(2008). They consist of boring autotrophic and heterotrophic microorganisms or euendoliths (boring microflora)
which actively penetrate (biochemical dissolution) the
coral substrate (Golubic et al., 1981). Species composition
of these communities varies between live and dead substrates. The species composition of those occurring in live
coral is positively phototrophic, fast growing taxa which
can keep up with accretion of the coral, and can stay in
the surface layers of the substrate to obtain sufficient light
for their growth (Tribollet and Payri, 2001). One species
of chlorophyte Ostreobium quekettii Bornet and Flahault,
1899, has been recorded widely from Atlantic and Pacific
corals but this may represent a suite of cryptic species
(pers. comm. H. Verbruggen as cited in Tribollet, 2008).
These algae form a distinct green band just below the coral
surface (Figure 1a). Infestation occurs as the coral polyp
is settling on the substrate and beginning to lay down
a coral skeleton. The same suite of microborers also colonize encrusting coralline algae growing over dead coral
substrates. Following death of the coral colony another
suite of algae colonize the substrate within a few days
(Hutchings, 1986; Gektidis, 1999) (Figure 1b). Early
colonists are short-lived opportunistic species. Within
6–12 months, these endolithic algal communities become
dominated by low-light specialists and heterotrophic fungi,
and are referred to as “mature communities” (Tribollet,
2007). Such changes in species composition are driven by
reduced light penetration as the surface of the substrate
becomes covered with epilithic organisms, which reduces
the amount of light able to penetrate into the substrate and
therefore species which can utilize these lower levels flourish. The species composition of the epilithic biota strongly
influences which species of boring microflora (euendoliths)
are present. For example, under turf algae (Figure 1b)
growing on dead Porites, colonies of the cyanobacteria
Mastigocoleus testarum are abundant, whereas a different
suite occurs under crustose coralline algae (Chazottes
et al., 2002). Substrates in turbid areas may be covered with
sediment which inhibits or reduces the density of boring
microflora (Osorno et al., 2005).
Method by which microborers bore
It was thought that the boring microflora penetrated the
substrate by dissolving its crystalline matrix (Tudhope
and Risk, 1985), but recent studies suggest that there
is a temporal separation between photosynthesis and boring activities (Garcia-Pichel, 2006). This involves active
transport of Ca
2þ from the apical cell of the filaments to
their trailing end occurs which would make dissolution
of the substrate around the apical cell feasible when interstitial pH is high due to photosynthesis. This may explain
why micrite and brucite are commonly seen precipitated
around the microflora filaments at the surface of dead substrates (Kobluk and Risk, 1977). But more studies are
required and it may be that the process of substrate dissolution varies depending on the taxa, type of substrate, and
environmental conditions (Tribollet, 2008).
Macroborers
Polychaetes
Diversity of polychaetes
Representatives of a variety of polychaete families are
found, including Eunicidae, Sabellidae, Spionidae, and
Cirratulidae, which are not closely related, suggesting that
the ability to bore into coral substrate has arisen several
times. Even within a genus not all members are borers.
In the Indo-Pacific, some species have been reported
as having wide distributions, such as Nematonereis
unicornis and Lysidice collaris, but this needs to be carefully checked using both morphological and molecular
data. Information on the boring species present in the
Caribbean is lacking. Boring polychaetes are primarily
found in dead coral substrate, when they occur in live coral
typically a few polyps have been damaged which presumably allows the larvae to settle and penetrate. The only
exception to this appears to be a species of Flabelligeridae
which is common in some live coral colonies in Hong
Kong Harbour (Hutchings, pers. observ.).
Succession of polychaetes
The first suite of macroborers to arrive are short-lived
polychaetes such as species of Polydora (Spionidae) and
Fabriciniids (Sabellidae), which can be extremely abundant. These species are either deposit or filter feeders
which feed on the sediment trapped in the irregularities
of the surface of the substrate or spread their feeding
crowns out into the water column above the surface where
they filter out food particles. Obviously these species are
susceptible to being removed when parrotfishes or echinoids graze on the substrate as they live in the surface
layers. Over the next year or so, other boring organisms
recruit to these substrates including a range of other longer
lived polychaetes belonging to the families Cirratulidae,
Eunicidae and Sabellidae (Hutchings et al., 1992; Pari
et al., 1998, 2002) which exhibit a range of feeding strategies including surface deposit feeders, filter feeders and
others are predators. One suspects that the predators feed
on the other macroborers but some may be more opportunistic and also feed on the microborers.
