Hutchings, 1994a, b). However, maximum recruitment of
this suite of polychaetes occurs during early summer
(Hutchings and Murray, 1982) and significant variations
occur between sites on a reef, with maximum recruitment
of most species occurring on windward and reef flat sites,
and least to a lagoonal patch reef (Hutchings et al.,
1992). Studies over several years have shown that as well
as seasonal and temporal variation there are also variations
between years, and it is suggested that local weather patterns are critical in the dispersal of these larvae (Hutchings
et al., 1992). Once the polychaetes are established within
the substrate they must retain an opening to the exterior
through which they obtain oxygenated water and their
food, discharge their waste products and gametes. The
only exception to this are some of the eunicids which
develop modified back ends full of gametes which become
detached from the rest of the body and leave the burrow
and swim up into the water column to spawn on particular
nights of the year. The most famous being the Palolo worm
(Eunice viridis) (Caspers, 1984). So basically once the
polychaetes have burrowed into the substrate they are
effectively entombed and never leave except for some of
the eunicids and then only posterior segments.
In addition to recruitment via pelagic larvae, members
of the genus Dodecaeria (F. Cirratulidae) can also undergo
asexual reproduction by splitting into individual segments
and with each segment developing a new head and tail,
and so inside the burrow an entire family group may be
found.
Mechanisms of boring by polychaetes
Polychaetes bore into the substrate using either chemical
secretion to dissolve the reef framework or perhaps
mechanically grind the substrate or use a combination of
these methods (Hutchings, 2008) but the precise details
still need to be worked out. Some of the boring polychaetes such as sabellids and cirratulids must dissolve
the substrate as they lack any structures with which to
mechanically bore, and sabellids at least have well developed glandular areas at the base of the crown which may
be responsible for secreting chemicals which dissolve
the substrate. Other groups such as Polydora spp. (F.
Spionidae) have thickened chaetae on segment 5, and it
has been suggested that they can use these to grind the
substrate. However, removal of these modified chaetae
did not impede the burrowing capacity of Polydora
websteri (Haigler, 1969). The same species boring into
mollusc shells secretes a viscous fluid which dissolves
the organic matrices of the shell and subsequently dissolves the exposed crystals (Zottoli and Carricker, 1974),
but the chemical composition of this fluid was not determined. More recently it has been suggested that this fluid
is secreted all along the body of the worm (Sato-Okushi
and Okoshi, 1993) and presumably a similar fluid is
secreted by other species which bore into coral substrate.
Examination of the walls of the burrows of eunicids
reveals bite marks which match the size of their welldeveloped jaws. Burrows of the larger polychaetes are
distinctive and can be recognized in sections of substrate
and those of Notaulux (F. Sabellidae) are lined with
a fine chitinous tube (Hutchings, 2008). Identifying the
burrows of the smaller early recruiting polychaete species
is far more difficult as their dimensions are similar to the
porosity of the coral substrate.
Molluscs
Diversity of molluscs
Representatives of six bivalve families are known to
bore into coral (Figure 1c). Of these the Petricolidae,
Pholadidae, and Clavagellidae are represented only by
a few species and generally bore into dead coral substrate.
Species of the Lithophaginae and the Gastrochaenidae
play a major role in bioerosion of dead coral with the
latter family the dominant one in both the Pacific and
the Caribbean. Another suite of species bore into living
coral belonging to the Mytilidae and some genera of
Lithophaginae. Within Lithophaga some species are
capable of living in a wide range of coral species, whereas
others are restricted to a single species. Species of
Leptoconchus and Magilopsis belonging to the gastropod
family Coralliophilidae bore into living coral (Soliman,
1969).
The Indo-Pacific and Atlantic coral reef faunas are
quite different, with only 7% overlap at the generic level
but none at the species level, 24 genera have been
recorded from the Atlantic and 87 in the Indo-Pacific.
The genera which overlap are those containing species
of the less specialized dead coral borers, and the other genera including those which bore into live coral evolved as
the corals themselves diversified (Rosen, 1984). For more
information on the evolution of boring bivalves and
a detailed list of the coral species bored by particular species of bivalves see Morton (1990).
Succession of molluscs
Experimental substrates have rarely been exposed for long
enough to demonstrate if any succession of species occurs
as the dead substrate ages. Obviously as the surface of the
dead substrate is eroded the bivalve has to continue to bore
deeper in the habitat whereas those boring into live coral
must reverse their direction of their boring as the coral
grows upwards enlarging its burrow posteriorly to keep
the entrance above open, and in these lithophagids, posterior pallial glands secrete an active chelating agent
(Morton, 1990).
