Anthozoans - One of the four classes of Cnidaria
(with the Hydrozoa, the Scyphozoa and the Cubozoa). Literally, "Animal-Flower".
The Anthozoa include a large number of organisms grouped into two subclasses:
the Octocorallia with 8 (or a multiple of 8) tentacles around the
mouth and the Hexacorallia (6 or a multiple of 6 tentacles). The
Anthozoa do not have a medusa stage in their life-cycle.
Anthropic - Result of human activity.
Ahermatypic Scleractinaria - do not take part in elaborating the
coral reef. The limit between hermatypic and ahermatypic is often difficult
to establish.
Aposymbiotic Scleractinaria - are often temporarily without zooxanthellae.
Asexual Reproduction - does not call for gonad intervention (the
familiar vegetative reproduction in plants). Asexual reproduction takes
place at the expense of the parent organism's tissue: simple scissiparity
(protists, sea-anemones ...), budding.
Azooxanthellate - do not have zooxanthellae. Some species have zooxanthellate
and azooxanthellate colonies at the same time.
B
Biomineralization - Process by which a living organism
transforms dissolved ions in the external environment into a highly organized
crystalline structure, the biomineral. This can take several forms :
- microcrystals in marine invertebrates (spicules of red coral or sponges
for example), otoliths of fish, otocones in our inner ear,
- but also structures which can be several hundreds of kilometres long,
the coral reefs.
The biomineral is a biological structure, even if it looks like a piece
of rock: its exceptional resistance (nacre, for example, has a breaking
point 10 times higher than that of concrete) is due to the presence of
a framework of organic macromolecules placed in the same way as the iron
in reinforced concrete. The importance of biomineralization goes beyond
the organism level, since it has a long-term effect on our ecosystem,
biomineralization models our geological landscapes and also our atmosphere.
The calcareous mountains around us (such as the Provençal Prealpes
and the Dolomites) are the result of deposition some 500 million years
ago of skeletons of the marine organisms, including coral reefs which
were present in our latitudes. Biomineralization also allowed for the
elimination, in our primeval atmosphere, of excess carbon dioxide and
so permitted life to develop.
Biomineralization also has applications in various fields: biomedical
(osteoporosis, for example, is one of the most costly illnesses for our
society), material technology...
Bleaching - This is a loss of coloration in corals which reveals the
white skeleton. It is consecutive to the loss of the symbiotic zooxanthellae,
or of photosynthetic pigments. The causes and mechanisms of this change
are still unknown. A histological study of bleached tissue does however
suggest three different bleaching mechanisms. These include the degeneration
of zooxanthellae in situ, the expulsion of zooxanthellae by the mesenteric
filaments and from the mouth, and expulsion via the cœlentron of
whole endodermal cells with their zooxanthellae. In the same colony, the
three processes can take place simultaneously. There is no separate mechanism
for each species. The expulsion of cells from the cœlenteron seems
to be the response to a thermal shock of tissue degenerating after being
exposed to extreme conditions, rather than an intermediary stage in bleaching.
Most hypotheses suggest that bleaching episodes are a result of high temperatures,
an abnormal ultraviolet intensity or the increase in the partial pressure
of CO2.
Certain authors suggest that bleaching could be an adaptive reaction rather
than a pathological one, stemming from an opportunity taken by the coral
to combine with other zooxanthellae more capable of adapting to a modified
environment.
Carbonate - Mineral formed by the precipitation
of inorganic dissolved carbon.(bicarbonate formed by dissolution in water
of carbon dioxide). Carbonate can be associated with calcium (limestone),
with magnesium and with calcium (dolomite), and with copper (malachite)
or even with strontium (strontianite).
Cnidaria - Branch of eucaryote (cells with a nucleus) metazoan (multicellular)
organisms. They are diploblastic, have a radial symmetry and have highly
specialized stinging cells, the cnidocytes. Being very polymorphic, some
are able to change from a medusa to a polyp stage during their life cycle.
They were initially listed with the Ctenaria (Ctenophora), as Coelenterata.
