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Family Acroporidae: (Gk. akron, extremity, summit; L. porous, pore)....relating to the presence of a corallite at the tip of each branch. Corallites (except Astreopora) are small with septa in two cycles (up to 12 septa) or less and without a columella (except Montipora). Coenosteum displays a spongy appearance.

Fossil evidence for the Jurassic origin of the Acroporidae is weak, being dependent on two Mesozoic genera. The extant genera are Cenozoic in origin except for Astreopora, which has been separate since the Cretaceous.
It is not only the most species-rich family of reef-building scleractinia with only four genera (i.e. Montipora, Anacropora, Acropora, and Astreopora) but taxonimically the most problematic and difficult to deal with. While Anacropora and Astreopora are represented with only a view species, the other two display a wide assemblage of controverse species - by far the most difficult genus is Acropora (A.robusta is an anarchist among corals - different parts of the same colony can have very different growth forms - Veron 1986).


Some members of the family Acroporidae (95kB)

Nothing seems to be more depressing to a coral ecologist, than seeing heaps of different kinds of Acropora growth forms in a seemingly diverse assemblage, yet not knowing for sure whether each form represents a distinct species or just a variation of a single species. It seems that the western archipellago of PNG and Irian Jaya of Indonesia can be considered the center of Acropora diversity, with numerous true species that have not yet been described.
All species belonging to the family Acroporidae that have been observed were found to be hermaphrodites. The majority are broadcast-spawners, while some are known to be involved in annual synchronous mass-spawning events on the GBR (Great Barrier Reef), on reefs in Okinawa (Japan), and in Java waters (Indonesia) as well. Extensive monospecific stands of some acropoids are primarily a result of fragmentation, a form of asexual reproduction. Higher predation by polychaete worms and zooplankton was observed in the close vicinity of coral polyps that are at the sea surface.

Key to the family Acroporidae No axial corallite Corallites <2mm in diameter, columella absent Branches without basal structures: Genus Anacropora
Branches with basal structures: Genus Montipora
Corallites obvious (>2mm), columella present: Genus Astreopora
Axial corallites on branch ends: Genus Acropora
Indo-Pacific genera: ----------------------------------------------------------------------------------------------------------

Anacropora (Gk. an-, without; akron, extremity; L. porus, pore):

Corallites structures are Montipora-like, the colony shape however is Acropora-like. Acropora may have evolved from an Anacropora-like ancestor. Because it now occurs in non-reef environments, distribution records are generally unreliable.
Delicate coral forms, that form small branched colonies which seldom reach 10cm in height. The branches may be clustered or more widely spreading, sometimes fused together. Calices are rounded and between 0.5-1mm in diameter. There are usually 6 main septa and additional secondary ones. Some septa are fully developed. There is no axial corallite present. The coenosteum between calices is porous and covered with small tubercles.
PRESENT DISTRIBUTION: western Indian Ocean to western Pacific, occur mostly in turbid waters.
GENERAL ABUNDANCE: uncommon, mostly non-reefal.
FOSSIL RECORD: none.
NUMBER OF EXTANT SPECIES: 7 known species.
Astreopora (Gk. aster, star; L. porus, pore): Colonies form dome-shaped to rounded heads or encrusting shapes; sometimes slightly protuberant, i.e. with leafy extensions; much larger than species of the genus Montipora.The colonies range from yellow, brown, green, pink, blue, and purple, but usually witish-blue. Corallites are distinct and separate; they are round in cross-section and range from 1-4mm in diameter. The skeleton is porous, with the coenosteum having a reticulate appearance. The tiny spines produce the rough appearance of the living coral. Septal development is extremely poor, giving this genus a cavernous appearance. Budding is extratentacular. Generally uncommon, but occur on reef flats and on reef slopes. Endolithic activity by polychaet- and Lithophaga species.
PRESENT DISTRIBUTION: Red Sea and western Indian Ocean to southern Pacific.
GENERAL ABUNDANCE: generally common, conspicuous.
FOSSIL RECORD: Cretaceous of the Tethys, Eocene of the Caribbean and Tethys. Fossil Astreopora are readily confused with Turbinaria; this has confused the fossil record.
NUMBER OF EXTANT SPECIES: 13 known species.
Montipora (L. mons, mountain; porus, pore): The often-highlighted taxonomic difficulties of Montipora stem from the number of species, and the presence of several species complexes. Otherwise, the genus does not deserve its taxonomic reputation; it has a wide array of useful, if not always conservative, skeletal characters. Many more species of Montipora, especially ramose species, occur in the Philippines than in PNG, Japan or Australia. Many of these are poorly known.
Members in this genus are usually thin corals that form leafy, encrusting, plate-like (ocasionally branching), or semi-massive colonies with numerous intermediates. Corals may be blue, brown, yellow, green, pink, purple, or red. Polyps are very small and appear empty, the septa consist of vertical rows of inwardly projecting spines. Calices are about 0.5mm (sometimes <2mm) in diameter and are usually well separated by the coenosteum (unlike Porites which has closely packed corallites). Walls are indistinct, septa are small or rudimentary - if present, 2 cycles are visible. Columella is rarely developed; skeleton is highly porous. The corallites are very small and well separated by coenosteum. Species that lack coenosteal surface projections are termed glabrous; some species have tiny smooth surface projections such as tuberculae (larger than the corallite), papillae (smaller that the corallite), or even verrucae-, or hydnophroid-like - useful in species identification (Veron 1986). Endolithic activity due to Lithophaga, Gastrochaena, and cirripeds. The group is present in all reef habitats, from the high-energy upper-reef slopes to deeper calm lagoonal habitats, therefore displaying a wide variaty in morphotypes (for morphological characteristics see table).
PRESENT DISTRIBUTION: Red Sea and western Indian Ocean to southern Pacific.
GENERAL ABUNDANCE: Extremely common, some species inconspicuous.
FOSSIL RECORD: Eocene (?) of the Pacific, Oligocene of the Tethys.
NUMBER OF EXTANT SPECIES: 75 known species.
Circum-tropical genera: -----------------------------------------------------------------------------------------------------

