Sexual reproduction in corals may occur in two ways: some corals are brooders, with fertilization and embryonic development occurring internally, in their coelenteric cavity; others are broadcasters, releasing their gametes synchronously into the water wliere fertilization and embryonic development occur (Atkinson and Atkinson, 1992). Depending on the species and its location, the reproductive period may be seasonal, monthly, or continuous (Atkinson and Atkinson, 1992). Synchronization of spawning is critical to the success of broadcast spawners since the tides rapidly wash their gametes away. On the Great Barrier Reef of Australia, a mass spawning event occurs once per year, after sunset about five days after the full moon in late spring (Veron, 1986). During this time it is estimated that 90% of the stony corals on the entire reef release their gametes!
In other areas of the Pacific, and in the Caribbean and Red Sea, corals spawn over a wider range of months, weeks, or days. There are numerous cues that can trigger coral spawnings and maintain synchronicity. Temperature, photoperiod, and nocturnal illumination all appear to be important. However, even within a species there can be a great deal of geographic variation in the spawning season (Richmond and Hunter, 1990).
AM Sammarco 1986
Growth series of a Fa via fragum that arose from a planula larva in the aquarium and settled on the glass. October, 1992; December, 1992; and February, 1993. J. Sprung.
The annual temperature range is thought to be directly correlated to the degree of multispecies synchronicity, such that areas that have the greatest annual temperature range exhibit the highest percentage of synchronicity (Richmond and Hunter, 1990). The affect on coral fecundity of the loss of these temperature cues in a closed system aquarium has not been explored.
While temperaaire is a seasonal cue, nocturnal illumination (lunar phases) can stimulate corals to spawn on a particular night (Richmond and Hunter, 1990). A coral species' spawning response differs according to the location. For example, Stylopbora pistillata colonies in Palau planulate according to a distinct lunar cycle, while Red Sea specimens do not (Richmond and Hunter, 1990).
Most corals (68% of those studied) are hermaphrodites, having both sexes in one individual (Richmond and Hunter, 1990). Some corals, however, have separate sexes (gonochorism). In hermaphroditic species, the eggs and sperm may be released separately, or together as an egg-sperm bundle, that breaks up rapidly after release before the eggs are ready for fertilization (Veron, 1986).
Hermaphrodites can also be brooders. Sperm enter the gastric cavity and fertilize the eggs within. These develop into planulae that are released when mature. Brooding appears to be the dominant form of sexual reproduction in Caribbean corals, while broadcast spawning seems to be more common in Pacific and Red Sea corals (Richmond and Hunter, 1990). Self-fertilization is suspected to occur, but has only been shown in Acropora tenuis (Richmond and Hunter, 1990). Barriers to self-fertilization appear to break down some time after spawning for some species (Richmond and Hunter, 1990).
The typical method for ahermatypic corals is internal fertilization and brooding of larvae (Veron, 1986). Those corals that brood their larvae do not need to synchronize spawns since the eggs are always retained in the polyps; only sperm are released (Veron, 1986). As a result, these corals can spawn year-round.
The larvae of corals are called planulae, and they may be produced asexually as well (see Ay re and Resing, 1986). This has been documented in at least one species, Pocillopora damicornis, but may be more widespread since it effectively maximizes production of larvae (Veron, 1986). Pocillopora damicornis, which is hermaphroditic and also able to produce planulae sexually, has been propagated for years by European aquarists. It has reproduced by means of asexually-produced planula larvae and polyp "bail-out" in many aquaria (Stiiber, 1990; J. Sprung, pers. exp.; S. Tyree, pers. comm.).
While sexual reproduction offers the species-preserving advantages of genetic variability and long distance dispersal (Veron, 1986), it is the least common means of duplication that corals employ. Far more regular is asexual reproduction, which comprises numerous strategies, including the production of planulae, as mentioned earlier. Sexual reproduction can be the primary means of recruitment for some coral populations, with asexual reproduction becoming the dominant or sole form of reproduction at the ecological limits of the coral's distribution (Richmond and Hunter, 1990). Temporal and regional variations in biotic and abiotic factors can cause differences in reproductive and recruitment patterns in coral communities (Richmond and Hunter, 1990).
Asexual reproduction strategies in corals include asexually produced brooded planulae, the formation of polyp "balls" or
Pocillopora damicornis from a planula larva spawned in the aquarium and settled on the glass.
Budding of polyps along the growing edge of the stony coral, Duncanopsammia axifuga. J. Sprung.
Polyps budding along the growing edge of the related coral Turbinaria. J. Yaiullo and F. Greco.
Acropora elseyi in an outdoor aquarium in June 1992. B. Carlson.
Same colony 7 months later. Growth rate was 13.2 cm (5 in.) per year, per branch. B. Carlson.
Acropora sp. fragment from D. Stuber in J. Sprung's aquarium, November 1992. J. Sprung.
January, 1993. J. Sprung
February, 1993. The clam had to be moved because of its own increase in size and the encroaching branches. J. Sprung
July, 1993. J. Sprung
• ... # •
t 1 Irr
ft m mm.
