Cast of characters

Scleractinian corals: anatomy

Scleractinian (stony) coral polyps consist of a simple closed sac made of two cell layers separated by a sheet of connective tissue (mesoglea).

The outer layer of cells (epidermis/ectodermis) is either: (1) exposed to the surrounding seawater (in which case it is called the oral or free epidermis), or (2) lies against the calcifying skeleton (in which case it is called the calicoblastic epidermis).

The closed sac is folded in on itself to form a mouth, pharynx and simple gut (in fact, the former name for the phylum Cnidaria (greek: "nettle"), was Coelenterata (greek: "closed gut"). The simple gut has many internal folds that aid in digestion by increasing surface area. This area also houses the reproductive organs (in the mesenteries).

The inner layer of cells (gastrodermis / endodermis) is flagellated and is in contact with the coral animal's internal gastrovascular circulation system (its "bloodstream"). This circulation connects adjacent coral polyps and allows some degree of interdependence between polyps in a colony.

Dinoflagellate algal symbionts (zooxanthellae) are located in the gastrodermis (endodermis), and are contained within specialized cell vacuoles (symbiosomes/perialgal vacuoles).

Zooxanthellae: taxonomy, life history and physiology

"Zooxanthellae" (greek: "yellow-brown animal algae") is a rather vague term that refers to a group of dinoflagellates of different evolutionary origins that occur in symbiosis with marine invertebrates.

Dinoflagellates (greek: "spinning whips") are relatively basal eukaryotes that are closely related to Alveolates and Plasmodium. They have permanently condensed chromosomes (the "dinokaryon") and (at first glance) appear to be very primitive organisms. In fact, that they were originally classified as "mesokaryotes", i.e., neither prokaryotes or eukaryotes, but somewhere between the two. Its is now clear that they are true (albeit unusual) eukaryotes.

Dinoflagellates are a weird and wonderful collection of organisms: some members are autotrophic (obtain energy from sunlight, i.e., they photosynthetically "fix" inorganic carbon), while others are heterotrophic (obtain energy by eating other organisms, i.e., they "steal" organic carbon previously fixed by other organisms).

Photosynthetic dinoflagellates are even more unusual because they have unique pigments (e.g., diadinoxanthin, peridinin), and unusual photosynthetic enzymes (e.g., form II Rubisco).

Free-living dinoflagellates can occur in a nonmotile, "coccoid" phase with no flagellae, or as a "dinomastigote" phase which has two flagellae and a characteristic swimming pattern (hence the "spinning whips").

Sex has never been observed in zooxanthellae, which complicates discussions of exactly how diverse this group is, because species boundaries cannot be firmly established. We do know, however, that (even in the scleractinian corals) they are an exceptionally diverse group (see Symbiont Diversity).