7.1 Introduction

The Cnidaria is a super-interesting and diverse phylum, containing both solitary and colonial animals, such as hydroids, jellyfish, sea anemones and corals. The animals are diploblastic; they have only two true tissue types (ectoderm and endoderm, and lack mesoderm and any type of coelom (sealed, internal body cavity). Particular functions are performed by epithelial, muscular, nervous and connective tissues that are differentiated within the primary body tissues. Cnidarians have no organs made up of specialized cells (like those found in triploblastic animals), and have unique features such as: Primary radial symmetry, specialized stinging cells (enidoblasts) that contain and release stinging structures (nematocysts) [it is the stinging cells that give the phylum its name], and polymorphism (different body forms occurring within the same species either sequentially, at different stages of the life cycle, or simultaneuosly within a colony).

Figure 9.1: Basic body forms in Cnidaria. A - simple polyp without pharynx, as in Hydrozoa; B -medusa- C - anthozoan polyp with pharynx. (Redrawn from Boardman et al., 1987)

Cnidarians also have no specific excretory, respiratory or circulatory structures. Food is obtained by the tentacles, which surround the mouth and are activated by a series of muscle cells in both cellular layers. From the mouth, the food enters the gastrovascular cavity (enteron), where digestion takes place both extra- and intracellulary. Finally, undigested material is ejected through the mouth. In colonial forms, nerve cells arranged in an irregular network (generally at the base of the tentacles) within individuals and throughout the colony.

The skeletons of cnidarians can be either external or internal (exoskeleton or endoskeleton respectively), and is either chitinous (a mucopolysaccharide) or calcareous. As you can imagine, since many groups within the phylum have no skeleton, or have one that is easily destroyed, the fossil record of cnidarians is spotty. Obviously, the most abundant cnidarian fossils are the corals because they secrete massive calcareous skeletons found in rocks from the Ordovician to the Holocene.

The coral skeleton is internal, and is secreted by the epidermis at the base of the polyp. The polyp rests initially upon a skeletal basal plate. With growth, the base of the polyp forms a series of radial folds, and within each fold a radially and vertically-oriented septum is secreted. As the polyp grows, the septa extend upward and join at their outer ends to a skeletal wall, the theca, that also extends upward with the septa. As the skeleton is growing upwards, it also lays down a series of new skeletal floors at its base. These can either be single transverse plates called tabulae or a series of smaller domed plates called dissepiments. During all this growth, the polyp only occurs in the calyx (the upper, cup-shaped depression on the top of the skeleton). The skeleton of one coral unit (solitary or colonial) is called a corallum and any one polyp in a colony is called a corallite.

All cnidarians have either radial, biradial or radiobilateral symmetry. Radially symmetrical organisms seem to be well adapted to both sessile and planktonic modes of life.

Living corals require water movement to supply nutrients and dissolved oxygen, as well as remove waste products. Like the sponges, corals can not usually tolerate heavy influxes of sediment (which is one of the reasons they are found associated with one another). Modern corals are divided into two ecologic groups: hermatypic or reef-building corals that have zooxanthellae (symbiotic algae) in their gastrodermal tissues, and ahermatypic or non-reef building corals that do not have zooxanthellae.

Hermatypic corals are restricted to shallow, tropical waters because their zooxanthellae require the constant and intense sunlight found in the tropics. However, these corals can survive in water as deep as 90m and a cold as 16 degrees or less.

Ahermatypic corals are less environmentally restricted. They can be found in shallow, tropical water like hermatypics, but can also be found in deep, cold waters (6000 m, 1 degree).

7.2 Classification and Taxonomy

7.2.1 Classification

There are three extant classes of cnidaria: Hydrozoa, Scyphozoa and Anthozoa, of which the Anthozoa have the longest and richest fossil record.

Class Hydrozoa

Marine and freshwater. Polyp stage usually dominates. Both polyps and medusae are radially symmetrical with simple, undivided enteron and no pharynx.

Class Scyphozoa

Exclusively marine. Most of life cycle is spent in medusa stage. Mineralized skeleton absent. Several fossil genera are known, including some from the Ediacaran stage. The best preserved fossils come from Konservat-Lagerstatte deposits from the Jurassic of Germany. Scyphozoans are exclusively marine and dominantly planktonic or nektonic. Medusae swim by rhythmic pulsations of the bell, but large-scale migration takes place by drifting in response to winds or currents.

Class Anthozoa

Exclusively marine. Solitary or colonial. Medusa absent, only free-swimming larva (planula) intervenes between repeated (sometimes asexual) stages. Skeletons external or internal- chitinous, collagenous, calcareous, or absent. The largest extant cnidarian class (> 6000 species). The number and symmetry of the skeleton (if present), are used to divide, this class into many groups (Orders). True corals are defined as those members of this class that secrete massive external calcium carbonate skeletons.

The Tabulata were some of the earliest corals. They had calcareous skeletons with no septa, but the tubes had very obvious transverse tabulae. Tabulates (Early Ordovician - Permian) were an exclusively marine colonial group, forming reefs in the Ordovician, Silurian and Devonian.

Rugosa were contemporaneous with the tabulates (Middle Ordovician - Permian), but they were both solitary and colonial. Their skeletons were more varied and had prominent septa that were grouped into four quadrats, often separated by gaps (fossulae). Rugose corals became important reef builders during the Devonian.

Figure 7.2: Patterns of septal insertion in rugose and scleractinian corals. A to H, serial transverse sections of early-to-late ontogenetic stages in rugose corals. I and J, scleractinian corals. (Redrawn from Boardman et al., 1987)

The Scleractinia (Middle Triassic - Recent) "replaced" the Paleozoic groups in the Triassic (though there are reports of a possible Late Paleozoic origin) as the major reef builders. In fact, scleractinians have built much larger reef complexes than their Paleozoic counterparts. This is due primarily to their lighter, more porous and complex skeletons in which the septa have 6-fold symmetry and no fossulae, and elaborate skeletal elements (coenosteum) are built between individual corallites. These more efficient skeletons may hve contributed greatly to the construction of modern atolls and barrier reefs (the largest structures ever made by non-human organisms).

7.2.2 Taxonomy Phylum: Cnidaria*
    Class: Hydrozoa* (Cam-Rec)
        Order: Hydroida (Cam-Rec)
        Order: Milleporina (Tert-Rec)
    Class: Scyphozoa* (pCam-Rec)
    Class: Anthozoa* (Cam-Rec)
      Subclass: Octocorallia (K-Rec)
      Subclass: Zoantharia* (Cam-Rec)
        Order: Rugosa* (Ord-?L. Tr)
        Order: Tabulata* (Ord-Perm)
        Order: Scleractinia* (Tr-Rec)

7.3 Terminology
oral disc enteron mesogloea cnidoblast corallite metasepta dissepiments hermatypic	mouth epidermis basal disc nematocyst calyx (calice) mesentery fossulae ahermatypic	tentacle gastrodermis pharynx corallum pro(to)septum tabulae epitheca growth form (e.g.massive, branching)