G. fascicularis is a hermatypic coral that grows in various forms depending on the size of the colony; low domes, cushion-shapes,  or irregular growths are typical of small colonies, while large colonies exhibit columnar or massive growth forms and can grow in excess of five metres across (Crabbe & Smith 2006; Veron & Pichon 1979).  Individual corallites typically vary in size reaching six to ten millimetres with the centre being a point of intersection for a number of exsert septa, thus columellae are either very fragile or not present. The septa can also be felt through the tissue of the polyp and observed in live specimens when the polyp retracts into the calyx (Figure 1), however during the day tentacles tend to be extended. Another defining characteristic of the corallites based on the species' inclusion in the genus Galaxea is that they are thin-walled and cylindrical with a blister-like coenosteum separating them. 

 

Figure 1. Two G. fascicularis polyps showing the difference between a polyp retracted into the calyx revealing septa (left) and a normal polyp with tissue fully covering the septa (right). 

Figure 2. The calcium carbonate skeleton secreted by and surrounding the four polyps can be clearly seen in this image.

In Figure 3a the same individual as Figure 1 is displayed retracted into the calyx, however one week later it was observed expanding its body and tentacles but not over the whole area of its skeleton like normal, which can be seen in Figure 3b. This may be indicative of it potentially resorbing its skeleton, possibly as a result of stressful or suboptimal conditions. While classified in general as a stony (scleractinian) coral, G. fascicularis colonies can also be differentiated into either soft or hard types based on morphology of their nematocysts; soft colonies are those with long shafted and wide capsuled nematocysts while nematocysts with short shafts and thin capsules are representative of hard colonies (Nakajima et al. 2015). Colonies with a mixture of these two forms can also be found.

G. fascicularis exhibits a range of morphotypes that can vary depending on a range of variables. Crabbe & Smith (2006) have shown that depth and incident light cause significant changes in various features of corallites of G. fascicularis such as height, width, density, and distance between corallites. It was found that with increasing incident light there was a decrease in corallite width, increase in corallite height, and decrease in inter-corallite distance. These conditions relate to colonies where light conditions are favourable such as on the reef crest or upper reef slope, and where the arrangement of corallites is in such a way that the surface area for photosynthesis is maximised as there is potential for more zooxanthellae to be present (Crabbe & Smith 2006). In contrast, in colonies found further down the reef slope or in turbid waters where light is a limiting factor, heterotrophy is favoured and thus larger corallites that are further apart may have more tentacles that they can also extend further to increase the capture of food (Crabbe & Smith 2006).

G. fascicularis can be found in a wide range of habitats owing to their high resilience to stress from bleaching and from sedimentation given their ability to rapidly remove sediment via ciliary action (Crabbe & Smith 2005; Junjie et al. 2014). They are a dominant species of inshore fringing reefs where they tend to prefer sheltered areas that do not receive a lot of strong wave action and may inhabit the reef crest, slope or deeper parts of the reef flat ranging from depths of three to 25 metres (Veron 2000; Crabbe & Smith 2005; Veron & Pichon 1979). Many large colonies tend to occur in habitats with lower levels of light, in particular water that is turbid and subject to tidal currents (Veron & Pichon 1979). 

It should be noted that the specimens of G. fascicularis spat found in the laboratory were attached to settlement plates that had been collected from Manly Boat Harbour. Given that coral larvae require a specific set of settlement cues in order for settlement to be induced, then it may be interesting to investigate this, as a settlement plate surely contains considerably different cues to those given off by a suitable location on a fringing reef. This may be indicative of a biofouling existence, which is plausible given the dominant nature of G. fascicularis in reef ecosystems. 

