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Alveopora sp. (de Blainville, 1830)

Taylah Bruce 2015


The genus Alveopora or ‘flowerpot coral’ is a submassive (boulder-like), scleractinian coral, traditionally of the family Poritidae, but also phylogenetically falls in Acroporidae (Veron 2000). This coenosarc colonial group is a relatively uncommon genus and is found in a range of places from turbid reef environments protected from wave action, to clear water on reef slopes (Veron 2000). It is typically found at the Houtman Abrohlhos Islands, south-west Australia, and less commonly throughout the Indo-Pacific and east coast of Australia (Veron 2000). As this genus is required to deal with moderate levels of turbidity and sediment in their natural environments, they have presumably adapted to successfully cope with the excess sediment. An observational study was conducted on a single colony of Alveopora and on a colony from the similar genus Goniopora, to better understand these coping mechanisms and determine if sediment size has differing effects on these organisms.
Figure 1

Physical Description

Colonies of Alveopora are made up of sessile, benthic polyps with a mouth-up orientation. These polyps have a cylindrical column arising from an aboral pedal disc, and an oral disc from which tentacles radiate (Ruppert 2004). The structure in which the polyps live is called the corallum (corallite + coenosteum), which is composed of very fine walls of rods and spines, highly perforated and made up of calcium carbonate (Veron 2000). The fleshy, greenish-brown to dusky pink polyps can be up to 100mm in length and 20mm wide when they are fully extended. The oral discs are tan and white in colour and are around 5mm in diameter when expanded [see Figure 2]. They each have 12 tentacles with swollen white tips, a distinguishing feature from Goniopora sp. which has 24 tentacles on each polyp (Veron 2000).
Figure 2
Figure 3


General Ecology

Scleractinian corals play a huge part in the construction and growth of reefs The lower end of the polyps’ body and basal disk secrete a calcium carbonate (limestone) skeleton that forms the corallite (Ginsburg 2007). The corallite + the coenosteum (common bone secreted by coenosarc) together form the corallum [see Figure 4]. These polyps continuously keep depositing to calcium carbonate beneath themselves and the skeleton continues to grow. The corallum houses and provides a solid substrate to protect the colony, and allows polyps under physical stress to retract right into the calyx. This hermatypic process is integral to ocean ecosystems as it builds the coral structures that supports each and every diverse reef community (Ginsburg 2007).
Figure 4


Adult colonial corals of this species are sessile, and predominantly grow in warm waters of tropical and subtropical areas such as the Indo-Pacific and Atlantic oceans.Their relatively small geographic distribution is restricted by their physiological demands for development, influenced by abiotic factors such as temperature, light, sediment, substrate and wave force. As all stony corals, Alveopora requires water temperatures of 23°-29°C to thrive and cannot survive for long in temperatures out of this range. Alveopora is generally found on upper reef slopes in turbid environments with low wave action (Ginsburg 2007, National Ocean Service 2008).   
Due to their symbiosis with zooxanthellae,
Alveopora like all scleractinians are restricted to the euphotic zone, more specifically at depths of 5-25 metres where light can penetrate sufficiently (Veron 2000). This is important as they harbor the photosynthetic algae, zooxanthellae [see Feeding & Associations]. They are normally distributed at high density within a small area, being a hermaphroditic brooding species, whose larvae tend to settle within hours (Veron 2000).


The most prominent association Alveopora, as with most corals, has is its mutualistic symbiosis with the dinoflagellate zooxanthellae. This unicellular algae lives in the cells of the coral’s gastrodermis and produces energy that is transferred into the coral tissue. The coral, in return provides habitat for the algae to thrive in [see Feeding] (Pearse 1971).  

Alveopora also has occasional direct association with clownfish. Whilst this is not an obligatory association, the clownfish will sometimes take refuge and "play" in the polyps as it does with anemones (Siegel 2002).
Figure 5

Life History and Behaviour


In the nature of true cnidarians, Alveopora will obtain much of its nutrients by using their tentacles to capture phytoplankton, zooplankton, dissolved organic and inorganic matter, particulate organic matter and bacteria floating through the water column (Goreau 1971). Their long tentacles contain nematocysts to stun and kill their prey before transferring it to its mouth.They also secrete large amounts of mucus that capture food particles which are transferred to the mouth via nematocysts and currents driven by cilia (Ruppert 2004) [see video below].

