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Student Project

Discosoma sp.
Mushroom Coral

Eleni Notaras 2016


Presently, Actinodiscus sp., belonging to the order Corallimorpharia, within Hexacorallia, are poorly understood taxonomically. They are commonly referred to by several different names including Mushroom Coral, Mushroom Anemones and Disc Anemones. Despite their hardiness and abundance, species are yet to be discerned from each other. Hopefully future study will illuminate distinguishable differences between these notoriously difficult to classify animals.   

Phylum Cnidaria
Class Anthozoa
Sub class Hexacorallia (Zoantharia)
Order Corallimorpharia
Family Discosomatidae
Genus Discosoma
Species undetermined

Physical Description

Members of the order Corallimorpharia are commonly referred to as skeleton-free corals because they do not secrete calcium carbonate frameworks internally nor externally (Hutchings, Kingsford and Hoegh-Guldberg, 2008). 

Corallimorpharians occur in two main polyp body shape forms. Family Sideractiidae and family Corallimorphidae have cylindrical bodies (Figure 1A). Family Ricordeidae and family Discosomatidae, to which my specimen belongs, have discoidal, plate-like, bodies (Figure 1B) (Cha, 2015).

The Discosomatidae are individual polyps, often colonial, ranging in size from 2.5 to 30cm across (Dye, 2012). “They have a broad, thin oral disc and short, featureless column. The mouth protrudes from the center of the oral disc” (Hutchings, Kingsford and Hoegh-Guldberg, 2008, p.207). My specimen, tentatively identified as Discosoma sp., has semi-translucent reddish brown colouration (Figure 2) and a white mouth (Figure 3).  

Corallimorpharian tentacles are either capitate or non-capitate. Capitate tentacles are longer than all non-capitate tentacle-types; simple, branched and dot-like, and end in a globule called an acrosphere, containing nematocysts (Fig. 4). Discoidal corallimorpharians, such as my specimen, are associated with the three defined non-capitate tentacle-types; branched, digitiform and papilliform (Figure 5). All three are non-retractile (Cha, 2001).  My specimen possesses dot-like discal tentacles (pappilliform) (Figure 6) and short, simple marginal tentacles (digitiform) (Figure 7). Many other Pacific mushroom corals lack tentacles entirely (Tullock, 1997) or they retain pseudo-tentacles.

Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7



“Corallimorpharians occur in all oceans, from polar to equatorial latitudes, and from the intertidal to at least 5 km in depth” (Fautin, Guinotte and Orr, 2009, p. 63). Corallimorpharians that exist in temperate regions or in deep-water are cylindrical in form whilst tropical shallow Corallimorphs are discoidal (eg. Discosoma sp.). 

Lagoons are shallow water environments in which Corallimorpharians frequently occur. Sands eroded from nearby reefs, containing elevated concentrations of organic materials, comprise lagoon bottoms. Corallimorphs prosper in these high nutrient conditions (Delbeek and Sprung, 2002).

My specimen was found attached to rock, in an aggregation with several other individual polyps, submerged in water several centimetres from the surface of a university aquarium, under light (Figure 8). 

Discosoma sp. is in fact widely recommended for beginner salt-water reef aquarium hobbyists due to their hardiness. For interested readers, they prefer to be positioned at the bottom of tanks where luminance is under 10 000 lux and the current is moderate (Tullock, 1997). 

Figure 8

Zooxanthellae Symbiosis

Discosoma sp. harbour zooxanthellae. Zooxanthellae are unicellular organisms capable of photosynthesis and they contribute in the supply of oxygen and other nutrients required for mushroom coral survival. Accordingly, the depth distribution of Discosoma sp. in shallow waters is critical for zooxanthellae to receive sufficient light to carry out the processes of photosynthesis (Dye, 2012). Furthermore, zooxanthellae are responsible for the colour variation in Discosoma sp. The symbiosis is mutualistic. The zooxanthellae acquire carbon dioxide and other necessary substances from the mushroom coral polyp.