Symbiotic dinoflagellates, Symbiodinium sp., are an integral part of the endodermal cells of hermatypic corals, with G. fascicularis relying mainly on the organic material produced by the algae’s photosynthesis to support most of their energetic requirements (Kunjie et al. 2014). Given this fact, and the high densities of zooxanthellae observed in specimens through a dissecting microscope, it was wondered whether the densities or distribution of zooxanthellae in the specimen’s tentacles changed as the polyp grew larger. As seen in Figure 6 of a dissected tentacle from a fixed specimen classified as being small in size (settled relatively recently), zooxanthellae occupy virtually all endodermal tissue. This was the same for the fixed specimen’s tentacles that came from a large polyp. Interestingly, tentacles from a polyp of intermediate size had a break in zooxanthellae just before the tip (Figure 7). The reason for this is not clear and it may be a morphological variation not related to size but rather different water conditions, as this specimen was fixed a week earlier than the others and came straight off the settlement plate, while the others were kept in a small plastic container in the university’s aquarium for a week before being fixed. Figure 8 depicting a planula coral larva that was found in association with the polyps but may not necessarily be of the same species was also found to have extremely high densities of zooxanthellae throughout the whole body.


Figure 6. A dissected tentacle from a fixed specimen that was small in size viewed under 10x magnification. Abundant zooxanthellae visible as brown dots throughout the whole tentacle.


Figure 7. A dissected tentacle from a fixed specimen that was medium in size viewed under 10x magnification. A section of reduced zooxanthellae is visible just before the tip of the tentacle. 


Figure 8. Fixed specimen of a coral planula larvae viewed under 10x magnification with abundant zooxanthellae visible throughout. 

Different clades of Symbiodinium sp. tend to be species specific with many corals being inhabited by only one clade. G. fascicularis colonies however, have been found to be host to both Symbiodinium clade C and clade D (Dong et al. 2009; Zhou et al. 2011). Huang et al. (2011) also found that latitudinal variation exists within this species where colonies from high latitudes in Japan contained only Symbiodinium clade C while lower latitude colonies were host to either Symbiodinium clade C or D sequentially or simultaneously. These trends are repeated in studies undertaken on G. fascicularis colonies from various locations within their distribution, with Symbiodinium clade D appearing to be confined to colonies of lower latitudes (Chen et al. 2005; Visram & Douglas 2006; LaJeunesse et al. 2004; McClanahan et al. 2005; Huang et al. 2011). The fact that G. fascicularis has the flexibility to be associated with two clades of Symbiodinium is ecologically important as these zooxanthellae taxa may confer an advantage against bleaching, especially clade D, whereby the ability to adapt to changing environmental conditions is enhanced and thus survival against mass coral bleaching may also be increased (Huang et al. 2011). This is of significant importance during this time of climate change where many species of coral are becoming increasingly bleached and there is fear of these populations not recovering. It may also be possible then, that the extremely high densities of zooxanthellae observed in the tentacles correlates to this advantage, with colonies containing higher densities possibly benefitting even more so than those with lower densities. Further research is needed to determine whether this may be the case.

As well as symbiotic algae, many coral species tend to have symbiotic relationships with other marine invertebrates, in particular epizoic acoelomorph flatworms. In the case of G. fascicularis, polyps have been found to be the host of flatworms from the genus Waminoa (Wijgerde et al. 2013). This relationship has been found to negatively affect the coral in a number of ways. For example, light-shading occurs when the presence of flatworms on coral polyps and coenenchyme limit the amount of light the coral’s zooxanthellae can utilise, thus hindering the holobiont’s productivity (Barneah et al. 2007). The coral’s ability to feed on zooplankton is also inhibited by the flatworms and can occur in four possible ways (Wijgerde et al. 2013). The first way involves the flatworm directly competing for zooplankton prey with the host by capturing this prey before the coral as it comes into close proximity (Wijgerde et al. 2013). The second way is by physically obstructing the coral polyp’s oral disc, which results in fewer tentacles responding to food stimuli thus decreasing feeding efficiency (Wijgerde et al. 2013). Another mechanism involves the flatworm removing mucus from the oral disc that hinders the polyp’s ability to capture prey (Wijgerde et al. 2013). This also potentially reduces the resistance of the coral to pathogens and environmental changes (Barneah et al. 2007). Lastly, kleptoparasitism directly decreases the rate at which the coral can retain prey as the flatworm directly steals it from the coral (Wijgerde et al. 2013). 