Acropora (Gk. akron, extremity; L. porus, pore):

This genus reigns supreme throughout the Indo-Pacific in almost all low-nutrient, high-energy reef environments and probably has done so since the Miocene. This overwhelming evolutionary success appears to be primarily based on three morphological characteristics: small corallites, allowing for fine detail in skeletal development; division of the roles of axial and radial corallites, allowing for highly deterministic growth forms; and porous skeletal microstructure, allowing maximum strength for weight. When combined, these characteristics produce morphologies of opportunity: they allow very rapid growth, very determinate growth, a high degree of colony integration and rapid local dispersion through fragmentation. With these characteristics, Indo-Pacific Acropora species are able to produce a much wider range of architectures, exploit a wider range of habitats and grow more rapidly than species of any other genus. Perhaps the greatest evolutionary achievements of coral are the 'plate'-and 'table' -forming species (A.palmata of the Caribbean, and the hyacinthus, divaricata and loripes species groups of the Indo-Pacific). These very highly integrated architectures are particularly suited to rapid exploitation of a wide range of environments, including high-energy ones: growth rate, substrate coverage, exposure to sunlight and the sieving of plankton are all maximised, while the quantity of skeletal material required to do so is minimal. These characteristics would be particularly advantageous during times of climatic change, when sea levels, wave turbulence, surface circulations and inorganic nutrients are all in states of flux. The paucity of Acropora in the Caribbean is in conformity with other coral genera, that is, owing to a lack of speciation. Whether this paucity also applied to the Tethys cannot be determined from the fossil record, although there is no suggestion that Tethyan Acropora were ever highly diverse or particularly abundant.
Intra-specific latitudinal change reaches an extreme in Acropora with the majority of species forming taxonomically meaningful geographic subspecies in most high-latitude regions. Thus, there is a greater degree of intra-specific similarity between the Acropora of the tropical northwestern shelf reefs of Australia and the GBR than there is between these regions and their southern neighbours. A similar situation occurs in Japan, where common species in most high-latitude localities are taxonomically separable from each other as well as from their tropical counterparts. Several apparently temporary populations of Acropora have been recorded in remote places: these include A.yongei at Rottnest Island near Perth, western Australia three species in Hawaii and A.valida in the far eastern Pacific. Relationships among species of Acropora cannot reliably be determined from morphological criteria, thus the genus has only one, small, subgenus (Isopora). To aid identification, some authors (Veron and Wallace) divided the genus into fifteen sub-generic groupings; these have no taxonomic status and the relative positions of the groups are somewhat arbitrary (see figure below).

Hypothetical representation of the taxonomic affinities of the sub-generic groupings of Acropora species. Affinities within groups indicated by a thick line are relatively clear, those indicated by a broken line are relatively unclear. Except for the sub-genus Isopora, these groups are not sufficiently well-defined to justify establishing sub-genera. The diameter of the circles is proportional to the number of species.
 
Thus, general features of this genus embrance abundance and high morphologica ldiversity: may be massive, encrusting, branching (staghorns), plate-like clusters, and tables. The branches of staghorns are usually over 10cm in length and may be 1.5cm or more in width. Corallites are of two types: at the branch tip is a cylindrical and usually larger axial corallite (bi-symmetrical axial corallite) whereas the lateral smaller corallites are mono-symmetrical (if not asymmetrical). Lobed or semi-massive colonies have scattered axial corallites. Corallites are round in cross-section and protrude several mm from the surface. The coenosteum is usually pitted or porous (sponge-like) and covered with tiny blunt spines. There are 12 fully developed septa. Endolithic activity predominantly due to Lithophaga species.
PRESENT DISTRIBUTION: Cosmopolitan.
GENERAL ABUNDANCE: Extremely common, very conspicuous, usually dominant in Indo-Pacific reefs.
FOSSIL RECORD: Eocene of the Caribbean, Pacific and Tethys.
NUMBER OF EXTANT SPECIES: 182 known species.
Seven distinct colony morphologies are recognized, namely
  • tabulate (e.g. Acropora hyacinthus, A.clathrata, A.cythera),
  • corymbose (e.g. A.nasuta, A latistella),
  • caespitose (e.g. A.horrida, A divaricata, A.valida),
  • hispidose (e.g. A.echinata, A.carduus, A.elseyi),
  • aborescent (e.g. A.formosa, A.pulchra, A.abrolhosensis),
  • massive (e.g. A.cuneata, A.robusta), and
  • digitate (e.g. A.bushyensis, A.gemmifera, A.digitifera).
Aiding in the identification of Acropora species, the group was divided into 15 non-taxonomic divisions (for morphological characteristics see table).