Time laps of nematocysts actively collecting material using a sticky mucus and barbs. Video by Khaled bin Sultan Living Ocean Foundation (2014). 

Another way in which they acquire nutrients is through their mutualistic symbiosis with zooxanthellae. The coral provides a habitat for these dinoflagellates and they in turn, perform photosynthesis to provide energy for the coral (Karako 2002). The products - glucose, glycerol and amino acids - of this photosynthesis supplied to the coral, are macromolecule "building blocks" important in the corals’ cell growth and function. The corals then  use these carbohydrates, proteins and fats to produce the fundamental component of all reefs, calcium carbonate. The zooxanthellae are also what gives most corals their remarkable colours. When bleaching events occur and corals lose their zooxanthellae, they also lose their colour (Karako 2002).


Alveopora, as with many stony corals are hermaphroditic, and can reproduce both sexually and asexually. Most species are thought to be brooders, meaning only the male sperm cells are released into the water column, and must enter the corals to fertilise the egg cells (Shlesinger 1998, Harii 2001).  Maturation of the male and female gonads will occur simultaneously and then spawning will take place almost continuously for 3-7 months. Planktonic, planula larvae are released into the water column and will take from 0.5 to 3 days to settle and then metamorphose into a juvenile polyp (Ruppert 2004). 
also has the ability to reproduce asexually. This occurs through corallite multiplication by budding and splitting (National Ocean Service 2009). If the polyp undergoes fission and splits at the mouth into two head, it is known as intratentacular budding. If a new polyp is produced between two or more older polyps then it is known as extratentacular budding [see figure 6] (Spotts 2001).

Figure 6


As all anthozoans, Alveopora lacks a medusae stage. It’s primitive life cycle begins with a polyp planula polyp (Ruppert 2004)[see figure 7]. Once the larvae has settled and attached itself to the substrate, it will begin metamorphosis into its adult form. The juvenile polyp then begins to deposit a calcium carbonate skeleton to form the corallite and buds additional polyps for colony growth (Ruppert 2004).

Figure 7


Alveopora is a sessile coral, however the individual polyps are highly flexible and active, constantly moving around and feeding. These polyps can extend to impressive distances and are sometimes mildly aggressive towards other corals, if too close (Thamrin 2001).

The retraction and extension of the polyps are generally in response to either feeding or defense in response to a disturbance. They can strongly retract their tentacles, oral disc and column towards the pedal attachment by simultaneously invaginating the oral disc and tentacles into the column, and then deflate the tentacles and column by expelling coelenteron fluid from its mouth (Ruppert 2004). This allows them to almost completely retract back into the corallum housing.
 Video of Alveopora colony of polyps waving around and feeding. Video by Taylah Bruce (2015)

Gas Exchange, Internal Transport & Excretion

Respiration takes place via gill surfaces on the  tentacles and general body wall. Ciliated epidermal cells create a current over the body surface and thus facilitates gas exchange. The energy required for respiration is provided by the zooxanthellae photosynthesizing (Riegl 1995). Internal transport is via a coelenteron, lined by gastrodermis, that extends into each individual polyp (Ruppert 2004). The coelenteron is important in roles such as extra-cellular digestion, circulation, excretion, reproduction, and hydro-static skeletal support (Ruppert 2004). The colony gains nutrition by releasing enzymes to break down prey and then circulating these digestive fluids through the coelenteron to be absorbed by a multitude of cells. Any indigestible material is expelled through the mouth (Ruppert 2004). The excretory product of all Cnidaria is ammonia which diffuses across the polyps’ body walls and is then dispersed by currents into the water column (Ruppert 2004).


Additional Observations

Observations of behaviour and feeding of a colony located in the University of Queensland’s marine aquarium were made over a four week period, in addition to thorough research into the already compiled literature. Alveopora was observed to be very active at most times, with polyps continuously extended and retracting when feeding. The polyps were never all extended at the one time, with approximately half remaining retracted into the corallum and oral discs shrunk. They responded rapidly to being touched by humans but did not seem to react to inanimate objects. This suggests they can distinguish between potential threats and may not react otherwise.   