Like all corals, G. fascicularis can reproduce both asexually and sexually. Asexual reproduction occurs in numerous forms and results in genetically identical individuals. Most commonly, asexual reproduction occurs by budding, where a new individual forms off the side of another, or fragmentation when a broken off part of the colony forms a new colony (Scott 1984). The primary type of budding exhibited by G. fascicularis is an identifying feature of the genus Galaxea and is known as extratentacular budding, which occurs at the colony margin where new corallites are formed outside of the corallite wall (Veron & Pichon 1979). It has also been observed in an aquarium setting however, that G. fascicularis colonies underwent intratentacular budding, whereby the corallite splits into two to form the new corallite and this occurs within the corallite wall (Nakano 2009).

The sexual reproductive mode of G. fascicularis is unique in that it is considered pseudo-gynodioecious, whereby colonies of this species can either be female or hermaphroditic (Harrison 1989; Keshavmurthy et al. 2012). Classified as broadcast spawners, mass spawning occurs twice a year after the full moon where female colonies release pink eggs, while hermaphroditic colonies produce white eggs that are very lipid-dense, as well as sperm (Keshavmurthy et al. 2012; Nakajima et al. 2016). While the pink eggs of female colonies are capable of being fertilized by the hermaphroditic sperm to produce planula larvae, the white eggs tend to remain unfertilized due to their large spheres of lipids inside them making it difficult for sperm to penetrate them (Keshavmurthy et al. 2012; Harrison 1989). Instead, these eggs appear to function in increasing the success rates of fertilization as they aid in lifting the bundles of sperm up to the water surface where the pink eggs float (Keshavmurthy et al. 2012). New research suggests that G. fascicularis may actually be gynodioecious however, as white eggs examined in this study were fertile and capable of undergoing embryogenesis in vivo (Keshavmurthy et al. 2012). This difference between the abilities of white eggs to be fertilized may simply be due to geographical variation between species. Nevertheless, this mode of reproduction results in a typical biphasic life cycle via production of a planktonic planula larvae and benthic adult coral colonies (Scott 1984; Rieger 1994).

G. fascicularis undergoes what is known as complex development and results in the development of free swimming planula larvae (Okubo et al. 2013). After fertilization has occurred, the embryos firstly undergo holoblastic cleavage (Okubo et al. 2013). Gastrulation then occurs as the embryo progresses through a compressed stage of two layers before becoming spherical with smoothening of the cell surface, thickening of the blastula by elongation at right angles of cells to the flattened disc and formation of the blastopore through invagination of the sides of the embryo (Okubo et al. 2013). This leads to larvae spherical in shape with cilia allowing for rotary swimming approximately 18 hours after fertilization (Okubo et al. 2013). 48 hours post-fertilization sees some larvae become elongated, with high mobility being observed on the third day, and larvae forming mesenteries and becoming pear-shaped by the fourth day (Babcock & Heyward 1986). G. fascicularis larvae may be lecithotrophic or planktotrophic but most tend to become competent and settle and metamorphose within four and a half to five days post-fertilization when settlement cues are exactly right (Scott 1984; Babcock & Heyward 1986). This is the dispersal stage of the life cycle that enables the large distribution of the coral in widespread geographic regions and also results in essential population connectivity (Scott 1984; Nakajima et al. 2016). After settlement the larvae becomes attached to the substrate and begins to grow into a polyp with out-pockets near the mouth producing tentacles and a calcium carbonate skeleton secreted at the base (Ruppert, Fox & Barnes 2004). 

