they are found to bore into, can stay on the surface of the
coral for days until they settle and bore into the coral. Scott
(1988) found that the larvae enter the coral via the coelenteron and she suggests that they then undergo metamorphosis and eventually bore through the walls into the
skeleton although she did not actually observe this.
This suggests that the larvae have morphological and
behavioral specializations so as to select their specific
coral species host and to penetrate its defences.
Mechanism of boring by molluscs
Many early studies (e.g., Yonge, 1963; Soliman, 1971)
suggested that boring in bivalves was largely mechanical
but more recent studies of species of Lithophaginae,
Pholadidae, and Tridacnidae which bore into live coral suggest that they are chemical borers, with acid-like secretions
being produced by the mantle folds. In the species of
Lithophaginae which bore into dead coral, calcium carbonate is used to smooth and fill the boring anteriorly. In those
more specialized species which bore into live coral the calcium is used to smooth and fill the boring posteriorly and to
form secondary extensions to the shell for predator defence.
The secretions are produced in this group by pallial glands
located in the middle folds of the mantle of lithophagids
and in other groups the boring glands are in the inner folds
around the pedal gape. Morton (1990) provides more details
on selected species across the families as well as details of
the structure of the tubes which can be easily assigned to
particular species. Detailed descriptions and illustrations
of some boring bivalves from the Maldivian coral reefs
are given by Kleeman (2008). Morton (1990) suggests that
in live coral borers over geological time have evolved from
mechanically boring ancestors toward chemical erosion and
selective relining of the burrow so that the occupant fits
snugly within it.
Bivalves living in live coral have glands on their
siphons which secrete a substance which either inhibits
nematocyst discharge or protects the siphons from them.
Species living in dead coral do not possess these glands
(Morton and Scott, 1980). But all boring species living in
either dead or live coral need to protect their siphons from
predation by predatory animals moving over the surface of
the substrate. Some species decorate the openings of the
siphon with detrital fragments, others can retract their
siphons back down the neck of the burrow and other species protect the posterior edges of the shells valves to help
minimize predation. For more details see Morton (1990).
Fossil record of molluscs
Boring bivalves are well represented in the fossil record,
and some of these ancient bivalves are almost certainly
ancestors of modern borers, they were uniformly circumtropical in distribution and primarily borers of dead coral.
Only in recent times with the evolution of modern coral
reefs was the close relationship between boring bivalves
and live coral really established, together with the strong
separation of Atlantic and Indo-Pacific fauna.
Sipunculans
Diversity of sipunculans
A number of species bore into coral reef substrates and
coral rubble representing several genera. In a study of
the distribution of sipunculans at Carrie Bow Cay, Belize,
Rice, and Macintyre (1982) found eight species, of
which six inhabited burrows within the coral substrate
and two which were found on crevices and crannies. Several genera were represented including Lithacrosiphon,
Aspidosiphon, Paraspidosiphon, and Phascolosoma. Similar genera were recorded from coral substrates in French
Polynesia with some additional ones, but they were not all
identified to species (Hutchings and Peyrot-Clausade,
2002). Eight species were recorded from studies along a
transect in North Queensland, and two species were shared
between the Caribbean and the Great Barrier Reef,
Paraspidosiphon steenstrupii and Phascolosoma perlucens
(Osorno et al., 2005) (Figure 1c). As with the polychaetes,
the habit of boring appears to have arisen independently
in several families of sipunculans and the so called widely
distributed species need to be carefully checked using morphological and molecular techniques.
Succession of sipunculans
Rice and Macintyre (1982), working in Belize, found that
sipunculan density was greatest in relatively unaltered
coral substrate which had relatively little secondary infill
of calcite cement and that the corals with uniform skeletal
framework like Porites and Acropora were favoured.
They found that highly eroded rocks of coral substrate
contained few if any sipunculans. Experimental studies
have clearly shown that sipunculans do not appear initially
but have not revealed any distinct patterns of succession,
only that individuals increase with size with increasing
exposure (Hutchings and Peyrot-Clausade, 2002).
Recruitment of sipunculans
Rice and Macintyre (1982) have shown that the species of
sipunculans which recruit depends on the time of year as
individual species have different breeding seasons. On
the Great Barrier Reef, Hutchings et al. (1992) found that
sipunculans exhibited spatial and temporal variations in
recruitment, with some years better than others, and that
recruitment was almost totally restricted to summer
months, suggesting that breeding occurs at this time,
although no information on their breeding cycles is available for these species on the Great Barrier Reef. They were
found to prefer reef front situations on the Great Barrier
Reef (Kiene and Hutchings, 1994a, b) which supports Rice
and Macintyre’s (1982) findings from the Caribbean that
sipunculans occur in greatest abundance in high-energy
reef crest areas, which is contrary to the findings of Bromley (1978) who found that low-energy lagoonal situations
in this region were preferred. No other information is available on other factors which may determine species composition or abundances. Peyrot-Clausade and Hutchings
(2002) found differences in species composition and
BIOEROSION
143
coral for days until they settle and bore into the coral. Scott
(1988) found that the larvae enter the coral via the coelenteron and she suggests that they then undergo metamorphosis and eventually bore through the walls into the
skeleton although she did not actually observe this.