Cuvier (1769-1832) classified these organisms as Zoophytes (or animal-plants),
also including the Protozoa, the sponges, the echinoderms and the bryozoans.
Cnidaria are like a bag with a single opening used as mouth and anus and
surrounded by tentacles. The mouth opens into a cavity, the coelenteron,
or gastrovascular cavity, which is digestive and circulatory at the same
time.
Cnidocytes (nematocytes, or Nessel cells) - During prey capture, the
Cnidaria use an organelle which is typical of the whole group - the cnidocyst.
This is a secretion of the cnidocyte, a sensorial secreting cell of which
30 types have been described for the Cnidaria. This highly specialised structure
is used to capture prey and to protect the organism. The cnidocyte includes
a capsule in which the stinging structure, the cnidocyst, is enclosed, rolled
around an axis and folded in like the finger of a glove, swimming in a liquid
matrix (generally toxic), which is injected into the eventual prey when
the cnidocyst is triggered. The capsule carries a cnidocil, oriented towards
the middle, and the mechanical stimulation of which sets off an electric
signal provoking an explosive exocytosis of the cnidocyst. In Hydra, this
exocytotic mechanism lasts less than 3 milliseconds. In the capsule, the
original osmotic pressure is 1,5 E+7 Pa, due to the presence of inorganic
cations (Mg2+ or Ca2+)
and of some rare polyanions. The cnidocysts only function once and cannot
regenerate. The cnidocytes from which the cnidocysts have been discharged
are replaced by new cells from the differentiation of pluripotential cells,
the interstitial cells.
Coelenterates - Ancient name for a branch grouping the Cnidaria and the Ctenaria (or Ctenophora).
Colony - Formed by fragmentation from an initial animal and whose individual parts remain connected and dependent on each other (sharing of resources...). They can be called a super-organism. The colony can, by fragmentation, produce new individuals forming a new colony. Very common in Cnidaria and Bryozoa or Ectoprocts). Quite different from a society of unconnected, sexually reproduced individuals.
Coenosarc - Coral polyp are interconnected by a living tissue, the coenosarc. It is constituted by the superposition of two tissues layers, the oral tissue and the aboral tissue whose demarcate the coenosarcal space, ie gastrovascular cavity.
Coral - This is a relatively imprecise name as it covers many organisms.
It was originally given to the red Mediterranean coral (jewellers' coral
or Corallium rubrum). The name includes several species of Cnidaria whose
anatomical and biological characteristics can be very different: some avoid
light and live at depth (red Mediterranean coral) and others, on the contrary,
live at the surface where their symbionts benefit from maximum sunshine.
They mainly belong to the class Anthozoa (Octocorallia: soft corals, red
corals, gorgonians, blue coral; Hexacorallia: coral reef constructers, black
coral) but also to the Hydrozoa (fire corals). Certain Bryozoa (or Ectoprocta)
are called false coral because of their resemblance to red coral.
Dinoflagellata (or dinophytes)
- Dinoflagellates (from the Greek "dinos," whirlpool) are unicellular
eucaryotes with two flagella placed perpendicular to each other. They have
particular characteristics differentiating them from other eucaryote organisms
and justifying their being grouped together. Usually of a brown colour,
they are characterised by having permanently condensed chromosomes in a
dinocaryon or mesocaryon nucleus. In many species, the cell is protected
by a theca of rigid cellulose plates. The dinoflagellates have a diameter
of 10 to 12 µm. They can be autotrophic, heterotrophic or mixotrophic. They
can be free-living and, in certain ecological conditions, some species (Gonyaulax,
for example) can proliferate and produce the red waters (or red tides),
secreting toxins (saxitoxine) able to provoke considerable mortality of
benthic animals and plankton or endanger human consumption of shell-fish
or fish. They can also live in symbiosis with many protists (ciliates, foraminiferans,
radiolarians) or marine invertebrates (sponges, Cnidaria, molluscs).