Corals are polytrophic, meaning they have the ability to acquire their nutritional requirements through either heterotrophic means or via their symbiotic zooxanthellae (Hii, Soo & Liew 2008). Heterotrophy is a vital part of the coral’s acquisition of nutrients as it supplies essential elements such as carbon, phosphorous and nitrogen to the coral and its zooxanthellae (Wijgerde et al. 2011). This is especially important during bleaching events where the corals are without zooxanthellae and must rely fully on heterotrophic feeding to obtain the nutrients they require (Grottoli, Rodrigues & Palardy 2006). The coral’s carnivorous process of feeding occurs by the polyp extending its tentacles into the surrounding environment in order to catch zooplankton, often producing mucus on the tentacles to increase capture efficiency (Hii, Soo & Liew 2008). When prey hit the tentacles, cnidocytes are also released to stun and ensnare the prey (Brusca, Moore & Shusca 2016). Mucociliary action then aids in moving the captured food to the oral disc where ingestion occurs (Wijgerde et al. 2013; Wijgerde et al. 2011). Food can either be digested internally or extracoelenterically (outside the coelenteron) by mesenterial filaments extending through the mouth of the polyp (Smith et al. 2016). In Figure 9 these filaments can be seen through the polyp’s mouth. The majority of G. fascicularis’s energy needs however, are supplied through the latter process by the photosynthetic production of organic compounds by their zooxanthellae (Junjie et al. 2014).


Figure 9. A live polyp with its mouth open displaying white mesenterial filaments located within the coelenteron.

Coral colonies are made up of a network of polyps that are typical of the class Anthozoa as depicted in Figure 10. The colonial form of G. fascicularis is classified as coenosarcal as each polyp is interconnected via the coenosarc (Brusca, Moore & Shusca 2016). With a body plan of radial symmetry and an oral-aboral axis, the body wall of the polyp consists of two germ layers that form during embryogenesis – the epidermis and gastrodermis – and a third middle mesoglea layer that is derived from the ectoderm in adults (Brusca, Moore & Shusca 2016). Beneath the epidermis is the coelenteron (gastrovascular cavity) lined by the gastrodermis, between which the mesoglea resides (Brusca, Moore & Shusca 2016). The coelenteron has a high degree of compartmentalisation by mesenteries in anthozoan polyps that extend from the body wall, which have a gastrodermal lining and mesenchyme within them, and can be complete or incomplete depending on whether they fuse with the pharynx or not (Brusca, Moore & Shusca 2016). Mesenterial filaments (depicted in Figure 11) have a thickened edge below the pharynx that contains cnidae, cilia and gland cells that aid in their use for defence and the capture of prey (Brusca, Moore & Shusca 2016). The mouth occurs in the centre of the oral disc and leads to a muscular pharynx from which the coelenteron extends (Brusca, Moore & Shusca 2016). A variable number of tentacles radiate from the oral disc of G. fascicularis as one of three kinds; most commonly seen are septal and lateral tentacles, however sweeper tentacles may also be present, which are much longer than the others and have a defensive function (Hidaka & Yamazoto 1984). 

Figure 10. The basic internal anatomy of an Anthozoan polyp (adapted from Brusca, Moore & Shusca 2016).


Figure 11. A histological section of a fixed specimen stained with H&E and viewed under 40x magnification. The arrow indicates the mesenterial filament running through the specimen that is stained light pink with a slightly darker pink thickened edge.

When a coral polyp settles on the benthos it begins to secrete a calcium carbonate exoskeleton individually known as a corallite. In terms of the whole colony, this skeleton is collectively called the corallum and the part between each corallite is called the coenosteum (Brusca, Moore & Shusca 2016). The corallite’s outer wall is the theca and it attaches to the substrate with a basal plate, with the whole structure being supported by the columella (Brusca, Moore & Shusca 2016). From this, septa radiate in a predictive pattern definitive of G. fascicularis. Four cycles of septa are typical with the first two usually being exsert and possibly of an irregular shape, however in some deep water colonies the fourth cycle may be absent, or those with large calices may have a partially formed fifth cycle (Veron 1986). For additional support to the polyps, longitudinal and circular muscles combined with the gastrovascular cavity act as a hydrostatic skeleton (Brusca, Moore & Shusca 2016). 

