PART-III Cnidaria (Gk. cnidos, stinging nettle) reproduction and growth of Scleractinia:

Scleractinian corals belong to the phylum Cnidaria. They form the basis of many tropical reefs ecosystems, but are also abundant in colder waters. There are four classes: Hydrozoa (hydroids), Scyphozoa (jellyfishes), Cubozoa (sea wasps), and Anthozoa (Scleractinian corals, corallimorpharians, sea fans, sea anemones, zoanthids and black corals), distinguished on the basis of life history and morphology. They are united by certain characteristics: radial symmetry, a central mouth surrounded by tentacles, a single opening through which food is ingested and expelled (coelenteron), a jelly-like middle germ layer (the mesoglea), and intracellular stinging structures called nematocysts. Members of the remaining class, Anthozoa, exist only as polyps, either solitary or forming colonies (for a more detailed insight on cnidarian taxonomy, try this link).



Graphic key to higher cnidarian taxa (200kB)

Knowledge of scleractinian coral reproduction has expanded greatly over the past 10 years into one of most intensely studied aspects of coral biology. Review by Richmond and Hunter (1990) provides overview of status of knowledge. Reproductive data are now available for about 210 of approx. 600 spp. of reef corals. What is most impressive is the variety and versatility of coral reproduction (according to region, varies even within same species) and can be both sexual and asexual (Veron, 2000). The individual coral polyp can be male, female, both or may not be reproductively active at all. If a polyp is just of one sex then it is termed gonochoric. A polyp that is both male and female is known as a hermaphrodite. Egg and sperm production can occur on the same mesentery or on differentiated mesenteries in same polyp, in different polyps of same colony, or at different times in same colony (i.e. sequential as well as simultaneous hermaphroditism).

Sexual Reproduction: corals are immobile organisms with separate sexes (25% of all known species; Veron, 2000). They rely on precise timing in order to bring their gametes together. Species which spawn must release their gametes into the water simultaneously. This is done in response to environmental cues, sexual reproduction offers two opportunities for new genetic combinations to occur:
     a) crossing over during meiosis, and
     b) the genetic contribution of two different parents when an egg is fertilized by a sperm.
To prevent self-fertilization, male and female gametes in hermaphroditic corals never mature at the same time.

Broadcasters (spawners): in hermatypic corals, spawners outnumber brooders; about 75% of all known coral species spawn positively buoyant gametes (eggs and sperm) at very specific times so as to ensure fertilization (Veron, 2000). Broadcasting is typical of corals in buttress and forereef zones of massive colonial forms with indeterminate growth; broadcasters have high larval mortality but successful recruits can invade new environments with lower competition with surviving colonies that can live 100s of years. Fertilization is external at the water surface. Many coral species mass spawn; i.e. within a 24 hour period, all the corals from one species and often within a genus release their eggs and sperm at the same time; e.g. Montastraea, Montipora, Platygra, Galaxea, Favia, and Favites (Wallace, 1994). Intraspecies fertilization is common but mass spawning raises the possibility of hybridization by congeneric species (Wallace, 1994 - breaking with the dogma of species as a biological unit). The zygote develops into a larvae (called planulae) which attaches itself to a suitable substrate, metamorphoses to a founder polyp, and grows into a new colony.
Brooders: most ahermatypic corals are brooders as are hermatypes living in disturbed, nearshore reef zones. Likewise, species in zones with high adult mortality, have high competition for spaces, and are exposed to intense bioerosion, require high rates of recruitment. In this strategy, sperm, but never eggs, are released into the water. Brooding produces mature, often negatively buoyant planulae ready to settle; (e.g. Favia fragum broods embryos for 3 weeks; brooding also found in Goniopora, Diaseris, and Agaricia). Asexually brooded planulae larvae may be developed by a kind of budding; i.e. internal fertilization, brooding of zygote, and release of a positively buoyant planulae. The larvae float to the top, sink back, settle, and metamorphose into a founder polyp, to become later on another colony. Species of Acropora release brooded larvae (vivipary).