Recruitment of molluscs
Bivalves recruit via pelagic larvae, and Morton (1990) suggests that those recruiting to dead coral substrate are similar to other bivalve larvae in their ability to find a suitable
substrate on which to settle and bore. Whereas larvae of
species recruiting to live corals have to have special adaptations. It appears that larvae settling inadvertently onto
a coral species, which they do not normally bore into,
are stung by the coral nematocysts and rapidly withdraw
their feet. Whereas larvae settling on a coral species which
142
BIOEROSION
this suite of polychaetes occurs during early summer
(Hutchings and Murray, 1982) and significant variations
occur between sites on a reef, with maximum recruitment
of most species occurring on windward and reef flat sites,
and least to a lagoonal patch reef (Hutchings et al.,
1992). Studies over several years have shown that as well
as seasonal and temporal variation there are also variations
between years, and it is suggested that local weather patterns are critical in the dispersal of these larvae (Hutchings
et al., 1992). Once the polychaetes are established within
the substrate they must retain an opening to the exterior
through which they obtain oxygenated water and their
food, discharge their waste products and gametes. The
only exception to this are some of the eunicids which
develop modified back ends full of gametes which become
detached from the rest of the body and leave the burrow
and swim up into the water column to spawn on particular
nights of the year. The most famous being the Palolo worm
(Eunice viridis) (Caspers, 1984). So basically once the
polychaetes have burrowed into the substrate they are
effectively entombed and never leave except for some of
the eunicids and then only posterior segments.
In addition to recruitment via pelagic larvae, members
of the genus Dodecaeria (F. Cirratulidae) can also undergo
asexual reproduction by splitting into individual segments
and with each segment developing a new head and tail,
and so inside the burrow an entire family group may be
found.
Mechanisms of boring by polychaetes
Polychaetes bore into the substrate using either chemical
secretion to dissolve the reef framework or perhaps
mechanically grind the substrate or use a combination of
these methods (Hutchings, 2008) but the precise details
still need to be worked out. Some of the boring polychaetes such as sabellids and cirratulids must dissolve
the substrate as they lack any structures with which to
mechanically bore, and sabellids at least have well developed glandular areas at the base of the crown which may
be responsible for secreting chemicals which dissolve
the substrate. Other groups such as Polydora spp. (F.
Spionidae) have thickened chaetae on segment 5, and it
has been suggested that they can use these to grind the
substrate. However, removal of these modified chaetae
did not impede the burrowing capacity of Polydora
websteri (Haigler, 1969). The same species boring into
mollusc shells secretes a viscous fluid which dissolves
the organic matrices of the shell and subsequently dissolves the exposed crystals (Zottoli and Carricker, 1974),
but the chemical composition of this fluid was not determined. More recently it has been suggested that this fluid
is secreted all along the body of the worm (Sato-Okushi
and Okoshi, 1993) and presumably a similar fluid is
secreted by other species which bore into coral substrate.
Examination of the walls of the burrows of eunicids
reveals bite marks which match the size of their welldeveloped jaws. Burrows of the larger polychaetes are
distinctive and can be recognized in sections of substrate
and those of Notaulux (F. Sabellidae) are lined with
a fine chitinous tube (Hutchings, 2008). Identifying the
burrows of the smaller early recruiting polychaete species
is far more difficult as their dimensions are similar to the
porosity of the coral substrate.
Molluscs
Diversity of molluscs
Representatives of six bivalve families are known to
bore into coral (Figure 1c). Of these the Petricolidae,
Pholadidae, and Clavagellidae are represented only by
a few species and generally bore into dead coral substrate.
Species of the Lithophaginae and the Gastrochaenidae
play a major role in bioerosion of dead coral with the
latter family the dominant one in both the Pacific and
the Caribbean. Another suite of species bore into living
coral belonging to the Mytilidae and some genera of
Lithophaginae. Within Lithophaga some species are
capable of living in a wide range of coral species, whereas
others are restricted to a single species. Species of
Leptoconchus and Magilopsis belonging to the gastropod
family Coralliophilidae bore into living coral (Soliman,
1969).
The Indo-Pacific and Atlantic coral reef faunas are
quite different, with only 7% overlap at the generic level
but none at the species level, 24 genera have been
recorded from the Atlantic and 87 in the Indo-Pacific.
The genera which overlap are those containing species
of the less specialized dead coral borers, and the other genera including those which bore into live coral evolved as
the corals themselves diversified (Rosen, 1984). For more
information on the evolution of boring bivalves and
a detailed list of the coral species bored by particular species of bivalves see Morton (1990).
Succession of molluscs
Experimental substrates have rarely been exposed for long
enough to demonstrate if any succession of species occurs
as the dead substrate ages. Obviously as the surface of the
dead substrate is eroded the bivalve has to continue to bore
deeper in the habitat whereas those boring into live coral
must reverse their direction of their boring as the coral
grows upwards enlarging its burrow posteriorly to keep
the entrance above open, and in these lithophagids, posterior pallial glands secrete an active chelating agent
(Morton, 1990).
Recruitment of molluscs
Bivalves recruit via pelagic larvae, and Morton (1990) suggests that those recruiting to dead coral substrate are similar to other bivalve larvae in their ability to find a suitable
substrate on which to settle and bore. Whereas larvae of
species recruiting to live corals have to have special adaptations. It appears that larvae settling inadvertently onto
a coral species, which they do not normally bore into,
are stung by the coral nematocysts and rapidly withdraw
their feet. Whereas larvae settling on a coral species which
142
BIOEROSION