Wednesday, January 16, 2013

Corallimorphs (Phylum Cnidaria: Order Corallimorpharia) of Singapore

Corallimorphs (phylum Cnidaria, class Anthozoa, subclass Hexacorallia, order Corallimorpharia) are solitary animals with radially symmetrical bodies, resembling sea anemones. However, as many species reproduce asexually by cloning, many individuals can usually be seen together, sometimes covering huge areas. They usually have an upturned mouth (which may or may not be surrounded by tentacles or pseudotentacles), unlike most sea anemones which have inward turning ones. Corallimorphs are sometimes called mushroom anemones, as they somewhat resembles mushroom with their wide oral disc sitting on a body column. Their internal anatomy, however, are more similar to those of the hard corals, even though they do not secrete a hard skeleton.

When exposed during low tide, the oral disc may retract into the body column. The body column houses a stomach (coelenteron). They do not have an anus, and hence the mouth performs both functions of ingesting food and removing waste. They have a specialised foot called a pedal disc to attach themselves to hard surfaces, and sometimes, buried in soft substrates. Sometimes, they may reproduce by cloning, forming clusters or even carpets of many individuals. Like other cnidarians, they possess explosive, harpoon-like cells called cnidocytes. Each cnidocyte contains a secretory organelle (cnidae), which can be a nematocyst that discharges a harpoon-like stinger carrying toxins, a ptychocyst that discharges sticky substances, or a spirocyst that discharges lasso-like threads. Hence while cnidocytes are often called "stinging cells", they do perform other functions apart from stinging.

Corallimorphs can be filter feeders (feed on plankton) or carnivores (using their tentacles to sting and paralyse small animals). Many corallimorphs also harbour the unicellular algae, zooxanthellae, which produce food through photosynthesis and share with the host corallimorphs, in return for shelter and nutrients (waste products of the corallimorphs). Sometimes, you may find tiny brown patches on the corallimorphs, which are actually acoel flatworms. They are believed to feed on the organic particles trapped by the mucus of the host animal.

The corallimorphs of Singapore are very understudied, and hence there are no proper published records of them as yet. Fortunately, I was able to get some information from Dr Daphne Fautin (thanks so much, Dr Fautin!) who studies both sea anemones and corallimorphs. Apparently, the shallow water tropical species are generally still not very well-studied and more work need to be done to sort out the genera. These are some of the corallimorphs that I have photographed in Singapore far.

A) Family Corallimorphidae

Members of this family lack zooxanthellae. The oral and pedal discs usually roughly similar in diameter, and hence the body column appears cylindrical. Every tentacle usually has a bulb-like structure at the tip.

Corallimorph (Corynactis sp.)
Corynactis spp. have long tentacles with a bulb-like structure at the tips. Dr Fautin has recently synonymised another genus, Pseudocorynactis, with this genus. As they generally lack zooxanthellae, they feed actively, either on plankton or small animals.

Corallimorph (Corynactis sp.)
The above image features a small cluster of Corynactis sp. The smaller ones are probably the clones of the bigger one.

Corallimorph (Corynactis sp.)
Sometimes, Corynactis corallimorphs can also occur in bigger clusters. When the tentacles are retracted, they appear very much like a cluster of sea anemones.

Corallimorph (Corynactis sp.)
Some Corynactis corallimorphs have bright colours, such as the orange ones above.

B) Family Discomatidae

Members of this family harbour zooxanthellae. The oral disc is much wider than the pedal disc, and hence they have a mushroom-like appearance. Many species have two types of tentacles: discal tentacles covering the oral disc, and marginal tentacles lining the edge of the oral disc. Sometimes, the marginal tentacles may be absent for some species. The discal tentacles are usually arranged in radial rows. They are often gregarious.

Most of the shallow-water species from this family are placed under the genus Discosoma. Dr Fautin had advised that more probably need to be done to sort out the genera.

Some resources that I have found proposed that Discosoma spp. generally have reduced marginal tentacles or none at all. The tentacles are extremely short, and there is no tentacle-free zone (i.e. an area near the edge of the oral disc with no tentacles at all).

The following corallimorphs that I have seen appear to fit the description above, but we certainly need the experts to look at them more closely to confirm the identity.

This species appears to have really short bumps for discal tentacles, and no obvious marginal tentacles.

This species has short, bulb-like pseudotentacles, and no marginal tentacles.

This species has very short, furry discal tentacles and somewhat reduced marginal tentacles.

This species has very short, finger-like discal tentacles. The marginal tentacles appear reduced in some parts, but appear well-developed in others. As such, I am not sure if it fits in here as well.

Another genus covered in some papers is Rhodactis, which usually have longer, usually branching, discal tentacles and finger-like marginal tentacles. They have no tentacle-free zone on the oral disc as well. Here are some of the corallimorphs I have seen which appear to fit the description, but again, we need the experts to look at the real animal to confirm their identities.

This species has relatively long, branching discal tentacles with white tips, and finger-like marginal tentacles.

This species has a furry appearance with brown branching tentacles and finger-like marginal tentacles on a bluish oral disc.

This species has somewhat relatively fatter branching tentacles and finger-like marginal tentacles.

This species also has long branching tentacles and finger-like marginal tentacles, but it appears that the tentacles can retract into the oral disc. This is weird as in some of the papers I have read, it was mentioned that corallimorphs of the family Discomatidae cannot retract their tentacles. Not sure if this paper was updated, or the above species is not from this family. The other features appear to fit the description rather nicely though.

I came across some resources on the Web which has identified specimens very similar to the ones above, with ridges and valleys on the oral disc, as Metarhodactis sp. Again, we will need the experts to look at the specimens to confirm the identity. According to some papers, Metarhodactis spp. have reduced marginal tentacles or none at all, while the discal tentacles may be bump-like or branched.

  • Cha, H. 2007. Systematics of the order Corallimorpharia (Cnidaria: Anthozoa). University of Kansas. pp. 83–85.
  • Erhardt, H. & D. Knop. 2005. Corals: Indo-Pacific Field Guide. IKAN-Unterwasserachiv, Frankfurt. 305 pp.
  • Fautin, D. G. 2011. Corallimorphus niwa new species (Cnidaria: Anthozoa), New Zealand members of Corallimorphus, and redefinition of Corallimorphidae and its members. Zootaxa, 2775: 37–49 .
  • Fautin, D. G. 2011. Hexacorallians of the World. Retrieved Jan 8, 2013, from
  • Fautin, D. G., T. Zelenchuk & D. Raveendran. 2007: Genera of orders Actiniaria and Corallimorpharia (Cnidaria, Anthozoa, Hexacorallia), and their type species. Pp. 183-244 In: Zhang, Z.-Q. & Shear, W.A. (eds) Linnaeus tercentenary: progress in invertebrate taxonomy. Zootaxa, 1668: 1–766.
  • Ruppert, E.E. and R.D. Barnes. 1991. Invertebrate Zoology (International Edition). Saunders College Publishing. U.S.A. 1056 pp.
  • Sprung, J. Aquarium invertebrates: mushrooms, elephants ears, and false corals: a review of the Corallimorpharia. Advanced Aquarists. Pomacanthus Publications.
  • World Register of Marine Species. 2012. Retrieved Jan 8, 2013, from

No comments: