Lab 3: Cnidaria II -- Medusozoa & Anthozoa - Department of Biology

ZOO 2203C / 6927 – LAB 3
CNIDARIA: MEDUSOZOA II & ANTHOZOA
Sections with bolded heading required, others optional for IZ, but required for AIZ.
SCYPHOZOAN DIVERSITY
CLASS STAUROZOA / ORDER STAUROMEDUSAE – Haliclystus – pickles on demonstration
Stauromedusae are unusual animals that mix the morphology of polyp and medusa stages in a single, solitary zooid
(Fig. 6). Their morphology is suggestive of the ancestral condition in the Medusozoa. The trumpet-shaped body is
attached by an adhesive disk, with which these animals attach to the substratum, typically algae or sea grasses. The
basal stalk is polyp-like, while the funnel-shaped distal part of the animal resembles a medusa. Tentacles are
clustered in 8 lobes. Between the tentacle lobes lie an equal number of adhesive organs (anchors) that are used in
locomotion (allowing the animal to inchworm along); these are viewed by some as homologous to rhopalia (see
below), as they are in the position, however they are morphologically quite different. Recent phylogenetic work
suggests that the unusual morphology of staurozoans may represent a condition before the evolution of free-living
medusae within the Medusozoa (Fig. 1).
CLASS CUBOZOA / ORDER CUBOMEDUSAE – Carybdea – pickle on demonstration
Cubomedusae are also sometimes relegated to their own class, because they possess a velarium (similar to, but not
homologous with, the velum of hydromedusae), and because their life cycle involves the wholesale conversion of
solitary polyps into medusae. This life cycle is suggestive of how medusae likely originated within the Medusozoa.
The groups name derives from the cubic, four-sided shape of their swimming bell. Tentacles arise from the four
corners of the bell, either singly (as in Carybdea – Fig. 6), or in clusters. Note the long manubrium and velarium.
Cubomedusae include the most dangerous cnidarians; some “sea wasps” (Chironex) can kill humans.
Fig. 6. Stauromedusae: Haliclystus oral view (right) (from: Kozloff, E. N. 1990. Invertebrates. Saunders,
Philadelphia); Cubomedusae: Carybdea (left; RF Myers photo)
CLASS SCYPHOZOA sensu stricto
ORDER CORONATAE
Nausithoe polyps – pickles.
Coronate medusae typically live in deep water and we have no material on hand. However we do have the unusual
polyp stage on demonstration. Coronate scyphopolyps are unusual in having an exoskeleton (unlike other
scyphistomae) and being colonial. The colonial polyps of Nausithoe (Fig. 9) are typically embedded within a
sponge, which provides added, chemical protection. These tiny polyps are well defended themselves; they wallop a
potent sting, and consequently are well known to divers on Pacific reefs. Note the four lobes developed around the
mouth giving them a characteristic appearance; the simple, fine tentacles arise along the length of this folded
perimeter.
ORDER SEMAEOSTOMAE
Aurelia – pickles and plastic mount. See above.

Chrysaora – sea nettles – pickles, live when available
Chrysaora is one of the common local scyphomedusae, with seasonal blooms on both coasts of Florida. Their
abundance and nasty stings make them a well-known hazard to swimmers and net fishermen. Note that both the
tentacles and oral arms are long – they serve as efficient fishing nets for these highly carnivorous animals.
ORDER RHIZOSTOMAE
Cassiopeia – pickles, live when available
The upside down jellyfish is a common inhabitant of calm, shallow, tropical waters in Florida and elsewhere. It is an
unusual medusa that typically rests upside down on the bottom (Fig. 9). Like other rhizostome (“root mouth”)
medusae, it is microphagous – feeding on tiny organisms captured by the enlarged and highly branched oral arms, the
tips of which open into large numbers of tiny mouths! It is also photosymbiotic and derives substantial nutrition
from unicellular algal symbionts (zooxanthellae).
Fig. 9. Coronatae: Nausithoe polyps, embedded in symbiotic sponge (left; GP photo); Cassiopeia (right; from
Brusca, R.C. & G.J. Brusca. 2003. Invertebrates. 2nd Ed..)