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February, 1994. The growth of Stuber's Acropora and other Acroporaspp. has injured, shaded, and nearly killed the adjacent colony of A. cervicornis that had previously grown well in this aquarium for three years. The front view of this aquarium has been obstructed as the encrusting growth is difficult to remove. J. Sprung.
Goniopora stokesi. M. Awai.
Same colony with polyps retracted to show daughter colonies. M. Awai.
Field of Goniopora stokesi on the reef flat in Palau, depth 1 m (3 ft.). M. Awai.
Satellite colony on a four year old aquarium specimen of Goniopora stokesi. J. Sprung.
Daughter colony after detaching from mother colony in previous photo. J. Sprung.
A newly introduced Platygyra in June, 1991. J.C. Delbeek.
The same coral in April, 1992. J.C. Delbeek.
A polyp "ball" forming in March 1993. J.C. Delbeek.
The resulting free-living polyp, complete with skeleton. J.C. Delbeek.
"satellites", polyp "bail-out", fission and fragmentation, outgrowths, and various means of "budding".
The process of growth for corals is a means of asexual duplication of polyps. Colonies of polyps that comprise a coral head are all clones from an original polyp that have arisen by budding. Such budding may occur by division of polyps, or by formation of new polyps between others. The distinction of some species of corals is based on the means of polyp duplication. Some corals are really composed of one large polyp that forms additional mouths as it grows.
The coral head need not have been produced by the settlement of an original polyp. It may, instead, have arisen from a fragment containing a few living polyps, broken off of a larger head. Such asexual reproduction is called fragmentation, and while it may be an accidental means of reproduction for some corals, it may be the most important means of dispersal on the reef for some species, Acropora being a primary example. Fragmentation increases recniitment in locations where sexual reproduction is also common, and it is an especially important means of forming new colonies during periods of environmental stress, when larval recniitment is low (Richmond and Hunter, 1990). In another example, the term fission refers to non-accidental fragmentation, as in the solitary coral Diaserisfragilis, which is closely related to the many Fungia species. It always develops weak spots in its round skeleton that cause it to fragment into several daughter colonies (Veron, 1986).
The formation of so-called polyp "balls" or "satellite colonies" among the polyps of the "mother" colony is another form of asexual reproduction that seems to combine clonal duplication of polyps and fragmentation. When the satellite colony of, for instance, Goniopora stoke si is heavy enough, its weight causes the tissue connecting it to the parent colony to tear, freeing the new colony which drops down next to its parent. This means of reproduction can result in vast monospecific fields of Goniopora.
A similar technique employed by faviid corals is the formation of a single polyp with a bit of skeletal material which, when heavy enough, separates from the parent colony and settles on the adjacent substrate.
Furthermore, large single-polyped corals such as Trachyphyllia, Euphyllia and Catalapbyllia, may produce unattached septae that may
Asexual reproduction in Catalaphyllia (see text for a complete description). J. Sprung.
Anthocauli production on Fungiasp. J. Sprung.
Anthocauli production on Herpolithasp. A.J. Nilsen.
drag a bit of tissue with them as they separate from the parent by their weight, forming a new colony (de Greef, 1990; J. Sprung pers. obs.).
Environmental stress plays a role in the stimulation of other means of asexual reproduction. In the case of polyp "bail-out" water temperature, oxygen concentration, pollution, or other environmental stimuli cause a polyp to separate from its skeleton and drift free (Sammarco, 1982). This affords great opportunity for dispersal away from the site of stress, without formation of planula larvae.
In aquaria, polyp separation from the skeleton can occur slowly as a result of numerous circumstances (see trouble-shooting section, chapter 10). This polyp separation is quite distinct from the bail-out method that occurs in nature. It is much slower, and is more akin to slow death than a quick escape. Such separation can lie healed if caught in time, and it is also possible for a separated polyp to form a new skeleton. Portions separated may form new skeleton and drop off as a new colony as in the Catalaphyllia pictured.
The other environmentally induced means of asexual reproduction is the formation of anthocauli in fungiid corals. The environmental stressor is typically an injury7 to the original polyp, such as burial, stinging by an adjacent anemone or coral, or predation by fish or invertebrates. The area of tissue loss soon produces tiny individual polyps called anthocauli. These form a skeleton of their own as i hey grow, and eventually they separate from the original, injured fungiid, forming a complete new coral. The small point of attachment, once broken, forms a new anthocaulus on the original Fungia, so the reproduction is perpetual. Hobbyists have used such
Small polyps forming at the growing edge of a specimen of Euphyllia divisa at the The New York Aquarium for Wildlife Conservation. J. Sprung.
A well-stocked reef aquarium dominated by soft corals requires frequent pruning. Terpenoids also accumulate unless removed by chemical filtration. J.C. Delbeek..
injured, reproductive Fungi a and new colonies for sale and trade.
specimens to propagate
Finally, Euphyllia, Catalapbyllia and Tracbypbyllia species may also produce new polyps like anthocauli between the septae when injured, and Euphyllia spp. regularly bud new polyps along the wralls pa
of the skeleton when they are healthy. These new polyps break off at the constricted point of attachment when they are large enough, or fuse with the main colony to form a new branch.
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