species from five extant classes Asteroidea (sea stars),
Ophiuroidea (brittlestars), Echinoidea (sea urchins),
Holothuroidea (sea cucumbers), and Crinoidea (feather
stars). They have a distinct radial symmetry based on
pentamery. There are usually five radii (e.g., arms of sea
stars), although multiarmed (6þ arms) asteroids and brittle stars are common. Asteroids are star shaped with the
arms (five or more) tapering from the disc (e.g., Linckia)
or a cushion-like pentagon shape lacking arms (e.g.,
Culcita). Ophiuroids have a round central disc and slender
flexible arms that are sharply set-off from the body. Their
body can have a simple (brittle stars or serpent stars)
or branched (basket stars) profile. Echinoids have a rigid
globose body covered by spines. Holothuroids are elongate sausage-shaped. Crinoids have an array of featherlike arms that range in number from five to more than
two hundred.
Echinoderms are an ecologically important component
of coral reef ecosystems. They are often the dominant
organisms on the sea floor. In shallow and intertidal coral
reef environments, this is particularly true of sea cucumbers (e.g., Holothuria) on reef flats and sandy areas. Several ecologically important keystone echinoderm species
exhibit boom and bust cycles in population density,
including sea urchins (Diadema) and the crown of thorns
sea star (Acanthaster planci) (see section). Echinoderms
provide crucial ecological services to reef systems, with
roles in nutrient cycling, predation, water quality regulation, and herbivory.
Most tropical sea cucumbers are benthic deposit feeders
in soft sediment inter-reefal and lagoon environments,
where they provide important ecosystem services. They
enhance the productivity of nutrient poor carbonate sediments through their bioturbation, burrowing, and feeding/
digestive activity. Taxonomically, these sea cucumbers
have long been a challenge, a problem of concern for the
management of poorly known commercial beche de mer
species, many of which are now locally extinct due to
overfishing.
Most sea urchins are grazers using their hard teeth to
remove algae and encrusting organisms from the surface.
As a member of the grazing guild on coral reefs, sea
urchins contribute to keeping the biomass of algae low,
thereby preventing the overgrowth of algae over coral
substrate. Loss of urchins, such as the disappearance
of Diadema in the Caribbean in the 1980s, resulted in an
ecological shift from coral to algal dominated reefs.
Tripneustes gratilla a commercially important tropical
sea urchin is a specialist on sea grass. The burrowing
activity of echinoids, particularly Echinometra species,
is important in bioerosion of coral reef and intertidal
habitats.
Crinoids and many ophiuroids are suspension feeders
extending their arms above the substrate to capture food
particles on their small tube feet that line the arms. The
food is captured from flow, and so they take up positions
on the reef to take advantage of ambient water movement.
Food particles are conveyed to the mouth in the centre of
the disc by food tracts. Some ophiuroids are predators
and scavengers.
Conclusion
Echinoderms are key members of the biodiversity of coral
reef ecosystems. Some species, such as Acanthaster,
Diadema, and Echinometra, are particularly important in
structuring coral reef communities. Some sea urchins
and many sea cucumbers are important as fishery species.
Bibliography
Birkeland, C., 1989. The influence of echinoderms on coral-reef
communities. In Jangoux, M., and Lawrence, J. M. (eds.), Echinoderm Studies. Vol. 3. Rotterdam: AA Balkema, pp. 1–79.
Byrne, M., Cisternas, P., Hoggett, A., O’Hara T., and S. Uthicke.,
2004. Diversity of echinoderms at Raine Island, Great Barrier
Reef. In Heinzeller, T., and Nebelsick, J. H. (eds.), Echinoderms:
München. London: Taylor and Francis Group, pp. 159–164.
Byrne, M., 2008. Echinodermata: In Hutchings, P., HoeghGuldberg, O., and Kingsford, M. J. (eds.), The Great Barrier
Reef, Biology, Environment and Management. Collingwood:
CSIRO Publishing, pp. 296–307.
Pawson, D. L., 2007. Phylum Echinodermata Zootaxa, 1668,
749–764.
Pearse, J. S., 2009. Shallow-water asteroids, echinoids, and
holothuroids at 6 sites across the tropical west Pacific, 1988–
1989. Galaxea, 11, 1–9.
Uthicke, S., Schaffelke, B., and Byrne, M., 2009. A boom and bust
phylum? Ecological and evolutionary consequences of large
population density variations in echinoderms. Ecol. Monogr.
79(1), 3–24.
Cross-references
Acanthaster Planci
ECO-MORPHODYNAMICS
Paul Kench
The University of Auckland, Auckland, New Zealand
Definition
Eco-morphodynamics is the interaction and co-adjustment
of coral reef structure, morphology, and physical hydrodynamic and ecological processes that is mediated by the
production, transfer, and deposition of calcium carbonate.