CLASS ANTHOZOA
Anthozoa lack a medusa stage and have complex polyps, with a large gastrovascular cavity partially subdivided by
numerous mesenteries and a pharynx (stomodaeum). This increased surface area allows anthozoan polyps to reach
much larger sizes (to 1m!) than the simple medusozoan polyps. Two subclasses are currently recognized, the
Hexacorallia, a heterogeneous group with highly variable polyp morphology and a tendency for six-fold symmetry,
and the Octocorallia, a group in which polyp (but not colony) morphology is highly conserved and polyps always
have 8 tentacles and mesenteries.
SUBCLASS HEXACORALLIA
HEXACORAL MORPHOLOGY: slides of transverse sections of Metridium, live Aiptasia.
We will use actinian anemones to introduce the morphology of anthozoan polyps. Note that anemones take polyp
complexity and size to its limits among cnidarians; some reach a meter in diameter or height, and have fleshy bodies
with large numbers of mesenteries. Start your study by noting the external morphology of live Aiptasia. Most
anemones are attached to hard bottom by a flat pedal disk and a cylindrical column, ending in wide oral disk. The
mouth is at the center of the oral disk, while tentacles encircle (as in Aiptasia), or cover, the disk. Note the radiating
lines on the oral disk of Aiptasia between the mouth and tentacles; these are the margins of the mesenteries that
partially subdivide the gastrovascular cavity. Note also that the tentacles always arise in spaces between mesenteries,
and the part of the gastrovascular cavity trapped between the mesenteries extends into the tentacle. The mouth of
hexacorals is usually compressed and slit-like, with one or two siphonoglyphs at its end(s). As a result hexacorals
tend to depart from perfect radial symmetry to biradial symmetry. Siphonoglyphs are ciliated furrows that pump
water into the gastrovascular cavity.
Study the transverse and longitudinal sections of Metridium next, with Fig. 10 as your guide. We have several slides,
and these differ substantially, so you may wish to look at more than one, as some features are clearer in some slides
than others. Note that the oral-disk invaginates around the mouth, forming a pharynx that extends down into the
column. The pharynx (or stomodaeum) is a characteristic of anthozoan polyps, absent in medusozoans. The
transverse sections we have cut across the portion of the column with the pharynx (as also illustrated in the diagram
below). You can clearly distinguish the outer epidermis, inner gastrodermis, and the sandwiched mesogloea on the

ody wall of the column. Note that circular muscle fibers run within the mesogloea, and permit contraction and thus
elongation of the column. The body wall is also visible in the pharynx, but in reverse: here the innermost layer is
epidermis, while the outermost layer is gastrodermis, as the pharynx is an invaginated structure. The gastrovascular
cavity is subdivided by a series of radiating, vertical, sheet-like mesenteries. Note that the mesenteries come in pairs
in actinians; however this is not the case in all anthozoans. The paired nature of actinian mesenteries may be
evidence that actinians evolved from corals that lost their skeleton, such that in the past, calcareous, skeletal septa
ran between such paired fleshy mesenteries, as they do in living corals (see later). Note the bulges (retractor muscle
bundles) on the paired mesenteries are arranged to either face outward on both, or inward on both. Those
mesenteries that reach and fuse with the pharynx are called complete; those that do not are termed incomplete. In
small Metridium, as featured in most slides, there are 6 pairs of complete mesenteries – demonstrating the 6-fold
symmetry of hexacorals. Note the numerous additional incomplete mesenteries of various sizes, again in pairs (these
also tend to occur in multiples of 6). Mesenteries are added in a specific order during growth, the shorter ones being
usually the most recently added. Mesenteries form as folds of the gastrodermis, with a mesogloeal core.
Study a large incomplete mesentery on the slide (Fig. 10). Note the bulging retractor muscles (absent from small
mesenteries), which can work as antagonists of the circular muscles of the column wall, with the gastrovascular
cavity used as a hydrostatic skeleton, with the animal closing its mouth tightly. You may also see developing
oocytes in more distal parts of the mesenteries, although the slides we have are largely from small, pre-reproductive
juveniles. The innermost margin of a free mesentery is thickened into a mesenterial filament. This is like the thick
cord sewn onto the edge of a curtain. The mesenterial filament is the results of three cord-like thickenings at the free
end of the mesentery in hexacorals (often appearing as a trefoil in transverse sections – Fig. 10): a pair of ciliated
bands that functions to circulate gastrovascular contents, and a cnido-glandular band that secretes digestive enzymes
and also subdues swallowed prey with nematocysts. At the base of the anemone, where the mesenteries attach to the
pedal disk, the mesenterial filament often does not terminate, but continues on, like a curtain cord that extend beyond
and dangles off the edge of a curtain. These free, nematocyst-laden mesenterial filaments are also called acontia.
