Cnidaria Part 3: Corals

Just a short introduction to corals today, I am heading off on a field trip this afternoon, so writing this just before I leave.  As it is a botany field trip, I may be blogging about plants as well as animals over the next few days.

Corals are in the same class as Sea Anemones, so Anthozoa (plant-animals), in the phylum Cnidaria.   We are all familiar with them from images such as the ones below

Tree Coral (Dendronephthya), from

Pillar coral (Dendrogyra cylindricus). Image from wikipedia

When I first saw corals, I thought they were rock formations on the sea floor, and indeed, some of them are very sharp, due to the Aragonite (calcium carbonate) they are made of (calcium carbonate is commonly found as chalk, or limestone, or limescale in the kettle, and can have different hardness depending on the way the crystals in the rock form).  Later, when I heard they were living, I assumed they must be some sort of underwater plant.  The reality is stranger than I possibly imagined, and I still have problems wrapping my head around it, even though I understand how they function, I still look at them and think “That cannot be how they work”

There are two types of coral, Hermatype or stony coral, and these build reefs, and Ahermatype corals which do not build reefs. The posts on corals will mostly be about the Hermatype coral, and Ahermatype corals will be covered under their other names (Sea Pen).

Corals have been around a VERY long time, with fossils recorded in the Cambrian period (this was approx 542 to 488 million years ago), although most existing fossils of coral are from later periods, the Ordovician (488-433 million years ago), and the Devonian (468-359 million years ago).  There is some evidence for soft corals in the Pre-Cambrian period, although whether these are corals, or polyps (one of the juvenile phases in Cnidaria) is debated, see Anthozoa Fossil Record for more details

Coral Fossil, possibly a tabulate coral (From UCMP Berkeley)

Rugosa (horn coral) fossil, from UCMP Berkeley

Next post will be about how corals are formed, and cover their life-cycle, and then later posts will be about feeding, and the different types of coral within each reef, the symbiosis they have with other organisms, and their place within the oceanic ecosystem (What they do in the ocean, and how they benefit the other organisms which live there), as well as the problems facing corals today in our oceans.  (See, I said there would be a lot of posts).