Life cycle of stony corals (50kB)


Life cycle of of a broadcaster (130kB)

Coral spawning: the long-term control of spawning (gonad maturation) appears to be temperature variances; the short-term control is lunar; i.e. rhythmicity in moon phase luminance (29.5 day periodicity) appears to be the most important environmental monthly spawning synchronizer. Brooding species can store unfertilized ova for weeks and thus require less synchrony for fertilization; and still they participate in mass spawning to increase the survival of their offspring.
Corals start their life as a free-swimming young (after spawning, the planulae larvae is only the size of the head of a pin) that are carried by ocean currents. Planula larvae are ciliated and up to 1.6mm long. Most already contain zooxanthellae when released from the parental polyp and can survive in plankton for up to 100 days. The larvae will drift with the current until it finds a hard bottom to attach itself. Helix experiment (Sammarco & Andrews, 1988) showed extensive degree of dispersal from an isolated reef in GBR. Once the larvae attaches to the bottom it quickly changes into a polyp (will never move again). It reproduces asexually by budding (in which an identical polyp sprouts out of the polyp's side).

Asexual reproduction: Through asexual reproduction, a coral can make a clone of itself. In this way, coral colonies are able to live for a few hundred years. Asexual reproduction is thus the main cause of colony growth.

Polyp bailout: a polyp abandons the parental colony and re-establishes on a new substratum; i.e. by becomming a solitary coral that further clones itself to an adult colony.
Budding: the coral colony expands in size by budding. In the process, a young corallite grows out from the adult (parent) polyp. The progeny polyp has two cell layers: ectodermis and gastrodermis. As the progeny polyp grows, it produces a coelenteron, tentacles and a mouth.
The distance between the polyps increases which causes development of coenosarc (the common body of the colony). New polyps grow on the coenosarc by producing new coenosteum, the colony's exoskeleton. Budding may be:


Intra- (left) vs. extra (right) tentacular budding (13kB)
  • Longitudinal division: in longitudinal division, the coral polyp begins to broaden; it then divides into a coelenteron and mesenteries; next, the mouth divides and tentacles encircle the new mouth. The difference from the above is that during budding, the parent polyp produces a smaller polyp, whereas after division, the two polyps are identical. Individual polyps divide according to the radial arrangement of septa. Every new part has to complete its missing parts of the body and exoskeleton.

  • Intra-tentacular budding, the new bud forms from the oral discs of the old polyp; both polyps (old and new one have the same size and are surrounded by the parental tentacular ring); typical of meandering corals such as Diploria, Platygyra, etc.
    Extra-tentacular budding in which the new polyp forms from the base of the old polyp (juvenile progeny polyps are smaller than parental polyps); e.g. as in Montastraea cavernosa.
  • Transversal division: individual corals are able to reproduce by transversal division. Polyps and the exoskeleton divides transversally into two parts. One of them has the basal disc. The second has the oral disc. The two new polyps must complete missing parts of the body and exoskeleton in order to function.

  • Fission: some corals (esp. mushroom corals among the family Fungiidae) are able to split into two or more colonies during the early stages of their development (also called strobilization).

Fragmentation: vegetative reproduction of coral colonies involves broken up individuals during storms. They can initiate many new colonies; it is quite common in branching forms and for species with limited distribution where conditions might not favor sexual reproduction or in stressful habitats without optimal regions. A piece of colony can actually be broken off to grow as a clone.

    Colonies can grow in the following manner:
  1. by producing stolons: stolons grow horizontally by attaching cell layers. They look like a system of roots that fix the whole colony to substrate. New polyps grow on stolons;
  2. by monopodial growth: this term means that the trunk of the colony is made by the oldest polyp. The trunk grows during growth of new polyps. The oldest polyp is always on top of the colony;
  3. by sympodial growth: this colony does not produce a trunk. New polyps offshoot along the edges of adult polyps. The youngest polyps are always on top of the colony;
  4. by dychotomic growth: it means that the corals divide symmetrically. Since all polyps grow simultaneously, neighboring polyps are of the same age.