They can be used to subdue prey within the gastrovascular cavity, as well as in aggressive and defensive interactions
when extruded from the body. Acontia are readily extruded by some hexacorals when they are disturbed, or when
they interact aggressively with competing neighbors or predators. They can emerge through the mouth, be forced
through the body wall, or come out through specialized pores in the body wall (=cinclides, see later).

Fig. 10. Anemone morphology: cut away of anemone (upper left); transverse section at level of actinopharynx
(upper right); transverse section of incomplete mesentery (lower left); transverse section through mesenterial
filament at end of free mesentery, with trefoil end (lower right) (from BIODIDAC)
Once you are done with the slides, go back to the live anemone, Aiptasia, to examine it further. Share animals in
groups of 4 to spare them. Start by studying the effectiveness of muscles vs. cilia by stimulating an extended animal
until it contracts using its musculature; time this response. Now time how long it takes for the animal to re-expand.
Which takes longer? Why? When you disturbed it, Aiptasia will likely discharge acontia in defense. Where do the
acontia emerge from the body? What color are they? Take a piece of acontia and place it on a microscope slide,
apply cover glass, and view under the compound scope to see the large nematocysts. By adding a drop of methylene
blue, you can get the nematocysts to discharge. You may also want to look at a tentacle snipped from Aiptasia (they
will readily regenerate it), to see the gastrovascular cavity running through it and the small (~8 μm) algal cells
(zooxanthellae) that occur intracellularly within the gastrodermis and give Aiptasia its brown color. The same algae
are the photosymbiotic partners of reef corals, soft corals, and many other anthozoans. Aiptasia is abundant in our
tanks as it buds asexually – it is actually a nuisance organism both in aquaria and in aquaculture settings (e.g. pearl
farms) because of its habit to cover everything by its rapidly spreading clones.
HEXACORAL DIVERSITY
The Hexacorallia includes stony corals, black corals, and a diversity of anemones, and are differentiated from the
other anthozoan group, the Octocorallia mainly on polyp morphology. While octocoral polyps are very conservative
in their form and size, having 8 pinnately branched tentacles and 8 mesenteries, hexacoral polyps are highly variable,
although they tend to have a 6-based symmetry in mesenteries and tentacles. The number of tentacles can vary from
six to hundreds, as a result of both a variation in the number of mesenteries and in the number of tentacles per
mesentery. Many hexacorals have but a single tentacle per mesentery and thus a ring of tentacles at the periphery of
the oral disk, while others have numerous tentacles per mesentery, covering the oral disk in a shag-carpet-like
manner. Polyps range from 1m in diameter, and may be solitary or colonial. The Hexacorallia include
two orders with well-developed skeletons: the Scleractinia (stony corals) with a calcareous skeleton and the
Antipatharia (black corals) with an organic skeleton. They also include four orders that lack a solid skeleton,
referred to broadly (sensu lato) as sea anemones: the Actiniaria, Corallimorpharia, Zoanthidea, and Ceriantharia.
Among anemones the actinians are the most diverse, both in terms of species and body form. In contrast members of
other orders are more conservative in form and thus more recognizeable. Corallimorpharians are closely related to
actinians; like actinians, but unlike other anemones, their mesenteries are arranged in pairs (a condition that may be
evidence of the ultimate origin of these groups from skeletonized ancestors). Most tropical corallimorpharians occur
in dense clonal aggregates, have very wide disks attached to often much narrower columns, and have short or even
just bump-sized tentacles arranged in rows radiating from the mouth. Zoanthids are usually clonal or colonial
anemones, with medium-sized (mostly 0.5-2cm) polyps, tentacles restricted to the periphery of the oral disk and
alternating between an inner, erect and an outer, reflexed series. Cerianthids are burrowing, tube-dwelling anemones
that usually live in sand or mud, with only their oral disk and tentacles emergent. They are readily differentiated
from burrowing actinians in the field by the possession of tubes and arrangement of tentacles in two tiers: an outer
ring of longer tentacles surrounding an inner cluster of shorter tentacles. Actinians, corallimorphs, and scleractinians
are closely related, even potentially not monophyletic relative to each other, while the other hexacoral orders are
quite divergent from these.