Platyhelminthe Parasitism

Platyhelminthes in their parasitic form vary in appearance from light grey to brown with a stripe down the length of their back (Figure 9) (Delbeek and Sprung, 2002). Flatworm infestation can be combated either by dipping the mushroom coral into fresh water, causing the flatworms to unattach, or by vigorous shaking so not to remove the mushroom coral from its salt-water environment.

Platyhelminthes (flatworms) infestation commonly affects Discosoma sp. Worms belonging to the genus Waminoa are often found on Discosoma sp. They are not particularly harmful however in large enough numbers they prevent the zooxanthellae harboured within the covered mushroom coral polyp from obtaining light (Dye, 2012).

Figure 9

Competition with Scleratinian Corals

Results obtained from a study by Kuguru et al. (2004) suggest that increased nutrient levels favour the prosperity of Corallimorpharians and are disadvantageous for Scleratinian Corals. The shallow water environments of many Corallimorphs (eg. Discosoma sp.) are vulnerable to frequent and rapid change as a consequence of human disturbance. The high tolerance of Corallimorphs to adverse conditions provides a competitive advantage over Scleratinian corals in an unstable environment and enables their subsequent dominance, even if only temporarily.  

Life History and Behaviour


Reproduction occurs both sexually and asexually. Oogenesis and spermatogenesis, the development of egg and sperm respectively, takes variable amounts of time within Corallimorpharian genera and between species. Also, asexual reproduction may occur by different processes including fission and budding. 

Corallimorpharians are notoriously difficult to identify based on external morphology. Chen et al. (1995) conducted extensive research on the reproduction of a Corallimorpharian. At the time of study it was believed to belong to Discosoma but more recently it has been identified as Rhodactis indosinensis. Although still within the family Discosomatidae, it is classified within a different genus. 

A common mode of asexual reproduction in Corallimorpharians is longitudinal fission (Fautin, 2002). Fission may initiate at the apical or the basal end of the polyp body (Figure 10) (Bocharova and Kozevich, 2011).

Figure 10

Anatomy and Physiology

Family Discosomatidae Diagnostics

Anatomical diagnostics of the family Discosomatidae include;

- the presence of abundant complete and incomplete mesenteries, often irregularly arranged 
- very weak or absent sphincter muscle 
- weak retractor muscles 
- weak or absent basilar muscle 
- very rare or usually absent spirocysts 
(Cha, 2001)

Nematocyst Structure

Specialised cells called cnidocytes are a defining characteristic of Cnidarians. They fire in response to mechanosensory and chemosensory input. Different cnidocyte cell types include; nematocytes which are found in all cnidarian classes, spirocytes which are found only in anthozoans, and ptychocytes which are found in some anthozoans (Degnan, 2016). They play roles in defence and prey capture. Nematocyte cells house undischarged nematocysts which consist of a capsule containing an ejectable thread that is typically spiny and/or releases toxin.  

Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM), of the apical structures of cnidae from across Cnidaria, was carried out by Reft et al. (2011). In nematocysts, a third apical cap morphology, in addition to the hinged cap (operculum) and flap morphologies already documented, was discovered in nonactiniarian anthozoans. Both SEM (Figure 11) and TEM (Figure 12) apical structure data generated showed that the apical structure of nematocysts within the mesenterial filaments of Discosoma sp. is of the third cap morphology. The observed nemanocyst types were holotrich and p-mastigophore. By definition a holotrichous isorhiza is a nematocyst with an open, spiny tubule (Maggenti, Maggenti and Gardner, 2005). A microbasic p-mastigophores defines when the discharged shaft is less than 3 times the length of the capsule and has a terminal tubule (Figure 13) (Williams, 1991). Scanning and transmission electron micrograph images of the apical end of the two nematocyst types were captured by Reft et al. (Figure 14).

