impact in structuring of reef communities. Bioerosion of
live coral skeletons and coral rock alters and destroys, ultimately reducing them to fine, unconsolidated sediments.
Bioeroders may be divided into the epilithic and the endolithic (Glynn, 1997). Using their reinforced radular teeth,
epilithic bioeroders abrade limestone in the process of
grazing on algae or other organisms. These include chitons, patellids, trochids, littorinids, and neritids (Glynn,
1997). Endolithic bioeroders bore into coral skeletons
using chemical and/or mechanical processes. Bivalves in
the genera Lithophaga, Gastrochaena, Fungiacava, and
Choristodon are active borers, whereas Tridacna and
Hippopus show a limited degree of downward penetration
(Glynn, 1997; Morton, 1983). Reef-building vermetid
gastropods in the genera Vermetus and Dendropoma can
be important contributors to the structure of coral reefs.
Bibliography
Caterall, C., 1998. Molluscs on coral reefs. In Beesley, P. L., Ross,
G. J. B., and Wells, A. (eds.), Mollusca: The Southern Synthesis.
Fauna of Australia, Vol. 5. Melbourne: CSIRO Publishing,
pp. 33À36.
Glynn, P. W., 1997. Bioerosion and coral reef growth: a dynamic
balance. In Birkeland, C. (ed.), Life and Death of Coral Reefs.
New York: Chapman & Hall, pp. 68–95.
Morton, B., 1983. Coral-associated bivalves of the Indo-Pacific. In
Russell-Hunter, W. D. (ed.), The Mollusca, Volume 6, Ecology.
Orlando: Academic Press, pp. 139À224.
Paulay, G., 1997. Diversity and distribution of reef organisms In
Birkeland, C. (ed.), Life and Death of Coral Reefs. New York:
Chapman & Hall, pp. 298–353.
Cross-references
Bioerosion
Sediment Dynamics
Sediments, Properties
MURUROA ATOLL
Lucien F. Montaggioni
University of Provence, Marseille Cedex 3, France
Definition
The atoll of Mururoa, also known as Moruroa or Aopuni,
lies in the south-eastern end of the Tuamotu Island Group
(21
50
0 S, 138
53
0 W) in the central Pacific. Politically
speaking, this island is part of the French Polynesia.
Geological setting
Mururoa is 28 km long and 11 km wide with a reef rim not
exceeding 15 km
2 in surface area. It falls into the openatoll type with a large natural pass (4,500 km wide,
1–9 m deep) on its leeward side. Its volcanic basement
has originated from the Pitcairn hotspot at present operating about 70 km to the south-east of Pitcairn Island
(Gillot et al., 1992). The cessation of volcanic activity
from about 11 to 10.5 million years favored prominent
reef building. Subsidence of the volcanic shield at an average rate of 7–8 mm per millennium has been accompanied
by deposition of a sedimentary pile varying between 130
and 570 m thickness (Buigues, 1997). From the end of
the 1960s, in relation to nuclear testing, intensive biological, geological, and geophysical surveys were carried out
on the modern reef and its carbonate foundations.
Modern reef morphology and zonation
The reef zonation, the composition and distribution of
major reef-building biological assemblages have been
described by Chevalier et al. (1969), Bablet et al.
(1995) and Camoin et al. (2001). Up to 50 coral species
have been described from the fore-reef to lagoonal areas.
The outer reef rim
The reef rim, 3 m in maximum elevation, consists of successive islets (« motus ») that form a continuous 400-m
wide line along the windward side, and a discontinuous
line, 1,100 m wide on the leeward side. The rim is made
up of coral rubble mixed with skeletal sand, locally resting
on the modern reef flat or overlying emergent, firmly
cemented coral conglomerates of mid-to late Holocene
age. The reef-flat zone is subdivided into three subzones.
The outermost subzone is a typical algal ridge, up to
0.50 m high, 20–50 m wide, mostly made up of coralline
algal crusts (Hydrolithon onkodes, Chevaliericrusta
polynesiae) associated with encrusting foraminifera
(Homotrema, Acervulina) and scarce robust branching
corals (Acropora humilis, Pocillopora verrucosa). Behind
the algal ridge, there is a coral-dominated subzone
inhabited by stunted branching, massive, and encrusting
colonies (A. humilis group, Pocillopora brevicornis,
Favia stelligera, Montipora caliculata, Porites lichen).
Behind the islets to the lagoon, there is an inner reef-flat
subzone mainly covered by loose detritus and a few corals
(Acropora muricata, A. corymbosa, A. microphthalma,
A. tenuis, and Alveopora allingi).
The fore-reef zone can be delineated into three subzones. The upper subzone (to about 10 m deep) is gently
dipping (less than 15
) and dominantly colonized by
robust branching and massive corals (Acropora robusta
group, A. humilis group, P. verrucosa, P. eydouxi,
F. stelligera, Platygyra daedalea). Between the depth of
10 and 40–50 m, the intermediate fore-reef subzone
becomes increasingly steeper (30–45
). The coral cover
rate decreases; the dominating coral species include
Acropora nasuta, Astreopora myriophthalma, Leptastrea
transversa, Echinopora gemmacea, Leptoria phrygia,
Pocillopora elegans, together with Montipora
and Cyphastrea. The lower fore-reef subzone below
40–50 m forms a sub-vertical drop-off, locally covered
by encrusting, tabular, and foliaceous corals (agariciids
mainly) associated with coralline algae and encrusting
foraminifera (acervulinids). In addition, the fore-reef zone
exhibits terraces at 10, 20, 40, 55, and 65 m and cave-like
MURUROA ATOLL
713
live coral skeletons and coral rock alters and destroys, ultimately reducing them to fine, unconsolidated sediments.