Anatomy and Physiology

Each individual polyp in the colony is linked by a common gastrovascular system through which they share food, water, and wastes with. This system is linked by the coenosarc, the soft tissue that stretches over the surface of the coral between all the polyps (Ruppert 2004). Coelenteric tubes are contained within this tissue, between the upper and lower epidermal layers.

The polyps comprise the epidermis and gastrodermis. Between these two layers is the mesoglea, a non-tissue, gelatinous layer. Within the polyps are mesenterial filaments which contain nematocysts, a pharynx, and the columella - the central axis of the corallite found directly below the oral disc (Budd 2001). The paired septa, typical of scleractinians, are calcareous plate-like structures that radiate from the wall of the corallum to the centre of the corallite [see Figures 8 & 9].
The most defining feature in all Cnidaria is the cnidocyte, a sensory-effector cell that has a key role in the defense and feeding of animals in this phyla. These powerful and toxic cells contain cnidae, the capsule that when stimulated appropriately, shoots out a long tubule that can sting or paralyse its prey (Ruppert 2004). The three types of cnidae include nematocysts, spirocysts and ptychocysts (Ruppert 2004).
Figure 8
Figure 9

Evolution and Systematics

Kingdom: Animalia Phylum: Cnidaria Class: Anthozoa Order: Scleractinia Family: Poritidae/Acroporidae Genus: Alveopora sp. Alveopora falls within the class Anthozoa, distinguished by its pharynx, septa, mouth-up orientated polyps and lack of medusae stage in the life cycle (Ruppert 2004). Within Anthozoa are the two major taxa: Alcyonaria and Zoantharia. Alveopora is within the Zoantharia, possessing hexamerous symmetry and spirocysts, a key synapomorphy. This monophyletic taxon consists of the sea anemones, stony corals and coral anemones (Ruppert 2004). As a stony coral, Alveopora falls in the order of Scleratinia, the hermatypic reef-building corals. Distinguishing features of this order include possession of paired septa, and secretion of calcium carbonate to form a corallite. Most lack siphonoglyphs and their retractor muscles tend to be sheet-like, unlike the localized, bulging retractor muscles possessed by sea anemones (Ruppert 2004).
The placement of Alveopora within a family is still controversial, being positioned in both Poritidae and Acroporidae. Whilst initially belonging to Poritidae due to its extreme similarity in ecology and polyp behaviour to Goniopora (Kitano 2014), there have been arguments based on recent molecular studies, that would place it genetically distant from this family. Veron (2000) states that Alveopora is isolated within Poritidae since it does not exhibit the pattern of septal fusion typical of that family, but instead has the poorly defined septa typical of Acroporidae.

Biogeographic Distribution

Alveopora sp. are generally found in the subtropical to tropical areas on the East and West Australian coasts. Particularly on the Great Barrier Reef and Houtman Abrohlhos Islands (Veron 2000) [see figure 10 below].
Figure 10

Conservation and Threats

Experiment - Poritidae Mechanisms of Sediment Removal

The mechanisms by which Alveopora sp. are able to tolerate and expel sediment are vital to the organisms, as a majority are found in habitats that are consistently exposed to moderate to high levels of turbidity, commonly a result of high amounts of total suspended solids (Kemker 2014), reflecting the sunlight so crucial to the corals’ zooxanthellae. Therefore, the more particles (total suspended solids) in the water column, the less light the coral is likely to receive. This can impose an energetic cost onto the corals, are less light means less photosynthesis and thus nutrient production. Riegl et. atl. (1995) found that this decrease in photosynthesis led to a decrease in respiration and an increase in mucus production, significantly interfering with the coral’s energy balance.

The size of the suspended sediment particles is also important. Smaller particles are less likely to settle and thus may linger in the water column, preventing light penetration for longer, whereas larger particles are more likely to settle sooner (Kemker 2014). General observed mechanisms of sediment removal include retraction of the colony of polyps, mucus production, or actual ingestion of sediment (Rosenfeld 1999 & Stafford-Smith 1992).

