been recorded
from fossil reefs although as Greenstein (1993) explains
they are not well conserved as their fragile skeletons do
not preserve well so that abundances may be severely
underestimated.
Molluscs
A high diversity of gastropods and a lower diversity of
chitons occur on reefs, with distinct faunas in the IndoPacific and Atlantic.
146
BIOEROSION