Method of grazing
Gastropod molluscs and chitons can be responsible for
considerable losses of substrate on intertidal reefs. They
use their radulae to remove the surface layers and the
embedded endolithic algae and like the parrotfishes and
echinoids they are able to break down the algal walls
and utilize the contents of the plant cells. Chitons excavate
a home scar to which they return to, after foraging, mainly
at night during low tide. They may be locally abundant on
intertidal reef flats.
Rates of grazing
A recent study at One Tree Island, Great Barrier Reef
based on Acanthopleura gemmata, estimated rates of erosion of 0.013–0.25 kg CaCO 3 m
À2 year
À1 at two sites on
the reef margin and on the beachrock platform (Barbosa
et al., 2008) (Figure 2a). While these figures are much
lower than those for echinoids and scarids they can be very
important in some habitats.
Grazing on live corals
The gastropod Drupella feeds on live coral (Shafir et al.,
2008; Lam et al., 2007) as does the Crown of Thorns Starfish (Acanthaster plancii) (DeVantier and Done, 2007 and
references therein) and the resultant dead coral then
becomes available for colonization by borers. Gastropods
belonging to the genus Duprella are obligate corallivores
and specialize on acroporid coral especially Acropora
and Montipora spp. (Morton et al., 2002). Recently large
colonies of Platygyra acuta and Platygyra carnosus were
observed in Hong Kong to be severely eroded at their
bases which makes them very susceptible to storm damage. This erosion was caused by the gastropod Drupella
rugosa which was feeding on the living coral tissue
and then by grazing of the newly available substrate
which had been colonized by endolithic algae by the echinoid Diadema setosum (Lam et al., 2007). A recent review by Morton and Blackmore (2009) suggests that the
dense concentrations of Drupella rugosa and another
corallivorous gastropod Cronia margariticola regularly
seen in Hong Kong Harbour are not plagues but rather
breeding aggregations and they actually doubt that these
gastropods pose a threat to the corals, contrary to the findings of Lam et al. (2007). However, there are welldocumented cases in which localized population outbreaks
of Drupella spp., as well as the starfish, Acanthaster planci
and can rapidly and severely reduce the percentage cover of
live coral, although some reefs subsequently recover
(Glynn, 1973; Colgan, 1987).
Parrotfishes
The Scaridae, a family of labroid fish, are highly characteristic of coral reef habitats. With few exceptions their geographical distribution is linked to tropical reef environments.
Compared to other tropical perciform fish, their diversity
on the reef is not that great, with about 25 described from
the Great Barrier Reef (Choat and Randal, 1986). While
some of these have Indo-Pacific distributions others have
restricted ranges. In the Caribbean, 15 species are present,
and those in the genera Cryptotomus, Nicholsina, and
Sparisoma are restricted to this area. The genus Scarus is
the dominant Indo-Pacific genus and there are six species
in the Caribbean that appear to be fairly recent colonizers;
Sparisoma has been there for much of the Tertiary. Only
one Caribbean parrot fish S. viride has the capacity to significantly bioerode calcareous substrata compared to several
species in the Indo-Pacific (Bolbometopon muricatum,
Cetoscarus bicolor, and the five large species of Chlorurus).
Additional species occur off the African coast and of Brazil
(Choat, pers.comm.).
Method of grazing
It is the presence of dense colonies of endolithic algae
which attracts numerous grazing scarids or parrot fish
to both live and dead coral (Figure 2b). Scarids or
parrotfish, now regarded as belonging to the Labridae
family (Cowman et al., 2009); can be divided based on
jaw morphology into excavators which remove pieces of
the substrate (Figure 2b) and scrapers (Figure 2e) which
have a nonexcavating bite just removing material from
the surface of the substrate. Schools of parrotfishes can
often be seen and heard feeding in the late afternoon in
shallow waters (Figure 2d) and distinctive grazing marks
are visible on the surface of both live and dead coral substrates (Figure 2e). While most parrotfishes feed on dead
coral substrates the large Bolbometopon muricatum has
a diet which consists of over 50% of live corals (primarily
Acropora species, Bellwood, 1986) (Figure 2c). Some
others that graze on Porites spp., occasionally, include
the excavators Scarus gibbus and Cetoscarus bicolor
and the scrapers Scarus frenatus and S. rivulatus on the
Great Barrier Reef, Australia. Such findings are contrary
to the traditional view that parrotfishes feed on dead coral
substrates and recent studies in the Caribbean have shown
that feeding on live coral can also be widespread. A study
on the back reef habitat at Carrie Bow Cay, Belize in
the Caribbean, found that parrotfish predation on the
reef building coral Porites astreoides was significant
with >13% of colonies exhibiting partial or total colony
mortality (Rotjan and Lewis, 2005). A suite of parrotfishes
were present but probably the most important species was
Sparisoma viride. Not only were the fishes targeting the
endolithic algae but in grazed areas of the colonies there
were significantly higher densities of macroborers namely
barnacles, polychaetes, and vermetids. Rotjan and Lewis
(2005) speculate that the parrotfishes were targeting such
areas to obtain additional nutritional benefits from these
macroborers. Subsequent studies by Rotjan and Lewis
(2006) investigated the spatial and temporal patterns of
parrotfishes across habitats on the Belize barrier reef.