This suggests that the larvae have morphological and
behavioral specializations so as to select their specific
coral species host and to penetrate its defences.
Mechanism of boring by molluscs
Many early studies (e.g., Yonge, 1963; Soliman, 1971)
suggested that boring in bivalves was largely mechanical
but more recent studies of species of Lithophaginae,
Pholadidae, and Tridacnidae which bore into live coral suggest that they are chemical borers, with acid-like secretions
being produced by the mantle folds. In the species of
Lithophaginae which bore into dead coral, calcium carbonate is used to smooth and fill the boring anteriorly. In those
more specialized species which bore into live coral the calcium is used to smooth and fill the boring posteriorly and to
form secondary extensions to the shell for predator defence.
The secretions are produced in this group by pallial glands
located in the middle folds of the mantle of lithophagids
and in other groups the boring glands are in the inner folds
around the pedal gape. Morton (1990) provides more details
on selected species across the families as well as details of
the structure of the tubes which can be easily assigned to
particular species. Detailed descriptions and illustrations
of some boring bivalves from the Maldivian coral reefs
are given by Kleeman (2008). Morton (1990) suggests that
in live coral borers over geological time have evolved from
mechanically boring ancestors toward chemical erosion and
selective relining of the burrow so that the occupant fits
snugly within it.
Bivalves living in live coral have glands on their
siphons which secrete a substance which either inhibits
nematocyst discharge or protects the siphons from them.
Species living in dead coral do not possess these glands
(Morton and Scott, 1980). But all boring species living in
either dead or live coral need to protect their siphons from
predation by predatory animals moving over the surface of
the substrate. Some species decorate the openings of the
siphon with detrital fragments, others can retract their
siphons back down the neck of the burrow and other species protect the posterior edges of the shells valves to help
minimize predation. For more details see Morton (1990).
Fossil record of molluscs
Boring bivalves are well represented in the fossil record,
and some of these ancient bivalves are almost certainly
ancestors of modern borers, they were uniformly circumtropical in distribution and primarily borers of dead coral.
Only in recent times with the evolution of modern coral
reefs was the close relationship between boring bivalves
and live coral really established, together with the strong
separation of Atlantic and Indo-Pacific fauna.
Sipunculans
Diversity of sipunculans
A number of species bore into coral reef substrates and
coral rubble representing several genera. In a study of
the distribution of sipunculans at Carrie Bow Cay, Belize,
Rice, and Macintyre (1982) found eight species, of
which six inhabited burrows within the coral substrate
and two which were found on crevices and crannies. Several genera were represented including Lithacrosiphon,
Aspidosiphon, Paraspidosiphon, and Phascolosoma. Similar genera were recorded from coral substrates in French
Polynesia with some additional ones, but they were not all
identified to species (Hutchings and Peyrot-Clausade,
2002). Eight species were recorded from studies along a
transect in North Queensland, and two species were shared
between the Caribbean and the Great Barrier Reef,
Paraspidosiphon steenstrupii and Phascolosoma perlucens
(Osorno et al., 2005) (Figure 1c). As with the polychaetes,
the habit of boring appears to have arisen independently
in several families of sipunculans and the so called widely
distributed species need to be carefully checked using morphological and molecular techniques.
Succession of sipunculans
Rice and Macintyre (1982), working in Belize, found that
sipunculan density was greatest in relatively unaltered
coral substrate which had relatively little secondary infill
of calcite cement and that the corals with uniform skeletal
framework like Porites and Acropora were favoured.
They found that highly eroded rocks of coral substrate
contained few if any sipunculans. Experimental studies
have clearly shown that sipunculans do not appear initially
but have not revealed any distinct patterns of succession,
only that individuals increase with size with increasing
exposure (Hutchings and Peyrot-Clausade, 2002).
Recruitment of sipunculans
Rice and Macintyre (1982) have shown that the species of
sipunculans which recruit depends on the time of year as
individual species have different breeding seasons. On
the Great Barrier Reef, Hutchings et al. (1992) found that
sipunculans exhibited spatial and temporal variations in
recruitment, with some years better than others, and that
recruitment was almost totally restricted to summer
months, suggesting that breeding occurs at this time,
although no information on their breeding cycles is available for these species on the Great Barrier Reef. They were
found to prefer reef front situations on the Great Barrier
Reef (Kiene and Hutchings, 1994a, b) which supports Rice
and Macintyre’s (1982) findings from the Caribbean that
sipunculans occur in greatest abundance in high-energy
reef crest areas, which is contrary to the findings of Bromley (1978) who found that low-energy lagoonal situations
in this region were preferred. No other information is available on other factors which may determine species composition or abundances. Peyrot-Clausade and Hutchings
(2002) found differences in species composition and
BIOEROSION
143