Morphology of a scleractinian corallite: After successful settlement of the planulae on a suitable hard substrate, the formation of a basal disk (precipitate of aragonite-CaCO3) takes place; due to the septal structure, the cavity (dissepiments and theca) is formed (septa are of ektodermal origin and are embedded in-between the mesenteries and are the Ca-precipitating organs). As soon as the basal disk is formed, the young coral begins with the structure of the side panels - the cup (calix, calice). A first radial partition (disseptiments), give the growing structure extra stability. As the young coral keeps growing, the calyx gains in height requiring an other set of dissepiments. In that process, the organic material that generated the first partition will die. In fact, only the very thin sheath of living tissue on top of the Ca-precipitate (coenosarc) is the responsible agent for the entire process of coral formation. In successive steps, more neighbouring corallites are formed while the space in-between adjacent corallites is loosely filled with exothecal dissepiments (coenosteum). Both corallite morphology and the coenosteum among them is one of the main criteria to assign species names to coral colonies.

    Structural elements of a corallite:
  • corallite: skeleton of a solitary individual or an individual within a colony;
  • calice: a cup-shaped depression on the corallite surface;
  • coenosteum (-a) [or peritheca (-ae)]: skeleton between corallites within a colony;
  • septum (-a): radially-arranged vertical partitions within a corallite; they can be either exsert, insert or even in regard to the corallite wall;
  • paliform lobe: an exsert protuberance of a septum at the center of the corallite;
  • wall [or theca (-ae)]: vertical structure enclosing a corallite (=wall);
  • theca: the sheath of "dura mater" which encloses a corallite;
  • costa (-ae): extension of a septum beyond the wall; a rib or riblike structure;
  • columella (-ae): central axial structure within a corallite; if present, it can be formed either as a
    solid columella: a central rod;
    spongy columella: formed by the inner ends of septa;
    papillose columella: many small rods; or
    lamellar columella: plate-like;
  • dissepiment: horizontal partition (flat or curved) within or outside of a corallite;
  • synapticulum (-ae): a conical or cylindrical supporting process, as those extending between septa in some corals;
  • coensarc: the living axial part of a coral colony (= peritheca);
  • peritheca: the living tissue surrounding or between corallites (= coenosarc);
  • coenenchyme (=coenosarc) the mesogloea surrounding and uniting the polyps in compound anthozoans;
  • mesentery: a fold of the peritoneum that connects the intestine with the posterior abdominal wall.


Corallite development (235kB)


Corallite morphology (265kB)

Morphology of stony coral colonies: If corallites of a colony have their own walls, they are termed plocoid (polyps with separate walls) or phaceloid (very prominent individual polyp). If they share a common wall, then they are called meandroid (valley forming polyp) or ceroid (adjacent polyps share their walls). Furthermore, corals are named flabello-meandroid (elongate polyps with separate walls) if they form valleys which do not have common walls.

    Corallite arrangement: corallites can be arranged in certain patterns, thus aiding in their classification:
  • plocoid: short stalked and isolated corallites (separated by coenosteum);
  • subplocoid: corallites sometimes separated by coenosteum - each corallite has its own wall;
  • cerioid: corallites juxtaposed, and are even while each corallite retains its own wall; massive corals that have corallites sharing common walls;
  • meandroid: corallites arranged in multiple series (due to intra-tentac. Budding); massive corals with coral mouths aligned in valleys separated by ridge; adjacent valleys share the same ridge;
  • flabelloid: corallites arranged in single series; corallites in long meandering rows or valleys that share a common base, however the walls (or ridges) of adjacent valleys are not connected;
  • flabello-meandroid: corallites in long meandering rows with common base; walls may be partially fused. This condition is also referred to as flabellate;
  • phaceloid: corallites separated by void space; corals that have corallites with distinct walls separated by coenosteum;
  • solitary: corallum formed by only one individual (single corallite);
  • dendroid: the corallites branch from each other in a dendritic pattern; derives from extra-tentacular budding (extinct);
  • thamnasterioid: the septa of adjacent corallites are confluent and often twisted or sinuous in form; plating coral with no walls surrounding corallites;
  • hydnophoroid: coral with cone-shaped protuberances between corallites;
  • fasciculate: the corallites are cylindrical but not in contact. It may be dendroid (with irregular branches) or phaceloid (with more or less subparallel corallites with connecting processes).


