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.
Polyps
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).
Tissues
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).
Sclerites
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
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
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.
