Evolution & Systematics

Phylum Cnidaria:

Cnidarians are radially symmetrical around the oral-aboral axis and are known as the flowers of the sea. Their most distinguishing feature is the presence of stinging cells called Cnidocytes. These stinging cells can be so potent that they can cause fatalities in humans, for example, the box jellyfish. Most cnidarians are carnivores that catch prey. There are both polyp (sedentary) and medusae (free swimming) forms. Cnidarians have true muscles, skeletons and a nervous system.  

The phylum of Cnidaria can be split up into many classes, the number of which is currently under debate. These classes include Anthozoa (corals and sea anemones), Cubozoa (box jellyfish, sea wasps), Scyphozoa (true jellyfish), and Hydrozoa (Portuguese man of war and firecorals).

Class Anthrozoa:

Anthrozoa is the largest cnidarian class with over 6000 species. This is the only cnidarian taxon to have all three types of cnidae which include nematocysts, spirocysts, and ptychocysts. Most anthrozoans can retract their tentacles and oral disk. A majority of anthrozoans are opportunistic feeders which means that they wait until a prey item swim or drifts into contact with their tentacles. Gas exchange and excretion occur via diffusion across the body including the tentacles. Gonochorism and hermaphroditism are common in anthrozoans. Reproduction can occur via fragmentation, budding and asexually through spawning.   

Order Scleractinia:

Scleractinians are known as the ‘stony corals’. They are the largest anthrozoan taxon with 3600 species. They produce a calcium carbonate exoskeleton. They can be solitary but the majority of scleractinians are colonial. Many corals have a mutualistic relationship with zooxanthellae which results in the exchange of nutrients between the two species. The zooxanthellae receives excess CO₂ and protection from the coral, while the coral receives N, C, and up to 50% of its energy needs from the zooxanthellae.  

There are limited studies on the evolution and speciation of scleratinian corals (Van Oppen et al 2004). However, it is believed that the scleractinian skeleton originated between 240 and 288 million years ago (Medina et al 2006).

Family Acroporidae:

Acroporidaes are commonly named staghorn corals. They are distinguished by the corallite present at the tip of each branch of coral (Madl 2001). Acroporidae corals are dominant reef building corals. They vary in colour, size, and shape, and are usually found in the Pacific and Indian oceans. All Acroporidae species are hermaphrodites (WoRMS 2012). Some Acroporidaes undergo fragmentation but most species are broadcast spawners.  

Genus Montipora:

Montipora is the second most species-rich coral genera found in Indo-Pacific reefs, numbering at 75 species (Vernon and Wallace 1984). It is currently unknown how old this genus is. Reports have speculated that this genus may be as old as the Oligocene of the Tethys, roughly 30 million years ago (Frost 1997), or the Eocene of the central Indo-Pacific (Gregory andTrench 1916).  

Species M. digitata:

Interestingly, M. digitata is more closely related to a species outside of its own genus, Anacropora, than a species in its own genus, Montipora aequituberculata (Van Oppen et al 2004).    

*Unless otherwise stated, all information above is adapted from Ruppert, E, Fox, R, Barnes, R 2004, Invertebrate Zoology, 7^th edn, Brooks/Cole Cengage Learning, Belmont, USA.