Animal Diversity Web

Geographic Range

The genus Acropora, in which Acropora millepora belongs, dominates the coral reefs of the Indian and western Pacific oceans. This particular species is known to occur throughout this region, in shallow tropical waters from South Africa north to the Red Sea, east through the tropical western Pacific (Hatta, 1999). (Hatta, 1999)

• Biogeographic Regions
• indian ocean
  • native
• pacific ocean
  • native

Habitat

Many reefs that have a high coral cover also have surprisingly turbid conditions, like fringing reefs around the inshore continental islands of the Great Barrier Reef Lagoon. This suggests that habitats may have turbid conditions without necessarily being detrimental to coral (Anthony, 1999).

A second issue that affects habitat is sedimentation. High sedimentation lowers coral diversity and allows the habitat to become dominated by sediment-resistant species. These reefs have slower colony growth rates, which results in reduced colony size and adaptations in the morphology of form as compared to reefs that experience lower levels of sedimentation. Sedimentation not only affects growth, but also metabolism and fecundity (Gilmour, 1999). One way in which sediment is a stress factor is that it reduces the amount of light that can penetrate to the coral for photosynthesis. Sediment also smothers coral tissues (Anthony, 1999).

Acropora millepora must have adequate light. This light is often regarded as the factor that limits maximum depth of coral growth. As depth changes, so does light intensity, spectral quality, and directional strength (The upper limits of growth are also increased with greater offshore distance of increased water clarity) (Mundy and Babcock, 1998). Studies of Acropora species show that light intensity may have an effect on settlement orientation. Planulae of Acropora millepora have shown an inclination to settle on upper surfaces rather than under surfaces (Dubinsky,1990). (Anthony, 1999; Dubinsky, 1990; Gilmour, 1999; Mundy and Babcock, 1998)

• Habitat Regions
• tropical
• saltwater or marine

• Aquatic Biomes
• benthic
• reef
• coastal

Physical Description

Acropora millepora is a hard coral. Starting from a single embryonic cell, it has been found to reach 5.1 mm in diameter during a period of 9.3 months (Dubinsky, 1990). This species grows mostly vertically, which leads to a bushy morphology that is semi-erect. Polyps extend from vertical branch tips on an average of 1.2 to 1.5 cm, and these polyps are nonreproductive. Laterally, though, most regions are reproductive (Hall, 1997). Polyps are on average about 1-2 mm in diameter (Anthony, 1999). Modules (in this case, polyps) that comprise a colony often show some degree of polymorphism (Hall, 1997). (Anthony, 1999; Dubinsky, 1990; Hall, 1997)

• Other Physical Features
• ectothermic
• heterothermic
• radial symmetry

Reproduction

Reef building corals, such as Acropora millepora, can reproduce sexually in an event called "mass spawning". This occurs once a year, around 3 nights in early summer when the moon is nearly full. Mass quantities of eggs and sperm are released simultaneously from the huge numbers of coral colonies, many belonging to different species and genera (Hatta, 1999). Colony size has no effect on the number of eggs or sperm per polyp, nor on the testes volume per polyp (Hall, 1996).

Acropora millepora eggs which have spawned have within them high levels of UV blocking agents. More than likely, this agent protects the eggs from UV radiation during the planktonic development stage (Dubinsky, 1990).

Within this hermaphroditic species, there is a striking difference in sex allocation. The ratio of total egg volume to total testes volume per polyp has a variability of 5 to 1. In every member of the genus Acropora, this ratio increases as colony size increases. In an attempt to explain this, it is now thought that an early investment that is mainly in the testes will allow sex to commence without having to spend the energy initially on egg production. Perhaps this allows colonies to grow larger and become safer before the expense of egg production is dealt with (Hall, 1996).

After the gametes are released into the water by adult coral, they must undergo 3 general stages of development before they may grow into newly settled coral. These stages are: 1) Fertilization and embryonic development; 2) Larval growth; 3)Settlement and metamorphosis. In each of these stages, the likelihood survival of each is low. This is due to both physical (wind, wave, salinity) and biological (predator abundance) factors (Gilmour, 1999).

One of the physical factors which affects these stages is suspended sediments. These sediments inhibit fertilization if their concentrations are high. However, they show no detectable effect on post-fertilization embryonic development (Gilmour, 1999).

Among settling and recently settled marine larvae, the mortality is very high. This suggests that this period of development is crucial in coral life. In the first 8 months of life, rates of mortality in juvenile Acropora millepora were as high as 86% (Dubinsky, 1990). In places where larval density was high, few larvae were able to survive exposure to the high and low sediment concentrations. However, where larval density was controlled, the larval survival stayed relatively stable. The sediment suspension and sediment layer were also linked to a significant decrease in larval settlement (not just larval survival) (Gilmour, 1999).

In almost all species of Acropora, individuals have a mandatory threshold size that they must attain before sexual reproduction will proceed. Once this size is met, reproductive output usually increases as a function of body size. The characteristic colony size at maturation usually corresponds to a minimum puberty age of 1-3 years (Hall, 1996). Like so many other sequential hermaphrodites, changes in sex often occur after a specific body size or age has been reached (Hall, 1996).

As mentioned before, many of the species and genera grow side by side and spawn simultaneously. Because of this, fertilization can occur between related but different species. This results in a significant number of hybrids. In a recent study, all of the hybrid embryos were active and developed into planula larvae normally. Some also metamorphosed into polyps. There was no difference in the metamorphosis frequencies between hybrid or full species larvae (Hatta, 1999).

Because clonal organisms, such as coral, are comprised of repeated polyps, a number of modes of asexual replication are present that are usually absent among solitary animals. Under favorable conditions, fragments of coral may survive, re-attach, and reproduce both asexually and sexually (Smith &Hughes, 1999). Asexual reproduction by fragmentation may be adaptive; evolved by natural selection to affect the shape and mechanical properties of branching colonies (Smith, 1999). However, asexual reproduction by fragmentation is a less important life-history trait for Acropora millepora than for other species (Smith & Hughes, 1999).

Fragmentation allows species to broaden their distribution areas and local abundance. It also allows colonization of such habitats which larvae would be unable to settle. An example is a sandy area, in which fragments are more likely than larvae to tolerate the unstable sediments because of their size (Smith & Hughes, 1999).

Acropora millepora have some of the smallest fragments in their genus. In a recent study, 8 of 15 were smaller than 6 cm, and only one of those was larger than 14 cm. These fragments had a 15% survivorship after 17 months. Larger fragments survived better than the smaller (about 30% versus 8%). Fragments that landed on the reef flat also survived better compared to the reef crest and the reef slope (32% vs. 14% vs 10%, respectively) (Smith & Hughes, 1999). (Dubinsky, 1990; Gilmour, 1999; Hall, 1996; Hatta, 1999; Smith and Hughes, 1999)

• Key Reproductive Features
• iteroparous
• simultaneous hermaphrodite
• asexual
• fertilization
  • external
• broadcast (group) spawning
• oviparous

• Breeding interval

  This coral spawns once per year, but can reproduce by fragmentation at any time.

• Parental Investment
• no parental involvement

Behavior

Like all coral, the polyps of Acropora millepora are social, sessile animals. Together they secret the minerals that form the skeleton of their colony.

In schleractinian corals, fitness has been shown to decline when the coral are in competition with macroalgae (Tanner, 1997). While reproduction is only minimally affected by competition, coral growth is substantially reduced. In fact, on a per polyp basis, reproduction may even marginally increase in colonies experiencing competition (Tanner, 1997). Although these studies were based on Acropora hyacinthies, they give general ideas about the genus Acropora. At the polyp level, competition had no effect on the coral. At the colony level, though, this was reversed. Growth rates were reduced, which led to smaller colonies, which led to less fecundity. This was partially compensated for by a decrease in the number of nonreproductive polyps in the colony. This was all due to competition (Tanner, 1997). As a response to competition, this species completes an infilling reaction. This involves both corals depositing a relatively undifferentiated skeleton pad along the contact area, which creates a bond between them so that no colony can dominate the other (Tanner, 1997). (Tanner, 1997)

• Key Behaviors
• nocturnal
• sessile
• colonial

Food Habits

The main carbon requirements of Acropora millepora are fulfilled by their symbiosis with unicellular algae (Anthony, 1999). Dinoflagellates, such as zooxanthellae, line the gastrovascular cavity of corals and contribute their photosynthetic products to the coral.

However, many studies have shown that hermatypic corals are able to capture and ingest particulate food from varied sources, including phytoplankton, zooplankton, and bacteria. Usually, this species extends its polyps during both the day and night (something that is uncommon among coral) (Anthony, 1999).

Coral also has the ability to be a suspension feeder. Usually, we think of fine suspended particulate matter (SPM) in high concentrations to be a stress on nearshore coral reefs. Because coral is able to be a passive suspension feeder, SPM can actually serve as a food source (Anthony, 1999). Various sources of SPM include suspended sediment, detrital matter, excretory products from other animals, and coral mucus (Anthony, 1999). These particles are also exposed to colonization by macroalgae and bacteria, which makes this a more organically valuable food source. The contribution of zooplankton feeding is not all that different from SPM feeding, in terms of maximum rate of SPM carbon assimilation. Also, when particle concentration is high, SPM feeding can cover half of the carbon and one-third of the nitrogen that is necessary for coralline tissue growth. As SPM concentrations increase, Acropora millepora ingestion rates increase linearly (Anthony, 1999). Successfully capturing and ingesting fine particles only increases 1-fold for every 8-fold increase in food availability (Anthony, 1999). (Anthony, 1999)

• Primary Diet
• herbivore
  • eats sap or other plant foods
• planktivore

• Animal Foods
• zooplankton

• Plant Foods
• phytoplankton

• Other Foods
• detritus
• microbes

• Foraging Behavior
• filter-feeding

Predation

One important predator of Acropora millepora is Acanthaster planci, the crown-of-thorns starfish. This starfish is regarded as a specialist corallivore. Acropora was the most preferred prey coral of Ancathaster planci, being favored over Porites (another hard coral) by 14:1. This could be due to A. millepora's branching morphology, as branching coral are favored about 7:1 over massives (De'ath and Moran, 1998). (De'ath and Moran, 1998)

• Known Predators

  • Acanthaster planci, the crown-of-thorns starfish

Economic Importance for Humans: Positive

A positive relationship has been found between the structural complexity of coral and the diversity in reef-fish. This diversity is concentrated in the Caribbean, East Asian, the Great Barrier Reef, and East Africa (Öhman & Rajasuriya, 1998). Studies suggest that the proportion of live coral cover affects species diversity and fish abundance in a positive correlation.