In the process of intracoelenteric digestion of G. fascicularis, ingested food passes through the pharynx and enters the coelenteron for extracellular digestion (Brusca, Moore & Shusca 2016). Here, digestive enzymes are produced by cells in the gastrodermis that break food down into a soupy mixture consisting of polypeptides, fats, and carbohydrates (Brusca, Moore & Shusca 2016). Circulation of digestive juices in the coelenteron may be aided by ciliary action on the lateral lobes of mesenterial filaments of the gastrovascular cavity (Brusca, Moore & Shusca 2016). These nutrients are then distributed throughout the coelenteron into other parts of the body to be fully digested intracellularly in food vacuoles in nutritive-muscular cells via phagocytosis or pinocytosis into the cell (Brusca, Moore & Shusca 2016). As the gut of the polyp is classified as being blind, all food enters and any undigested wastes exit through the one opening. While G. fascicularis also digests food extracoelenterically via mesentarial filaments, the specific process describing how this occurs has not been recounted in depth and therefore cannot be discussed further.

Due to the radial symmetry of the animal, the nerve net of cnidarian polyps is diffuse and non-centralised with sense organs being distributed radially (Brusca, Moore & Shusca 2016). Two nerve nets exist with one being bounded by epidermis and mesenchyme and the other gastrodermis and mesenchyme (Brusca, Moore & Shusca 2016). Due to the neurons and synapses of cnidarians being non-polar, impulses generated by stimuli can extend in every direction throughout the nerve net (Brusca, Moore & Shusca 2016). Minuscule hair-like structures located on the body surface, in particular in higher densities on the tentacles and where cnidae are more abundant, may act as mechanoreceptors or chemoreceptors, influencing body movement and movement of tentacles toward prey or predators (Brusca, Moore & Shusca 2016). While polyps display photosensitivity, there is no receptor known to actively facilitate this, and instead it is thought to be associated with the epidermal cell’s translucent surfaces that contain concentrations of neurons within or just below the surface (Brusca, Moore & Shusca 2016). 

While no true mesoderm exists in cnidarians, longitudinal and circular fibrils that originate from the epithelium and reside in the mesenchyme tend to be loosely termed as muscles (Brusca, Moore & Shusca 2016). Within the tentacles and oral disc, epitheliomuscular cells enable movement, which particularly aids in the capture and transport of prey to the mouth during feeding, or the movement of tentacles in defence (Brusca, Moore & Shusca 2016). While extension of the polyp is mainly achieved through manipulating the amount of water in the hydrostatic skeleton, the column of the polyp can be contracted via longitudinal fibres acting as retractor muscles that are located in bundles lining the mesenteries (Brusca, Moore & Shusca 2016). The gastrodermis produces circular muscles that are found in the tentacles, oral disc and mouth and are especially useful in holding the mouth closed (Brusca, Moore & Shusca 2016). Distension of the polyp body and tentacles is also achieved in combination by the hydrostatic skeleton and circular muscles (Brusca, Moore & Shusca 2016). 

All cnidarians lack specialised organs for gas exchange, thus respiration occurs via diffusion across the internal and external cell surfaces of the polyp’s coelenteron, tentacles and oral disc (Brusca, Moore & Shusca 2016). This supplies the polyp with sufficient gases due to the thin body wall of the polyp (Brusca, Moore & Shusca 2016). As these are sessile creatures, they rely on water currents to bring essential gases to them. Shallow water colonies often have a constant supply of new water brought to them, however deeper water colonies may be in more stagnant water and increase efficiency by creating a water current themselves. This is done by the beating of epidermal cilia, which create a current that draws water over and into the body (Ruppert, Fox & Barnes 2004). 

