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Fimbriaphyllia paraancora

Hanna Wood 2019


Fimbriaphyllia paraancora, commonly known as the “hammer coral”, is a branching colonial scleractinian coral from the family Euphylliidae (Luzon, 2018). Formally classified within the subgenus Euphyllia, recent molecular phylogenetic analyses lead to its classification within the newly recognised genus Fimbriaphyllia (Luzon, 2018). Each colony is composed of large polyps with tubular tentacles and hammer or anchor-shaped tips (Veron, 2000). Tentacles typically radiate from the corallite centre and form a concentric pattern (Veron, 2000). Colonies are phaceloid with exsert septa which plunge near the corralite centre and extend to the outside wall to form costae (Veron, 2000). Each branch is formed from one corralite (Veron, 2000). No columellae are present. F. paraancora is found in the shallow waters of the Indo-West Pacific reefs, with a strong presence in Indonesia (Veron, 2000). Many colour and tentacle shape variations exist within the species, ranging from bright green to gold. 

Physical Description

Colonies of F. paraancora are made up of sessile polyps with hammer or anchor-shaped tipped tentacles that are extended during the day and night. Some polyps may exhibit long 'sweeper' tentacles that are capable of extending to extreme lengths in order to compete with neighbouring corals (Luzon, 2018). Within the centre of each polyp is a single opening, which serves as both a mouth and anus. F. paraancora share similar physiological characteristics to Fimbriaphyllia ancora but can be easily distinguished as each polyp (corralite) forms a single branch (phaceloid skeletal plan). The underlying colonial skeletal structure is often hidden during full polyp extension. Each corralite is approximately 20-40 millimetres in diameter.  Figures 1 to 3 below show a variety of colouration patterns observed within the species. 

Figure 1
Figure 2
Figure 3


F. paraancora is a scleractinian coral, producing a calcium carbonate skeleton that forms the foundation of coral reef systems (Hickman et al., 2014). Coral reefs are highly diverse systems and are among the most productive (Hickman et al., 2014), supporting the existence of many species. Without scleractinian corals and their foundation building skeletons, our reefs would simply not exist to the extent they do today. As F. paraancora house photosynthetic symbionts, their presence is limited to shallow reef systems (the euphotic zone) that are exposed to adequate amounts of sunlight and low turbidity.

Life History and Behaviour


Like other anthozoans, F. paraancora feed through the extension of tentacles that are lined with stinging cells (cnidocytes) which stun and capture passing phytoplankton, zooplankton, and small prey (Hickman et al. 2014). Once captured, food is drawn to the mouth and passes through the gastrovascular cavity for digestion. Other cells of the tentacle ectodermis secrete a slimy mucus which coats the animal and trap passing food particles (Veron, 2000). The mucus is moved around the polyp by microscopic cilia (Veron, 2000). The zooxanthellae that live within the tissue of F. paraancora provide most of the required sugars and amino acids required for protein synthesis and other cellular processes (Barnes, 1987; Sumich, 1996). Up to 90% of the organic material produced by the zooxanthellae during photosynthesis are provided to the host corals tissue (Sumich, 1996).


F.paraancora reproduce both sexually and asexually. Asexual reproduction occurs through budding, whereby a parent polyp divides into two or more daughter polyps. These daughter polyps form new elongated corallites, which are attached to the colony at the base forming a single branch. Budding appears to be intratentacular, occurring within the tentacle ring of the parent corralite (Veron, 2000). Figure 4 below shows F. paraancora at two different stages of budding. Sexual reproduction occurs through the synchronised release of eggs and sperm into the water column, where fertilisation takes place (Barnes, 1987; Hickman et al., 2014; Veron, 2000). This is common among zooxanthellate corals and is known as broadcast spawning (Veron, 2000). The release of gametes into the water column allows for greater dispersal among reefs (Veron, 2000). A vast majority of zooxanthellate corals are hermaphroditic, with polyps containing both male and female gonads. However, some species are gonochoric (have separate sexes). Fimbriaphyllia ancora, a close relative of F.paraancora was discovered to be gonochoric, with each colony producing either male or female gametes. Veron (2000) states that the sexuality of corals tends to be consistent within species and genera, therefore it is highly likely that F. paraancora colonies are also gonochoric. A video posted by Monsoon Aquatics from Darwin Australia (provided below) shows colonies of F. paraancora storing egg bundles within tentacle tips and releasing them through the mouth cavity. Staff from Monsoon reported that some colonies released sperm, and others released both sperm and eggs (suggesting hermaphroditism). However, investigations into the phylogenetic classification of F. paraancora by Luzon et al. (2018) supports gonochory. 