Likewise, coral habitat structure can influence fish communities (Öhman & Rajasuriya, 1998). An example is how coral feeders use branching corals like Acropora millepora. Coral feeders were correlated with live coral cover in a recent study. It showed that coral feeders actually used the branching corals for protection. This study showed a significantly direct correlation between these feeders and the density of Acropora colonies (Öhman & Rajasuriya, 1998). Not only does coral benefit humans by providing us a beautiful reef to enjoy, but it also increases the diversity of fish which we use both for amusement and enterprise. (Öhman and Rajasuriya, 1998)

• Positive Impacts
• pet trade

Conservation Status

Coral colonies may be damaged through either natural or human causes. Examples of natural damages include predation, competition, storm, and cyclone damage. Human activities such as overfishing, anchoring, diving, mining, and pollution (including sewage and sediments) can also damage the coral reefs (Hall, 1997).

How can diving affect coral? Communities of Acropora at 18-24 meter depths were the most susceptible to diver damage in a recent study (Riegl & Riegl, 1996). Acropora austera is similar to A. millepora in that it too is a branching species, so we can use A. austera as an illustration of how A. millepora might be affected. A. austera was especially susceptible to breakage by dives and dislocation in high wave energy conditions (Riegl & Riegl, 1996). However, tissue damage was not critical in this study, and it always remained way below the 5% of all scleractinian colonies.

Most of the tissue damage described above was related to natural causes. In fact, of all the factors that contribute to reef degradation, the most immediately significant are the dramatic increases in eutrophication and sedimentation (Gilmour, 1999).

This species is rated "Near Threatened" by the IUCN, based on general decline in reef coral populations and predictions of increasing ocean temperature, that causes harm to acroporine corals. (Gilmour, 1999; Hall, 1997; Riegl and Riegl, 1996)

• IUCN Red List

  Near Threatened
  More information

• IUCN Red List

  Near Threatened
  More information

• US Federal List

  No special status

• CITES

  No special status

Contributors

Amanda Ziglinski (author), Western Oregon University, Karen Haberman (editor), Western Oregon University.

References

Anthony, K. 1999. Coral suspension feeding on fine particulate matter. Journal of Experimental Marine Biology and Ecology, 232(1): 85-106.

De'ath, G., P. Moran. 1998. Factors affecting the behavior of crown-of-thorns starfish (Acanthaster planci) on the Great Barrier Reef: 2: Feeding Preferences. Journal of Experimental Marine Biology and Ecology, 220(1): 107-126.

Dubinsky, Z. 1990. Ecosystems of the World 25: Coral Reefs. New York, NY 10010 USA: Elsevier Science Publishers B.V..

Gilmour, J. 1999. Experimental investigation into the effects of suspended sediment on fertilization, larval survival, and settlement in a scleractinian coral. Marine Biology (Berlin), 135, no. 3: 451-462.

Hall, V. 1997. Interspecific differences in the regeneration of artificial injuries on scleractinian corals. Journal of Experimental Marine Biology and Ecology, 212(1): 9-23.

Hall, V. 1996. Reproductive strategies of modular organisms: comparative studies of reef-building corals. Ecology (Washington DC), 77, no. 3 (1996): 950-963.

Hatta, M. 1999. Reproductive and genetic evidence for a reticulate evolutionary history of mass spawning corals. Molecular Biology and Evolution, 16(11): 1607-1613.

Mundy, C., R. Babcock. 1998. Role of light intensity and spectral quality in coral settlement: implications for depth-dependent settlement. Journal of Experimental Marine Biology and Ecology, 223(2): 107-126.

Riegl, B., A. Riegl. 1996. Studies on coral community structure and damage as a basis for zoning marine reserves. Biological Conservation, 77, no. 2-3: 269-277.

Smith, L., T. Hughes. 1999. An experimental assessment of survival, re-attachment and fecundity of coral fragments. Journal of Experimental Marine Biology and Ecology, 235, no. 1: 147-164.

Tanner, J. 1997. Interspecific competition reduces fitness in schelactinian corals. Journal of Experimental Marine Biology and Ecology, 214, no. 1-2: 19-34.

Öhman, M., A. Rajasuriya. 1998. Relationship between habitat structure and fish communities on coral and sandstone reefs. Environmental Biology of Fishes, 53, no. 1: 19-31.