Figure 11
Figure 12
Figure 13
Figure 14

Histology - My Specimen

My mushroom coral Discosoma sp. specimen was fixed, sectioned and hematoxylin and eosin (H&E) stained. Hematoxylin binds to and stains cytoplasmic proteins pink. Eosin binds to and stains DNA and RNA purple. The dye made mesenterial filaments (Figure 15) and cnidoglandular bands (Figure 16) distinguishable.
Figure 15
Figure 16

Biogeographic Distribution

Corallimorpharians occur widely at latitudes in tropical regions extending to polar regions (Figure 17) (Cha, 2001). Discosoma sp. distribution is restricted to “the tropical shallow waters of the Indo-West-Pacific and the western Atlantic” (Den Hartog, Ocana and Brito, 1993, p. 4).  

Figure 17

Evolution and Systematics

Relationship to Orders Actinaria and Scleratinia

Currently, Corallimorpharia are poorly known taxonomically. The systematics of Corallimorpharia is under great contention. Corallimorpharians are an intermediate between, and share attributes with, orders Actinaria (=Sea anemones) and Scleratinia (=hard/stony corals). Multiple hypotheses of the phylogenetic relationship between Scleratinia and Corallimorpharia exist, dependent on the character on which the analysis is based on (Cha, 2015). Like Actinaria, Corallimorpharians do not possess a skeleton but the internal anatomy and histology of Corallimorpharia, including features such as mesentery, muscles and the cnidom, resembles that of Scleratinian polyps (Torres-Pratts et al., 2010). Studies based on morphological characteristics as well as gene-based studies have been executed. Based on morphological characteristics Corallimorpharia is a monophyletic, sister taxon of Scleratinia. Phylogenetic relationships inferred from the analysis of 18S rDNA disputes this proposed affinity. Instead, it suggests that Corallimorpharia is within Scleratinia (Cha, 2015). According to further research and analysis of mitochondrial and nuclear genes by Fukami et al. (2008) it seems the most parsimonious scenario is that Corallimorpharia and Scleratinia are derived from single evolutionary lineages. 

Classification of My Specimen

Molecular data has provided limited insight into the taxonomy of Corallimorpharia. It supports that Corallimorpharia can be divided into four families:
- Family Corallimorphidae Hertwig, 1882
- Family Discosomidae Verrill, 1869
- Family Ricordeidae Watzl, 1922
- Family Sideractinidae Danielssen, 1890
(Dye, 2012)

My specimen has been identified as belonging to Discosomatidae and is cautiously descrived a Discosoma sp. “The taxonomy beyond genus is very problematic (Dye, 2012, p.24)”

Family: Discosomidae Verrill, 1869
Genus Name: Discosoma Rüppell & Leuckart, 1828 
Synonymised Names
Actinodiscus de Blainville, 1830
Phialactis Fowler, 1888

Genus Discosoma has been synchronised down to 11 species from an original 42. These species are:
- Discosoma album (Forsskål, 1775)
- Discosoma carlgreni (Watzl, 1922)
- Discosoma dawydoffi Carlgren, 1943
- Discosoma fowleri (den Hartog, 1980)
- Discosoma fungiforme (Verrill, 1869)
- Discosoma molle (Couthouy in Dana, 1846)
- Discosoma neglecta (Duchassaing & Michelotti, 1860)
- Discosoma nummiforme Rüppell & Leuckart, 1828
- Discosoma rubraoris Saville-Kent, 1893
- Discosoma unguja Carlgren, 1900
- Discosoma viridescens (Quoy & Gaimard, 1833)
(Fautin, D 2012) 

Following correspondence with authorities on Corallimorphians, the species classification of my specimen remains undetermined.

Conservation and Threats

Genus Discosoma, Family Discosomatidae and Order Corallimorpharia have not been assessed for the IUCN Red List of Threatened species (, 2015). There are no conservation strategies currently in place for them and they face no known threats.

Corallimorpharians are resillient animals. The destruction of coral reef habitats may be detrimental to their survival but they are highly adaptive and can likely overcome difficult circumstances. 


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