Like other anthozoans, the release of gametes by F. paraancora is synchronised, with gametogenesis (the production and maturation of gametes) and mass spawning events being triggered by environmental cues. These environmental cues include the phase of the moon, sea temperature and day length (Veron, 2000). A phenomenon known as split spawning, where colonies undergo multiple spawning events, is believed to occur when there is a disconnect between lunar and seasonal cues (Foster et al., 2018). Split spawning was observed within my own aquarium, with F. paraancora colonies releasing eggs over two events within two consecutive months.

Histological analysis revealed that F. ancora gametogenesis occurs in the mesenterial mesoglea between the mesenterial filaments and the re-tractor muscle bands (Shinya et al., 2012). As F. ancora shares very similar physiological characteristics to F. paraancora, and the two species are very closely related, it is likely that gametogenesis occurs in the same location.

Fimbriaphyllia paraancora
spawning in captivity. Video by Monsoon Aquatics (2016). 
Figure 4
Figure 5

Anatomy and Physiology

Skeletal and Tissue Structure

Each polyp is housed within a ‘cup’ called a corallite (Figure 8), which is composed of calcium carbonate (Veron, 2000). Calcareous plate-like structures, called septa (Figure 9.D) , radiate from the wall of the corallite and extend to the outside surface to form costae (Figure 6) (Veron, 2000). The skeletal formation of F. paraancora is known as phaceloid, where corallites are elongated and branch off at the base to form a colony (Figure 7) (Veron, 2000). Each branch is formed from one corallite and no columellae are present (Veron, 2000). The sterome is a solid sheet that forms the inner lining of the corallite wall, and gives the skeleton a porcelain appearance (Veron, 2000). The calcium carbonate skeleton is secreted by the epidermis at the base of the polyp and is characteristic of all Scleractinia corals (Hickman et al., 2014).

The body wall of F. paraancora, like all Cnidaria, is made up of two cell layers, the ectodermis and the gastrodermis. These two layers are separated by the mesoglea, a gelatinous matrix composed primarily of water (Veron, 2000). The body cavity of the individual polyps, called the coelenteron (Figure 9.C), has a single slit opening that acts as a mouth and anus. The coelenteron is linked to adjacent polyps by tubes,which allow for the circulation of fluids and nutrients (Veron, 2000). By linking polyps within a colony, competition for resources is minimised. The coelenteron is partitioned by mesenteries, which increase the surface area of the gastrodermis for efficient digestion. Packed within the margins of the mesenteries are coiled mesenterial filaments (Figure 9.B) (Veron, 2000). These filaments are lined with stinging cells called nematocysts, and aid in feeding, digestion and defence (Figure 10) (Veron, 2000). During defence and when under stress, the mesenterial filaments are often protruded from the mouth cavity (Veron, 2000).

Figure 6
Figure 7
Figure 8
Figure 9
Figure 10


Like all cnidarians, the surface of the tentacles and mesenterial filaments of F. paraancora are lined with unique stinging cells known as nematocysts (Veron, 2000). Nematocysts are tiny capsules composed of a chitinous material and contains a coiled tubular “thread” or filament (Hickman et al., 2014). The thread may bear barbs or spines that penetrate prey. The nematocysts of anthozoans are equipped with mechanoreceptors that trigger the nematocyst to discharge when stimulated by passing prey (Hickman et al., 2014). Figure 11 below shows a unfired (A) and fired (B) cnidae from F. paraancora.

