The South Atlantic Bight
Octocoral Morphology

Gorgonacean Bauplan

List of Species

published version (DeVictor & Morton, Zootaxa 2599)
see this for keys

Occurrence Table

Notes on the Species
Carijoa riisei
Scleranthelia rugosa
Telesto fruticulosa
Telesto nelleae
Telesto sanguinea
Bellonella rubistella
Pseudodrifa nigra
Nidalia occidentalis
Iciligorgia schrammi
Diodogorgia nodulifera
Titanideum frauenfeldii
Muricea pendula
Thesea nivea
Bebryce cinerea
Bebryce parastellata
Scleracis guadalupensis
Leptogorgia hebes
Leptogorgia punicea
Leptogorgia cardinalis
Leptogorgia virgulata
Leptogorgia setacea
Leptogorgia euryale
Viminella barbadensis
Renilla reniformis
Sclerobelemnon theseus
Stylatula elegans
Virgularia presbytes

References Cited

Suggested Reading/Viewing


Octocoral Morphology

A brief overview of octocoral morphology is provided here for reference.

The Octocorallia (=Alcyonaria) are anthozoan cnidarians with polyps bearing eight pinnate tentacles and eight complete septa. Most species have a skeleton or tissue containing calcareous sclerites, and an axis that is horny or calcified to varying degrees.

Colony form
Octocoral colonies worldwide take on many forms, such as highly branching, encrusting, whip-like, feather-like, fleshy or even completely calcareous structures resembling their scleractinian (stony coral) counterparts. Many of the cost common octocorals in the SAB have a branching colony form, so octocorals here are visualized as such. The types of branching displayed by octocorals in the SAB are illustrated in Figure 2. In some species, branching (or lack thereof) easily distinguishes species. Non-branching colony forms found in the SAB include whip-like, clavate, encrusting, leaf-like, club-shaped and pen-like (Figure 3). Growth form of the colony is affected by the environment (Bayer 1961) and is often variable between localities.

Although there is one octocoral species that exists as a solitary polyp, all of the species in the SAB are colonial and contain multiple polyps. Colonies that have one type of polyp are termed monomorphic; colonies that have two types of polyps are called dimorphic. Some octocorals are trimorphic and quadrimorphic, but none of these are found in the SAB. Some terminology used here may differ among groups. For instance, when referring to polyps of the monomorphic Alcyonacea, the term ‘anthocodia’ is often used, while the terms ‘autozooid’ or ‘siphonozoid’ are used when describing the dimorphic Pennatulacea. The octocoral polyp is divided into the anthocodia, which is usually the visible portion that can extend and retract, and the anthostele, which is the extension of the gastrodermal canal into the coenenchymal mass. The polyps have eight tentacles which generally have pinnules, finger-like projections that serve to increase surface area. The tentacles house stinging cells and, in some species, symbiotic algae. Some groups, such as the Plexauridae, have a strong armature of sclerites that form a crown and points (Figure 4). Octocoral polyps may be: a) contractile and capable only of pulling the tentacles into the basal area of the polyp, leaving the crown exposed or b) retractile, in which the entire anthocodia can be retracted into the calyx.

In the Pennatulacea, the colony form is actually a modified primary polyp that gives rise to the peduncle (stalk) and rachis, which bears the secondary polyps (autozooids and siphonozooids) (Figure 5). Other groups have secondary polyps arranged in leaves (Figure 6).

The outer layer of polyp tissue in contact with the external environment is the epidermis which contains various specialized cells, such as nematocysts. The inner tissue layer, gastrodermis, lines the gastrodermal cavity, pharynx and eight mesenteries. The coenenchyme is the tissue surrounding the axis and includes the calyx. This tissue is perforated by many solenia, canals that transport fluids between the polyps. In species that have a spiculated axis, the coenenchyme is often referred to as the cortex (Figure 8).

A sclerite is the term that encompasses all calcified microscopic elements embedded in the coenenchyme of octocorals. More specific names for each shape are capstan, spicule, spindle, double head, rod, radiate, plate, club, etc. Bayer et al. (1983) provided images that represent the different morphologies of over 150 types of octocoral sclerites.

With the exception of few species, sclerite morphology plays an important role in the classification and identification of octocorals. Bearing that in mind, it is sometimes difficult to identify specimens based on sclerites alone because there is some degree of variance within species populations, and visualizing the sclerites can be challenging without a compound or scanning electron microscope. Specimens such as the Gorgoniidae and some members of the Plexauridae have small sclerites that need to be viewed under at least 100x magnification to properly examine the features. Sclerite morphology often varies within colonies, with different forms present in various layers or regions of the coenenchyme and polyps. In some cases it is not necessarily the morphology of the sclerites that is important but the aspect ratio or comparative lengths or sclerite types that distinguishes the species, making a measuring device important during examination. Lastly, there are species in which the orientation of the sclerites within the soft tissue (such as the tentacles) is important for proper identification, so examining live or freshly dead specimens is helpful.

Axis morphology is a character that separates octocorals (specifically the Alcyonacea) into suborders, although the taxonomic significance of the suborders has been diminished (Bayer, 1981, Fabricus and Alderslade, 2001) since earlier works. Most octocoral species in the SAB have an axis, which may be spiculated, horny, or calcified, but they are limited to only five of the eleven families. The Plexauridae and Gorgoniidae have a horny axis that has a hollow, cross-chambered inner core, and a dense outer core (composed of gorgonin) which has varying degrees of loculation (Figure 7). The Anthothelidae have an axis that consists of densely packed sclerites that are often very different than the cortical sclerites (Figure 8). The axis of the Ellisellidae is solid, calcified and radially patterned.
Species without an axis, such as true soft corals, depend on hyrostatic pressure to maintain their shape, forming what is called a hydroskeleton. Other species without an axis may have dense spiculation that allows for a rigid form.

Color is rarely a diagnostic characteristic and is often highly variable, but many octocorals are noted as being vibrantly pigmented. Because their sclerites have pigment incorporated during their accretion, octocorals often maintain some or all of their color after preservation, even after being bleached. Although many of the octocorals in the SAB do not have symbiotic zooxanthellae, this association is also attributable to some octocoral pigmentation and is not retained after preservation.

A bauplan of gorgonacean octocorals and a glossary are provided for reference.


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