This experiment aims to observe any differing responses of each colony to the different sizes in particles and see if there is a significant difference in time taken to expel the sediment between each size treatment. To observe these mechanisms and examine how they may deal with these fluctuating levels of suspended solids present, a simple experiment was conducted testing Alveopora’s response to different particle sizes. Over three weeks, a colony of Alveopora and a nearby colony of the similar genus Goniopora were subjected to three small plumes of sand/silt, each differing in particle size. The plume sand grain sizes measured approximately 2mm, 0.5mm or 0.1mm. Once exposed to these “plumes”, the time it took for the colonies to rid themselves of the majority of the sand was recorded.

In the first week, each colony of Alveopora and Goniopora - which were permanently located in a small aquarium - were exposed to sediment plumes with 2mm particles. Using small test tubes, 10 mL of the sediment was measured for each of the colonies, and then simultaneously poured onto the polyps. The behaviour and response of the polyps was then observed until most or all of the sediment had fallen off the colonies or been ingested and the time taken to do so was recorded. This was repeated for the following two weeks with the 0.5mm and 0.1mm particle plumes.

The results observed showed no significance between times [see figure 11]. When graphed, there appears to be a slight trend in the times taken to expel sediment, with the 2mm particles taking on average (between the two colonies) 47 minutes versus 44 minutes for 0.5mm and 32 minutes for 0.1mm. Ingestion of the sediment by either colony was not observed, but may explain this slight difference in times as smaller particles are easier to uptake. When treatments were applied, colonies were also observed for behavioural responses. Alveopora showed immediate responses by retracting some polyps back, whilst extended ones became slightly more aggressive and began waving around. This response, whether or not a mechanism to assist in sediment removal, initially accelerated the process. Most of the polyps remained retracted for the whole 30-40 minutes, even after the sediment had been expelled. Goniopora’s only initial response was increased movement of the polyps, no retraction was observed. Throughout the three treatments Goniopora removed the sediment at a faster rate than Alveopora, however the colony was larger and thus more polyps were present to assist in the removal.

Due to time and experimental constraints, one replicate for each genus of each treatment is far from what is needed to draw any valid conclusions. The fact that only one colony of Alveopora and Goniopora were available in an in-situ environment, replications of each treatment were limited so as not to permanently harm the colonies nor other surrounding corals present in the aquarium. Pseudoreplication, although could not be avoided, was minimised by leaving long periods of time between each treatment and allowing the colonies to recover from any effect the plumes may have had. Whilst these results should not be considered biologically meaningful without more thorough investigation, they may perhaps give some preliminary insight into what we may expect from further studies. Future studies into the effects of sediment overload on coral reefs should be conducted to explore the true effects that varying sediment types may have, particularly on the fundamental hermatypic reef-building corals (see Future Conservation).
Figure 11

Future Conservation

Scleractinian corals face many major threats that are contributing to the rapid decline of coral reefs around the world. Increased amounts of ambient suspended and settling sediment caused by the breakdown of coral reefs and anthropogenic activity in the oceans are having detrimental effects on the corals’ growth and calcification rates (Stafford-Smith 1992). These increased concentrations of sand and silt disturb the energy budget of corals by interfering with prey capture as well as increasing demand for active sediment rejection (Stafford-Smith 1992). Additional sediment on substrates may also inhibit the settlement of juveniles.

Corals are also under threat from bleaching as a result of ocean acidification. The symbiosis between corals and zooxanthellae is delicate and even a slight increase in ocean temperature and subjection to stressful conditions can result in the expulsion or loss of photosynthetic algae. Due to the coral’s high dependence on zooxanthellae for nutrients, they can not survive for long after a bleaching event unless the temperature drops back to normal and are able to re-recruit the algae. These bleaching events have been on the increase with the threat of global climate change being exacerbated by anthropogenic means. Alveopora is particular is highly susceptible to this phenomenon and data from past mass bleaching events has shown Alveopora to have the highest bleaching response in the Indian Ocean (McClanahan 2007). This makes them a highly susceptible species to extinction.


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