A defining feature of the phylum Cnidaria and a useful defensive mechanism is the cnidae. A fully formed cnidae is termed the more commonly known cnidocyte.  Three types of cnidocytes exist within the Cnidarians; true nematocysts, spirocysts, and ptychocysts. Within scleractinian corals and thus G. fascicularis, only nematocysts and spirocysts occur. Nematocysts are defined as having capsules that are double-walled and a barbed tubule to increase the efficiency of penetration and anchorage into the prey (Ruppert, Fox & Barnes 2004). A mixture of phenols and proteins constitute the toxins contained within the capsule, which may be transferred into a wound by being on the tubule surface, or it may be injected through a terminal pore on the thread (Ruppert, Fox & Barnes 2004). In contrast, spirocysts have capsules with only a single wall and their tubules are adhesive rather than barbed and have no apical pore due to the fact that they do not pierce the prey but rather wrap around it (Ruppert, Fox & Barnes 2004). The capsules in this case also contain mucoprotein or glycoprotein rather than toxins (Ruppert, Fox & Barnes 2004). A nematocyst or spirocysts’ release may be triggered by mechanical and chemical stimuli, resulting in the tripartite apical flap covering the coiled barb opening, releasing the cnidae into the prey (Ruppert, Fox & Barnes 2004). Figure 12 depicts these cnidocytes in the coelenteron and tentacle where they are most abundant. Whether these are nematocysts or spirocysts however, could not be determined.


Figure 12. Histological sections of a fixed specimen stained with H&E and viewed under 40x magnification. Arrows indicate the cnidocytes (stained light pink) within (a) the coelenteron and (b) a tentacle.

Corals can deal with interspecific competition for space through use of sweeper tentacles by discharging nematocysts at their opponent, by expelling mesenterial filaments for extracoelenteric digestion, or through overgrowth (Hidaka 1985). When intraspecific competition occurs between two colonies of G. fascicularis, sweeper tentacles have been the main described mechanism of defence, however interestingly are only utilised when the competing colony is of a different colour morph (Hidaka 1985; Hidaka & Yamazoto 1984). The composition of nematocysts of the sweeper tentacles is also different to those of regular tentacles, and it is thought that sweeper tentacles form from regular tentacles, due to the presence of tentacles at intermediate states in terms of their outward appearance and nematocyst composition (Hidaka & Yamazoto 1984).

The ability to fluoresce has been observed in many different marine invertebrates with corals being considered one of the most fluorescent animals in the sea. The significant applications of fluorescence however, in particular in molecular and cellular biology, have only recently become recognised. One of the most well known fluorescent proteins, the green fluorescent protein (GFP), was first discovered in the jellyfish Aequorea victoria in 1961 and has since been used for a myriad of techniques such as molecular markers, fusion tags, transcriptional reporters and biosensors (Ben-Zvi, Eyal & Loya 2015; Karasawa et al. 2003). These pigments have the ability to autocatalytically produce a specialised molecule called a chromophore that causes fluorescence by absorbing light that is of a short wavelength and then re-emitting it at a longer wavelength thus giving off the green colour (Ben-Zvi, Eyal & Loya 2015; Vogt et al. 2008). The specific function of GFP in coral has been widely discussed with possible roles including antioxidant function (Bou Abdallah et al. 2006), photoprotection (Salih et al. 2000), attraction of zooxanthellae (Hollingsworth et al. 2004), and photosynthesis facilitation (Dove et al. 2008).

In G. fascicularis, GFP has been identified and exists in different amounts depending on the colour morph of the coral colony (Nakaema & Hidaka 2015). Figure 13 displays the biofluorescence displayed in one of the specimens when viewed under blue light with a wavelength of 475nm. Within this species, GFP was proposed to have the function of being photoprotective in the study conducted by Ben-Zvi et al. (2015), however the study by Nakaema and Hidaka (2015) concluded that the GFP of the corals in their study did not have a role of photoprotection. Thus further investigation is still needed in order to determine what the specific role may be of GFP in G. fascicularis.


Figure 13. Live polyp viewed under blue light to display biofluorescence caused by a green fluorescent protein.