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.
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.
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.
Intra- (left) vs. extra (right) tentacular budding (13kB)
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.
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.
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.
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 (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
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;
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.
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).
Precipitation reaction (according to Schumacher - 60kB)