Bioeroders may be divided into the epilithic and the endolithic (Glynn, 1997). Using their reinforced radular teeth,
epilithic bioeroders abrade limestone in the process of
grazing on algae or other organisms. These include chitons, patellids, trochids, littorinids, and neritids (Glynn,
1997). Endolithic bioeroders bore into coral skeletons
using chemical and/or mechanical processes. Bivalves in
the genera Lithophaga, Gastrochaena, Fungiacava, and
Choristodon are active borers, whereas Tridacna and
Hippopus show a limited degree of downward penetration
(Glynn, 1997; Morton, 1983). Reef-building vermetid
gastropods in the genera Vermetus and Dendropoma can
be important contributors to the structure of coral reefs.
Bibliography
Caterall, C., 1998. Molluscs on coral reefs. In Beesley, P. L., Ross,
G. J. B., and Wells, A. (eds.), Mollusca: The Southern Synthesis.
Fauna of Australia, Vol. 5. Melbourne: CSIRO Publishing,
pp. 33À36.
Glynn, P. W., 1997. Bioerosion and coral reef growth: a dynamic
balance. In Birkeland, C. (ed.), Life and Death of Coral Reefs.
New York: Chapman & Hall, pp. 68–95.
Morton, B., 1983. Coral-associated bivalves of the Indo-Pacific. In
Russell-Hunter, W. D. (ed.), The Mollusca, Volume 6, Ecology.
Orlando: Academic Press, pp. 139À224.
Paulay, G., 1997. Diversity and distribution of reef organisms In
Birkeland, C. (ed.), Life and Death of Coral Reefs. New York:
Chapman & Hall, pp. 298–353.
Cross-references
Bioerosion
Sediment Dynamics
Sediments, Properties
MURUROA ATOLL
Lucien F. Montaggioni
University of Provence, Marseille Cedex 3, France
Definition
The atoll of Mururoa, also known as Moruroa or Aopuni,
lies in the south-eastern end of the Tuamotu Island Group
(21
50
0 S, 138
53
0 W) in the central Pacific. Politically
speaking, this island is part of the French Polynesia.
Geological setting
Mururoa is 28 km long and 11 km wide with a reef rim not
exceeding 15 km
2 in surface area. It falls into the openatoll type with a large natural pass (4,500 km wide,
1–9 m deep) on its leeward side. Its volcanic basement
has originated from the Pitcairn hotspot at present operating about 70 km to the south-east of Pitcairn Island
(Gillot et al., 1992). The cessation of volcanic activity
from about 11 to 10.5 million years favored prominent
reef building. Subsidence of the volcanic shield at an average rate of 7–8 mm per millennium has been accompanied
by deposition of a sedimentary pile varying between 130
and 570 m thickness (Buigues, 1997). From the end of
the 1960s, in relation to nuclear testing, intensive biological, geological, and geophysical surveys were carried out
on the modern reef and its carbonate foundations.
Modern reef morphology and zonation
The reef zonation, the composition and distribution of
major reef-building biological assemblages have been
described by Chevalier et al. (1969), Bablet et al.
(1995) and Camoin et al. (2001). Up to 50 coral species
have been described from the fore-reef to lagoonal areas.
The outer reef rim
The reef rim, 3 m in maximum elevation, consists of successive islets (« motus ») that form a continuous 400-m
wide line along the windward side, and a discontinuous
line, 1,100 m wide on the leeward side. The rim is made
up of coral rubble mixed with skeletal sand, locally resting
on the modern reef flat or overlying emergent, firmly
cemented coral conglomerates of mid-to late Holocene
age. The reef-flat zone is subdivided into three subzones.
The outermost subzone is a typical algal ridge, up to
0.50 m high, 20–50 m wide, mostly made up of coralline
algal crusts (Hydrolithon onkodes, Chevaliericrusta
polynesiae) associated with encrusting foraminifera
(Homotrema, Acervulina) and scarce robust branching
corals (Acropora humilis, Pocillopora verrucosa). Behind
the algal ridge, there is a coral-dominated subzone
inhabited by stunted branching, massive, and encrusting
colonies (A. humilis group, Pocillopora brevicornis,
Favia stelligera, Montipora caliculata, Porites lichen).
Behind the islets to the lagoon, there is an inner reef-flat
subzone mainly covered by loose detritus and a few corals
(Acropora muricata, A. corymbosa, A. microphthalma,
A. tenuis, and Alveopora allingi).
The fore-reef zone can be delineated into three subzones. The upper subzone (to about 10 m deep) is gently
dipping (less than 15
) and dominantly colonized by
robust branching and massive corals (Acropora robusta
group, A. humilis group, P. verrucosa, P. eydouxi,
F. stelligera, Platygyra daedalea). Between the depth of
10 and 40–50 m, the intermediate fore-reef subzone
becomes increasingly steeper (30–45
). The coral cover
rate decreases; the dominating coral species include
Acropora nasuta, Astreopora myriophthalma, Leptastrea
transversa, Echinopora gemmacea, Leptoria phrygia,
Pocillopora elegans, together with Montipora
and Cyphastrea. The lower fore-reef subzone below
40–50 m forms a sub-vertical drop-off, locally covered
by encrusting, tabular, and foliaceous corals (agariciids
mainly) associated with coralline algae and encrusting
foraminifera (acervulinids). In addition, the fore-reef zone
exhibits terraces at 10, 20, 40, 55, and 65 m and cave-like
MURUROA ATOLL
713