Recruitment of polychaetes
Following successful colonization of newly available substrates by a suite of endolithic algae, viruses etc. and turf
algae, pelagic larvae of boring polychaetes settle on the
surface and turf algae may provide some protection for
these larvae from small scale water movement which could
wash them off the substrate as they metamorphose and
begin to bore. Observations on experimental substrates
suggest that larvae tend to settle in small depressions
(Hutchings, pers. observ.). High rates of mortality of such
larvae must occur at this time, but experimental studies
have shown that some recruitment of this initial suite of
polychaete macroborers occurs throughout the year and
to all habitats (Hutchings et al., 1992; Kiene and
140
BIOEROSION
which actively penetrate (biochemical dissolution) the
coral substrate (Golubic et al., 1981). Species composition
of these communities varies between live and dead substrates. The species composition of those occurring in live
coral is positively phototrophic, fast growing taxa which
can keep up with accretion of the coral, and can stay in
the surface layers of the substrate to obtain sufficient light
for their growth (Tribollet and Payri, 2001). One species
of chlorophyte Ostreobium quekettii Bornet and Flahault,
1899, has been recorded widely from Atlantic and Pacific
corals but this may represent a suite of cryptic species
(pers. comm. H. Verbruggen as cited in Tribollet, 2008).
These algae form a distinct green band just below the coral
surface (Figure 1a). Infestation occurs as the coral polyp
is settling on the substrate and beginning to lay down
a coral skeleton. The same suite of microborers also colonize encrusting coralline algae growing over dead coral
substrates. Following death of the coral colony another
suite of algae colonize the substrate within a few days
(Hutchings, 1986; Gektidis, 1999) (Figure 1b). Early
colonists are short-lived opportunistic species. Within
6–12 months, these endolithic algal communities become
dominated by low-light specialists and heterotrophic fungi,
and are referred to as “mature communities” (Tribollet,
2007). Such changes in species composition are driven by
reduced light penetration as the surface of the substrate
becomes covered with epilithic organisms, which reduces
the amount of light able to penetrate into the substrate and
therefore species which can utilize these lower levels flourish. The species composition of the epilithic biota strongly
influences which species of boring microflora (euendoliths)
are present. For example, under turf algae (Figure 1b)
growing on dead Porites, colonies of the cyanobacteria
Mastigocoleus testarum are abundant, whereas a different
suite occurs under crustose coralline algae (Chazottes
et al., 2002). Substrates in turbid areas may be covered with
sediment which inhibits or reduces the density of boring
microflora (Osorno et al., 2005).
Method by which microborers bore
It was thought that the boring microflora penetrated the
substrate by dissolving its crystalline matrix (Tudhope
and Risk, 1985), but recent studies suggest that there
is a temporal separation between photosynthesis and boring activities (Garcia-Pichel, 2006). This involves active
transport of Ca
2þ from the apical cell of the filaments to
their trailing end occurs which would make dissolution
of the substrate around the apical cell feasible when interstitial pH is high due to photosynthesis. This may explain
why micrite and brucite are commonly seen precipitated
around the microflora filaments at the surface of dead substrates (Kobluk and Risk, 1977). But more studies are
required and it may be that the process of substrate dissolution varies depending on the taxa, type of substrate, and
environmental conditions (Tribollet, 2008).
Macroborers
Polychaetes
Diversity of polychaetes
Representatives of a variety of polychaete families are
found, including Eunicidae, Sabellidae, Spionidae, and
Cirratulidae, which are not closely related, suggesting that
the ability to bore into coral substrate has arisen several
times. Even within a genus not all members are borers.
In the Indo-Pacific, some species have been reported
as having wide distributions, such as Nematonereis
unicornis and Lysidice collaris, but this needs to be carefully checked using both morphological and molecular
data. Information on the boring species present in the
Caribbean is lacking. Boring polychaetes are primarily
found in dead coral substrate, when they occur in live coral
typically a few polyps have been damaged which presumably allows the larvae to settle and penetrate. The only
exception to this appears to be a species of Flabelligeridae
which is common in some live coral colonies in Hong
Kong Harbour (Hutchings, pers. observ.).
Succession of polychaetes
The first suite of macroborers to arrive are short-lived
polychaetes such as species of Polydora (Spionidae) and
Fabriciniids (Sabellidae), which can be extremely abundant. These species are either deposit or filter feeders
which feed on the sediment trapped in the irregularities
of the surface of the substrate or spread their feeding
crowns out into the water column above the surface where
they filter out food particles. Obviously these species are
susceptible to being removed when parrotfishes or echinoids graze on the substrate as they live in the surface
layers. Over the next year or so, other boring organisms
recruit to these substrates including a range of other longer
lived polychaetes belonging to the families Cirratulidae,
Eunicidae and Sabellidae (Hutchings et al., 1992; Pari
et al., 1998, 2002) which exhibit a range of feeding strategies including surface deposit feeders, filter feeders and
others are predators. One suspects that the predators feed
on the other macroborers but some may be more opportunistic and also feed on the microborers.
Recruitment of polychaetes
Following successful colonization of newly available substrates by a suite of endolithic algae, viruses etc. and turf
algae, pelagic larvae of boring polychaetes settle on the
surface and turf algae may provide some protection for
these larvae from small scale water movement which could
wash them off the substrate as they metamorphose and
begin to bore. Observations on experimental substrates
suggest that larvae tend to settle in small depressions
(Hutchings, pers. observ.). High rates of mortality of such
larvae must occur at this time, but experimental studies
have shown that some recruitment of this initial suite of
polychaete macroborers occurs throughout the year and
to all habitats (Hutchings et al., 1992; Kiene and
140
BIOEROSION