INVERTEBRATES: CORALS

Corals – animals with radial symmetry

Phylum Cnidaria, Class Anthozoa

Corals are important components of many marine ecosystems, both ancient and modern and hundreds of fossil corals are held in the Geology Museum collection. Members of the phylum Cnidaria (sometimes called Coelenterata) are multicellular, generally tentacle-bearing, mostly marine animals that possess stinging nematocysts for defence and for capturing food. Their chief diagnostic feature is the possession of radial symmetry.

Cnidarians are divided into three classes: Hydrozoa, Scyphozoa (including familiar soft-bodied jellyfish), and exclusively marine Anthozoa which includes sea anemones, corals, sea fans and sea pens.  Corals may be solitary or colonial – most New Zealand fossil corals are solitary although a few rare examples of colonial fossils from southern Zealandia give evidence for much warmer sea temperatures at various times in the distant past.

The oldest fossil corals known from New Zealand are of Ordovician age. A few examples in the Geology Museum collections are from rocks of Permian and Triassic and Jurassic age, but the majority are from the mid Cenozoic.

These belong to several different groups of corals, mostly scleractinian or stony corals, but with many examples of octocorals which are a diverse but not well-studied group.

The scleractinian or stony corals have skeletons composed of calcium carbonate. Each coral consists of a central cup which provides the home for the soft polyp, a surrounding wall for protection and an arrangement of radial septa which differ from genus to genus. The septa function to provide additional support and to increase the surface area for food digestion.