Corallite arrangement (65kB)


The septa of Pavona explanulata (Agariciidae) extend beyond the wall of one corallite to the next, giving it a thamnasteroid-like appearance (155kB)
Main observable morphological characteristics of coral colonies:
Branched: the fingerlike appearance with a typical bifurcating pattern; this pattern can be
    arborescent- colonies typically composed of tree-like branches;
    bottlebrush - colonies have small branchlets coming out from the sides of the main branch;
    caespitose - colonies are "bushy", consisting of possibly fused branches inclined at various angles;
    corymbose - colonies are composed of horizontal (possibly fused) branches, with short vertical branchlets;
    digitate - Colonies are composed of short, non-dividing branches, similar to fingers;
  • Needle coral: fragile, very slender needles are a common feature of this group (Seriatopora sp.);
  • Staghorn/Finger coral: Fingerthick, digitating branches characterize these genera (Paulastrea sp., Pocillopora sp., Stylophora sp., Montipora sp., Anacropora sp., Acropora sp., even some Porites sp., Hydnophora sp., Cyphastrea sp., Echinopora sp., Tubastrea sp.);
  • Elkhorn coral: asymmetrically branching corals with one side considerably wider than the other (Pocillopora sp., Psammocora sp.);
  • Knobby coral: bluntly shaped but very thick projections that aggregate densly (Porites sp., Goniopora sp., Alveopora sp.) or irregular shaped hillhocks or ridges (Pavona sp., Favia sp.); corals of the genus Blastomussa reveal phaceloid (very prominent individual polyp) colony structure;
  • Hibiscus coral: uprightly branched corals with elongate valleys and walls (Pectinia sp.);
Boulder: massive corals that do not develop fingerlike projections, but reveal a compact appearance; i.e. mounding, mound-shaped or encrusting colony; similar in all dimensions;
  • Brain coral: dome-shaped corals with meandroid colonies (Symphyllia sp., Goniastrea sp., Platygyra sp., Oulophyllia sp., Physogyra sp.)
  • Honeycomb coral: hemispherical coral with polygonal corallites up to 3-6mm in diameter (Pseudosiderastrea sp., Favia sp., Goniastrea sp., Montastrea sp., Diploastrea sp.);
  • Golfball coral: Hemispherical coral colonies with evenly spaced corallites (Astreopora sp., Favia sp., Favites sp, Goniastrea sp., Leptastrea sp., Cyphastrea sp.);
  • Lunar coral: Large (often several meters in diameter), spherical colonies with very small corallites (<2mm in diameter); often perforated with Christmas tree worms (Spirobranchus sp.) such as in Porites;
  • Bouquet coral: Dome-shaped corals with phaceloid (very prominent) polyps (Lobophyllia sp., Caulastrea sp., Euphyllia sp.);
Plate: Laminar corals that spread out mainly in fanlike plates; polyps may be present unilaterally or at both sides.
  • Table coral: colonies are flat (spread horizontally) and attached either with a central foot or on one side to the substrate; (Acropora sp.);
  • Basket coral: Dome-shaped, solitary, free-living corals (Cycloseris sp., Fungia sp.);
  • Mushroom coral: Flat, solitary, free-living corals (Diaseris sp., Fungia sp.);
  • Slipper coral: Elongate, solitary, and free-living coral (Fungia sp., Herpolitha sp.);
  • Sheet (Vase/Cabbage/Leaf) coral: leaf-like, thin, folded plates or spires extending upward by developing flat (vertical or horizontal) plates or whorls; calices on only one (monofacial) or both side (bifacial);
  • Plates generally are horizontal, upward facing side with polyps. Whorls tend to be "double-sided" with polyps and are vertical; flat plated corals; plates sometimes arranged in whirls with unifacial polyps (present only on one side - Montipora sp., Coscinarea sp., Leptoseris sp., Turbinaria sp.) or bifacial as among Astreopora sp., Pavona sp., Pachyseris sp., Echinophyllia sp., Oxypora sp., Merulina sp., Echinopora sp., Turbinaria sp.;
  • Encrusting coral: colony adheres to the substrate and grow over hard substrate by developing a thick covering (Montipora sp. Leptoseris sp., Podabacia sp. Galaxea sp. Echinophyllia sp., Oxypora sp., Acanthastrea sp., Hydnophora sp., Turbinaria sp.);