Figure 11


Living within the tissue of most anthozoans, are microscopic dinoflagellate zooxanthellae (Symbiodinium spp.) (Veron, 2000). These zooxanthellae are photosynthetic, unicellular organisms that share a unique mutualistic relationship with corals (Hickman et al., 2014). These organisms live within the gastrodermis and provide corals with the nutrients required for cellular processes (Veron, 2000). The coral in return provides the zooxanthellae with a protected environment and compounds required for photosynthesis (Hickman et al., 2014). The most prominent zooxanthellae species associated with F. paraancora is Symbiodinium clade D. Figures 12 and 13 below show the extent of zooxanthellae living within the tissue of a tentacle tip of F. paraancora.

Figure 12
Figure 13

Digestion and the Nerve Net

Digestion occurs within the gastrodermis, both intracellularly and extracellularly (Hickman et al., 2014). Intracellular digestion takes place within specialised cells that line the gastrodermis and extracellular digestion occurs as mesenterial filaments secrete a digestive fluid to break down food within the cavity (Hickman et al., 2014).

The nerve net of cnidarians is composed of both ectodermal and gastrodermal cells (Veron, 2000). These cells permeate the body wall and are connected to specialised cells that respond chemical and mechanical stimuli (Veron, 2000). The muscular system of anthozoans allows polyps to retract and extend their tentacles in response to signals from the nerve net which is transmitted across polyps. When disturbed, F. paraancora retracts its fleshy body into the corallite for protection.  

Biogeographic Distribution

F. paraancora is found in the shallow waters of the Indo-West Pacific reefs, with a strong presence in Indonesia (Veron, 2000). F. paraancora typically prefer areas of the reef with low to moderate wave activity. 

Figure 14

Evolution and Systematics

Kingdom: Animalia Phylum: Cnidaria Class: Anthozoa Subclass: Hexacorallia Order: Scleractinia Family: Euphylliidae Genus: Fimbriaphyllia Species: Fimbriaphyllia paraancora

F. paraancora falls within the class Anthozoa, distinguished by its characteristic body plan (mouth-up orientated polyps with a “flower-like” appearance) and lack of a medusae stage within the life cycle. Within Anthozoa are three subclasses: Hexacorallia; Octocorallia; and Ceriantipatharia.  F. paraancora is within Hexacorallia, displaying hexamerous symmetry with paired septa and tubular tentacles arranged in circlets on the oral disc. This monophyletic taxon consists of the sea anemones and hard corals (Hickman et al., 2014). As a stony coral with a hard calcium carbonate skeleton (secreted by the lower epidermis), F. paraancora is in the order Scleractinia, the reef-building corals. Until recent advancements in molecular phylogeny, F. paraancora was classified within the subgenera Euphyllia, along with other species such as Euphyllia glabrescens, Fimbriaphyllia ancora, Fimbriaphyllia divisa, Fimbriaphyllia paradivisa, and Fimbriaphyllia yaeyamaensis. This classification was based upon similarities in colony skeletal structure (Luzon et al., 2017). The analyses of multiple genetic markers by Luzon et al. (2017) revealed that there were two distinct groups within Euphyllia that originated from different ancestors. Euphyllia glabrescens formed one of these groups, while F. paraancora, F. divisa, F. ancora, F. paradivisa, and F. yaeyamaensis formed the other, having diverged from a separate common ancestor. The above findings highlight the issues associated with relying on skeletal features to reflect evolutionary relationships within families of Scleractinia, in that it does not take the convergence of similar skeletal structures into account. 

Conservation and Threats

Like other anthozoans, F. paraancora is under serious threat of coral bleaching. Environmental stressors are impacting the mutualistic relationship shared between anthozoans and Symbiodinium spp. Increasing levels of atmospheric greenhouse gases has led to tropical oceans warming at approximately 70% of the global average rate (Lough, 2012). Other factors such as ocean acidification, pollution and changes in salinity also result in coral bleaching. Human induced global warming has resulted in large-scale mass bleaching events that have devastated entire reef systems, resulting in the death of many coral species (van Oppen and Lough, 2018). 

Another threat to F. paraancora is its desirability within the aquarium industry. The exact impact imposed on the species as a result of collection is unknown. By removing colonies from the reef, genetic diversity is reduced, impacting the resistance of the species to environmental change. 

F. paraancora is currently listed as vulnerable on the IUCN Red list. 


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