The most common dinoflagellates in marine symbiosis with Cnidaria, belong
to the genus Symbiodinium. Although initially there only seemed
to be one species of Symbiodinium (S. microadriaticum), genetic variations
have been detected between different Symbiodinium and at present several
genetically different dinoflagellates have been identified. Certain Cnidaria
that seem phyletically close have different symbionts. Recent studies
show that the same Cnidaria species can contain several different types
of dinoflagellates. Considering the unknown taxonomic position of most
of the Symbiodinium, it is wiser to list dinoflagellates of different
hosts and from different places, as Symbiodinium spp.
When not in a symbiotic association, dinoflagellates are mobile, each
cell having two flagella which emerge at the junction between a transverse
and longitudinal furrow at the surface of the cell, and extend the whole
length of this furrow. In the symbiotic association, the cells do not
produce flagella although the flagellum bases are present. Each individual
has four main organelles: nucleus, chloroplast, pyrenoid and accumulation
body. The lobed chloroplast, made up of thylakoids, is peripheral. The
principal pigments are the chlorophylls a and c2, ß-carotène
and the xanthophylls (peridinin, diadinoxanthin, dinoxanthin). The pyrenoid,
attached to the internal face of the chloroplast by a short peduncle,
contains ribulose 1,5-diphosphate carboxylase oxygenase, enzyme responsible
for CO2 fixation in photosynthesis. The accumulation body is a vacuole,
specific for symbiotic dinoflagellates, and its function is not clear,
although it is most pronounced in aged cells.
Diploblastic (ou diblastic) - An organism formed from two epithelial
layers (ectoderm and endoderm): Cnidaria, Ctenophores.
Eutrophication - Modification of aquatic ecosystems,
due to an enrichment of nutrients (mineral salts) and recognised by an increase
in algal production and a reduction of water transparency.
Greenhouse effect - Climatic phenomenon in which
the lower layers of the atmosphere (principally water vapour, carbon dioxide,
methane) hold back the warm infra-red wavelengths of solar radiation emitted
by reflection from the surface of the earth.
Hermatypic Scleractinaria - take part in elaborating
the coral reef.
Hexacorallians - Subclass of Anthozoa, with the mouth surrounded by
6 or a multiple of 6 tentacles. Can be solitary or colonies. This subclass
includes sea anemones (Actinaria), the reef-building corals (Scleractinaria
or Madreporia), the Cerianthia, the Zoantharia and the Antipatharia (black
coral).
Limestone - Rock containing at least 50 % calcium
carbonate (CaCO3). This is an important constituent
of the relief, sedimentary rocks resulting from sedimentation of living
organisms, mainly corals, foraminiferans (nummulites, globigerines), unicellular
calcareous algae ……. The cliffs at Etretat and the material used on the
Egyptian pyramids are made up of an accumulation of the skeletons of unicellular
organisms (calcareous algae, coccoliths, foraminifera). Calcium carbonate
can be crystallized in three forms: calcite (rhomboedric crystals), aragonite
(orthorhombic crystals) and vatherite. Calcareous rocks are easily recognized
by their solubility in acid and emission of carbon dioxide.
Mesocosm -Closed experimental system of varying size destined to study the effects of environmental parameters or of pollutants on a complete ecosystem or on a few isolated elements of this ecosystem.
Mesoglea - In Cnidaria, the two epithelial layers (ectoderm and endoderm) are separated by a gelatinous layer, similar to the basal layer in vertebrates. The mesoglea is rich in collagen and elastic fibres. The matrix in which the fibres float is essentially made up of a protein-polysaccharide hydrated complex. This composition offers elasticity and rigidity to the whole polyp. The mesoglea contains 95% water in medusa forms and slightly less in polyp forms.
Octocorallaria - Subclass of the Anthozoa, with a mouth surrounded with 8 or a multiple of 8 pinnate tentacles (small expansions, or pinnules, disposed in two rows). They are colonial. The skeleton can be organic axial or organomineral, or spread in the tissue (mesoglea) as calcareous grains (spicules) with a specific form for any given genus. Includes the Gorgonidia (gorgonians), the Corallidia (red corals), the soft corals(or Alcyonaria), the Helioporidia or blue coral ), the Pennatulidia (sea feathers, veretille), the Stolonifera (Tubipora).