They found that parrotfishes were selective in the live
corals on which they grazed. The most heavily targeted
species were all members of the Montastrea annularis
species complex, and colonies of M. cavernosa, Agaricia
agaricites, Diploria strigosa, Porites astreoides, and
Porites porites were not heavily targeted. Parrotfishes also
BIOEROSION
147
Gastropod molluscs and chitons can be responsible for
considerable losses of substrate on intertidal reefs. They
use their radulae to remove the surface layers and the
embedded endolithic algae and like the parrotfishes and
echinoids they are able to break down the algal walls
and utilize the contents of the plant cells. Chitons excavate
a home scar to which they return to, after foraging, mainly
at night during low tide. They may be locally abundant on
intertidal reef flats.
Rates of grazing
A recent study at One Tree Island, Great Barrier Reef
based on Acanthopleura gemmata, estimated rates of erosion of 0.013–0.25 kg CaCO 3 m
À2 year
À1 at two sites on
the reef margin and on the beachrock platform (Barbosa
et al., 2008) (Figure 2a). While these figures are much
lower than those for echinoids and scarids they can be very
important in some habitats.
Grazing on live corals
The gastropod Drupella feeds on live coral (Shafir et al.,
2008; Lam et al., 2007) as does the Crown of Thorns Starfish (Acanthaster plancii) (DeVantier and Done, 2007 and
references therein) and the resultant dead coral then
becomes available for colonization by borers. Gastropods
belonging to the genus Duprella are obligate corallivores
and specialize on acroporid coral especially Acropora
and Montipora spp. (Morton et al., 2002). Recently large
colonies of Platygyra acuta and Platygyra carnosus were
observed in Hong Kong to be severely eroded at their
bases which makes them very susceptible to storm damage. This erosion was caused by the gastropod Drupella
rugosa which was feeding on the living coral tissue
and then by grazing of the newly available substrate
which had been colonized by endolithic algae by the echinoid Diadema setosum (Lam et al., 2007). A recent review by Morton and Blackmore (2009) suggests that the
dense concentrations of Drupella rugosa and another
corallivorous gastropod Cronia margariticola regularly
seen in Hong Kong Harbour are not plagues but rather
breeding aggregations and they actually doubt that these
gastropods pose a threat to the corals, contrary to the findings of Lam et al. (2007). However, there are welldocumented cases in which localized population outbreaks
of Drupella spp., as well as the starfish, Acanthaster planci
and can rapidly and severely reduce the percentage cover of
live coral, although some reefs subsequently recover
(Glynn, 1973; Colgan, 1987).
Parrotfishes
The Scaridae, a family of labroid fish, are highly characteristic of coral reef habitats. With few exceptions their geographical distribution is linked to tropical reef environments.
Compared to other tropical perciform fish, their diversity
on the reef is not that great, with about 25 described from
the Great Barrier Reef (Choat and Randal, 1986). While
some of these have Indo-Pacific distributions others have
restricted ranges. In the Caribbean, 15 species are present,
and those in the genera Cryptotomus, Nicholsina, and
Sparisoma are restricted to this area. The genus Scarus is
the dominant Indo-Pacific genus and there are six species
in the Caribbean that appear to be fairly recent colonizers;
Sparisoma has been there for much of the Tertiary. Only
one Caribbean parrot fish S. viride has the capacity to significantly bioerode calcareous substrata compared to several
species in the Indo-Pacific (Bolbometopon muricatum,
Cetoscarus bicolor, and the five large species of Chlorurus).
Additional species occur off the African coast and of Brazil
(Choat, pers.comm.).
Method of grazing
It is the presence of dense colonies of endolithic algae
which attracts numerous grazing scarids or parrot fish
to both live and dead coral (Figure 2b). Scarids or
parrotfish, now regarded as belonging to the Labridae
family (Cowman et al., 2009); can be divided based on
jaw morphology into excavators which remove pieces of
the substrate (Figure 2b) and scrapers (Figure 2e) which
have a nonexcavating bite just removing material from
the surface of the substrate. Schools of parrotfishes can
often be seen and heard feeding in the late afternoon in
shallow waters (Figure 2d) and distinctive grazing marks
are visible on the surface of both live and dead coral substrates (Figure 2e). While most parrotfishes feed on dead
coral substrates the large Bolbometopon muricatum has
a diet which consists of over 50% of live corals (primarily
Acropora species, Bellwood, 1986) (Figure 2c). Some
others that graze on Porites spp., occasionally, include
the excavators Scarus gibbus and Cetoscarus bicolor
and the scrapers Scarus frenatus and S. rivulatus on the
Great Barrier Reef, Australia. Such findings are contrary
to the traditional view that parrotfishes feed on dead coral
substrates and recent studies in the Caribbean have shown
that feeding on live coral can also be widespread. A study
on the back reef habitat at Carrie Bow Cay, Belize in
the Caribbean, found that parrotfish predation on the
reef building coral Porites astreoides was significant
with >13% of colonies exhibiting partial or total colony
mortality (Rotjan and Lewis, 2005). A suite of parrotfishes
were present but probably the most important species was
Sparisoma viride. Not only were the fishes targeting the
endolithic algae but in grazed areas of the colonies there
were significantly higher densities of macroborers namely
barnacles, polychaetes, and vermetids. Rotjan and Lewis
(2005) speculate that the parrotfishes were targeting such
areas to obtain additional nutritional benefits from these
macroborers. Subsequent studies by Rotjan and Lewis
(2006) investigated the spatial and temporal patterns of
parrotfishes across habitats on the Belize barrier reef.
They found that parrotfishes were selective in the live
corals on which they grazed. The most heavily targeted
species were all members of the Montastrea annularis
species complex, and colonies of M. cavernosa, Agaricia
agaricites, Diploria strigosa, Porites astreoides, and
Porites porites were not heavily targeted. Parrotfishes also
BIOEROSION
147