Introduction
Coral reefs and reef sedimentary landforms are unique in
that they are composed predominantly of calcium carbonate (CaCO 3 ) that is generated almost entirely from ecological processes. Reef scientists have long recognized the
fact that the structure of coral reefs reflects a balance
between the constructive processes of framework building
corals and other primary and secondary carbonate
ECO-MORPHODYNAMICS
359
Ophiuroidea (brittlestars), Echinoidea (sea urchins),
Holothuroidea (sea cucumbers), and Crinoidea (feather
stars). They have a distinct radial symmetry based on
pentamery. There are usually five radii (e.g., arms of sea
stars), although multiarmed (6þ arms) asteroids and brittle stars are common. Asteroids are star shaped with the
arms (five or more) tapering from the disc (e.g., Linckia)
or a cushion-like pentagon shape lacking arms (e.g.,
Culcita). Ophiuroids have a round central disc and slender
flexible arms that are sharply set-off from the body. Their
body can have a simple (brittle stars or serpent stars)
or branched (basket stars) profile. Echinoids have a rigid
globose body covered by spines. Holothuroids are elongate sausage-shaped. Crinoids have an array of featherlike arms that range in number from five to more than
two hundred.
Echinoderms are an ecologically important component
of coral reef ecosystems. They are often the dominant
organisms on the sea floor. In shallow and intertidal coral
reef environments, this is particularly true of sea cucumbers (e.g., Holothuria) on reef flats and sandy areas. Several ecologically important keystone echinoderm species
exhibit boom and bust cycles in population density,
including sea urchins (Diadema) and the crown of thorns
sea star (Acanthaster planci) (see section). Echinoderms
provide crucial ecological services to reef systems, with
roles in nutrient cycling, predation, water quality regulation, and herbivory.
Most tropical sea cucumbers are benthic deposit feeders
in soft sediment inter-reefal and lagoon environments,
where they provide important ecosystem services. They
enhance the productivity of nutrient poor carbonate sediments through their bioturbation, burrowing, and feeding/
digestive activity. Taxonomically, these sea cucumbers
have long been a challenge, a problem of concern for the
management of poorly known commercial beche de mer
species, many of which are now locally extinct due to
overfishing.
Most sea urchins are grazers using their hard teeth to
remove algae and encrusting organisms from the surface.
As a member of the grazing guild on coral reefs, sea
urchins contribute to keeping the biomass of algae low,
thereby preventing the overgrowth of algae over coral
substrate. Loss of urchins, such as the disappearance
of Diadema in the Caribbean in the 1980s, resulted in an
ecological shift from coral to algal dominated reefs.
Tripneustes gratilla a commercially important tropical
sea urchin is a specialist on sea grass. The burrowing
activity of echinoids, particularly Echinometra species,
is important in bioerosion of coral reef and intertidal
habitats.
Crinoids and many ophiuroids are suspension feeders
extending their arms above the substrate to capture food
particles on their small tube feet that line the arms. The
food is captured from flow, and so they take up positions
on the reef to take advantage of ambient water movement.
Food particles are conveyed to the mouth in the centre of
the disc by food tracts. Some ophiuroids are predators
and scavengers.
Conclusion
Echinoderms are key members of the biodiversity of coral
reef ecosystems. Some species, such as Acanthaster,
Diadema, and Echinometra, are particularly important in
structuring coral reef communities. Some sea urchins
and many sea cucumbers are important as fishery species.
Bibliography
Birkeland, C., 1989. The influence of echinoderms on coral-reef
communities. In Jangoux, M., and Lawrence, J. M. (eds.), Echinoderm Studies. Vol. 3. Rotterdam: AA Balkema, pp. 1–79.
Byrne, M., Cisternas, P., Hoggett, A., O’Hara T., and S. Uthicke.,
2004. Diversity of echinoderms at Raine Island, Great Barrier
Reef. In Heinzeller, T., and Nebelsick, J. H. (eds.), Echinoderms:
München. London: Taylor and Francis Group, pp. 159–164.
Byrne, M., 2008. Echinodermata: In Hutchings, P., HoeghGuldberg, O., and Kingsford, M. J. (eds.), The Great Barrier
Reef, Biology, Environment and Management. Collingwood:
CSIRO Publishing, pp. 296–307.
Pawson, D. L., 2007. Phylum Echinodermata Zootaxa, 1668,
749–764.
Pearse, J. S., 2009. Shallow-water asteroids, echinoids, and
holothuroids at 6 sites across the tropical west Pacific, 1988–
1989. Galaxea, 11, 1–9.
Uthicke, S., Schaffelke, B., and Byrne, M., 2009. A boom and bust
phylum? Ecological and evolutionary consequences of large
population density variations in echinoderms. Ecol. Monogr.
79(1), 3–24.
Cross-references
Acanthaster Planci
ECO-MORPHODYNAMICS
Paul Kench
The University of Auckland, Auckland, New Zealand
Definition
Eco-morphodynamics is the interaction and co-adjustment
of coral reef structure, morphology, and physical hydrodynamic and ecological processes that is mediated by the
production, transfer, and deposition of calcium carbonate.
Introduction
Coral reefs and reef sedimentary landforms are unique in
that they are composed predominantly of calcium carbonate (CaCO 3 ) that is generated almost entirely from ecological processes. Reef scientists have long recognized the
fact that the structure of coral reefs reflects a balance
between the constructive processes of framework building
corals and other primary and secondary carbonate
ECO-MORPHODYNAMICS
359