Polyp - This is the basic unit of a coral. It is a fixed sac with only one orifice, which functions as mouth and anus, and is surrounded by several crowns of hollow tentacles ending or not with a swelling, the acrosphere. The mouth is prolonged by a short śsophagus, the stomodeum, terminating in the gastrovascular cavity or cślenteron, itself prolonged by a narrow canal to the end of each tentacle. The stomodeum consists of an ectodermic pharynx and gastric divisions radially orientated, alternating with the tentacles. The upper part of the animal carrying the tentacles is called the oral disc and the lower part above the skeleton is called the basal or aboral disc. The oral disc is connected to the basal disc by the mural column. The polyp can be isolated (as in the sea anemone or the solitary coral Fungia), or colonial, forming a super-organism. In certain cases, the polyps can have specialisations (for the capture of food, gastrozooids; for reproduction, gonozooids…).
Asexual Reproduction - does not call for gonad intervention (the familiar vegetative reproduction in plants). Asexual reproduction takes place at the expense of the parent organism's tissue: simple scissiparity (protists, sea-anemones ...), budding.
Ahermatypic Scleractinaria - do not take part in elaborating the coral reef. The limit between hermatypic and ahermatypic is often difficult to establish.
Aposymbiotic Scleractinaria - are often temporarily without zooxanthellae.
Hermatypic Scleractinaria - take part in elaborating the coral reef.
Scleractinaria zooxanthellate - have symbiotic dinoflagellates,
the zooxanthellae.
Symbiosis
The German botanist, Anton de Bary (1831 - 1888) suggested the word symbiosis
in 1879, this word was initially defined by him as life in association
with different organisms (thus including parasitism). The notion of symbiosis
is commonly restricted to associations with mutual benefit (or mutualism).
Symbiosis is an association which lasts for at least one part of the
biological cycle, between two or several specifically distinct organisms.
This association leads to the formation of a new biological entity, the
symbiocosm, itself subject to natural selection. Physiologically, symbiosis
allows for a better adaptation to the environment and, on the genetic
level, it is a sophisticated way to obtain new genes by lateral transfer.
The integration of the symbiont in the host's metabolism is sometimes
so specialized that the symbiont is like a new cytoplasmic organelle.
For certain researchers this is one of the most powerful evolutionary
powers.
The appearance of higher forms of life during early evolution is due
to symbiosis. In fact, it is now strongly admitted that developed cells
are a result of bacteria incorporation some 1 to 1.5 billion years ago
in the primitive cells of living organisms benefiting mutually from this
association. These bacteria became so well integrated in their new environment
that they can now no longer live in isolation: these are the mitochondria,
the energy producing centres of our cells.
The chloroplasts, organelles at the origin of plant photosynthesis, also
have an endosymbiotic origin. This theory developed over the last 30 years
by Lynn MARGULIS, originated from work done in Monaco at the beginning
of the 21st century by Paul PORTIER, a collaborator of H.R.H. the Prince
Albert 1st and author of the book "Les Symbionts" published
in1918.
But the role of symbiosis is not limited to cellular evolution and symbiotic
associations are important agriculturally: leguminous plants (green haricot
bean, broad beans, soja, lucerne …) for example are successful because
of a symbiotic association with bacteria.
The enormous biodiversity represented by coral reefs is also due to a
symbiosis between the animal coral cell and a vegetal protist (dinoflagellate).
This dinoflagellate, called a zooxanthella, when photosynthesizing, provides
the energetic molecules for its host, which then has less need to feed
itself. In exchange the zooxanthella is housed and receives waste nitrogen
and CO2 from the host which is the basis of its own food.
Symbiont - Partners in symbiosis. Generally, the larger of the organisms is called the host and the smaller one the symbiont.