Colony morphology arrangement (95kB)


Top: various growth forms; bottom: Acropora sp. (prob. A.millepora - Acroporidae 190kB)


Top: various growth forms; center: Montipora monasteriata with Acropora montipora (Acroporidae); bottom: Seriatopora hysterix (Pocilloporidae - 280kB)


Top: scleractinians among soft corals; center: Porites sp. (Poritidae) bottom: Pectinia lactuca (Pectiniidae - 230kB)

Physiology of scleractinian corals - Coral/Algae Symbiosis: Coral reefs are the result of a most intricate and subtle relationship between the coral polyp and the minute single-celled algae which live symbiotically within the cells of the polyp. The colouring of the coral colony is a result of the enclosed zooxanthellae (Gymnodium = Symbiodinium protists) from the group of the dinoflagellates, e.g. S.microadriaticum. These algae belong to a group of unicellular brown plants known as dinoflagellates. Like land plants, the zooxanthellae are able to use the process of photosynthesis to capture the sun's energy and use it to make their own organic food from CO2, inorganic nutrients, and water. Doing so, these algae fulfil life-supporting function and aid indirectly in the calcium-carbonate precipitation. Thus, it is essential that scleractinian corals be exposed to light in order to guarantee their long-term survival.
Symbiosis is a close association of two species. Both species live closely together. In the case of corals the algae lives inside the coral host (endosymbiosis - endosymbionts). In certain aspects, it is even a mutualistic bond as both species derive benefit from this association). These zooxanthellae are minute, spherical unicellular algae, are already acquired during the planulae stage and proliferate as the coral colony grows (if algae are isolated from host and cultured on their own, they transform into typical biflagellate, motile dinos). Algae can be expelled from host under stressful conditions such as seen in recent coral bleaching event. Likewise, are the corals able to re-incorporate them back into their tissue. Since algae also reside in the gastrodermis of the coral host, incorporation or "infected" of these algae is still somewhat unclear but probably occurs through ingestion. Whether more species of algae acts as the symbiont to all corals (as well as other hosts) or a single species is not clear, although there is evidence of different genetic strains in different hosts.
Reef-building corals precipitate up to 6 tons CaCO3/(km2/day). By night, a polyp captures plankton with its tentacles. By day, the zooxanthellae photosynthesize. The polyp benefits from the photosynthate (product of photosynthesis), while the alga benefits from the nitrogenous wastes of the polyp.
All reef-building corals are zooxanthellate, but not all zooxanthellate corals are reef builders. Non-zooxanthellate corals are not reef builders (ahermatypic). Other marine invertebrates that harbor zooxanthellae include Tridacna (giant clam), nudibranchs, Cassiopeia jellyfish, sponges, anemones, etc.

However, zooxanthellae aid in skeletal excretion by removing phosphorus as a metabolic waste product from coral as dissolved phosphate may inhibit calcification, this may benefit growth of coral skeleton. Being autotrophic, zooxanthellae also remove respirative CO2 from the coral host, thus enhancing coral calcification.

     1. Photosynthesis reaction: 6CO2 + 6H2O ® C6H12O6 + 6O2
     2. Hydrocarbonate reaction: CO2 + H2O ® H2CO3 ® H+ + HCO3-
     3. Calcification reaction: Ca++ + 2HCO3- ® Ca(HCO3)2 ® CaCO3 + H2O + CO2
     4. Removal of CO2 through photosynthesis will enhance calcium carbonate precipitation.

Nearly 90% of the carbon fixed by zooxanthellae is released to the coral host primarily as glycerol, glucose, and alanine. Nitrogen and phosphorous derived from captured plankton are shared between symbiont and host (Gladfelter, 1985).

Symbiodinium microadriaticum (70kB)


Precipitation reaction (according to Schumacher - 60kB)