Order Actiniaria: Calliactis, Condylactis, Epicystis Bunediopsis, Aiptasia, etc. – live.
Actinians are by far the most diverse (1000+ species) group of sea anemones, and often considered to be sea

anemones in the narrow sense (sensu stricto). They are soft-bodied, solitary anthozoans that generally lack a
secreted skeleton, although a few can form a chitinous cuticle under their pedal disk. They resemble the three other
groups of anemones: corallimorpharians, cerianthids, and zoanthids, and are most easily differentiated by the unique
characters of these other, smaller, and more homogeneous groups (see above and below). Actiniarians have an oral
disk that bears the mouth and tentacles, a column, and, in all but burrowing species, an adhesive, pedal disk that
attaches to hard bottom. The tentacles range widely in shape, can be simple, branched, or more complex, and there
may be more than one type of tentacle in an animal. Tentacles may be restricted to the margin of the oral disk, or
cover most of it. The column may have verrucae, wart-like extensions that can be adhesive and help anchor the
column to the substratum, or adhere sediment grains to the body. Although the nematocysts of most anemones do
not affect humans, several species have nasty stings, and some readily feed on fishes foolish enough to brush against
them. The sticky feeling to the tentacles of many anemones is due to the discharge of nematocysts (try this with
some of the larger species – don’t worry, none of our pets are harmful stingers). Because of their stinging power
anemones are favored as shelter by numerous animals, including copepods, shrimp, porcellanid crabs, and fish.
Most larger anemones that live on coral reefs are photosymbiotic, harboring symbiotic dinoflagellate algae
(zooxanthellae) that contribute to their nutrition. Although actinians are solitary (with a single, recently discovered
exception), many species are clonal and live in aggregations produced by a variety of modes of asexual reproduction.
Examine the diverse anemones provided; note that some are in the large display aquaria in the back of the lab and
downstairs. Which species do you think are zooxanthellate? How many circles of tentacles do each have?
Calliactis is an unusual anemone in two senses. First, it commonly lives in symbiotic associations with hermit crabs,
attached to their shells, giving the crab protection and benefiting from their messy eating habits. Second, it has
unusual, pimple-shaped perforations on the sides of its column, called cinclides. These are specialized openings
through which acontia are extruded when the animals are bothered. Calliactis is easily irritated; watch the
demonstration of acontial discharge given by your TA. What color are its acontia?
Order Corallimorpharia: live Discosoma, Ricordea
Corallimorpharians include two rather different groups of anemones. One group (Discosomatidae & Ricordeidae) is
comprised of large, zooxanthellate, warm-water species characterized by short, disc-shaped, flaring columns and
wide oral disks, with radiating rows of short to barely bump-sized tentacles; these tend to live in large, clonal
aggregates. This group lives on coral reefs and is represented by live specimens in the lab. The second group
(Corallimorphidae) is characterized by azooxanthellate polyps sporting elongate tentacles with bulbous ends,
arranged again, in radiating rows. These are known from temperate to tropical waters, including deep water, but are
less common. Corallimorphs are so named because they resemble stony corals in their polyp anatomy, but lack a
skeleton.
Order Scleractinia: Acropora, Fungia, Manicina, Astrangia, etc. skeletons, plus live Oculina, Solenastrea,
Euphyllia, Caulastrea.
The Scleractinia are the “stony corals”. The word “coral” is applied more broadly to cnidarians with a solid
carbonate skeleton, while “stony coral” refers to scleractinians. The vast majority of corals are scleractinians, with
>1300 described species. Scleractinian corals are characterized by an aragonitic skeleton, with each polyp
occupying a cup-like depression (calice) with radiating calcareous septa – skeletal elements that lie between fleshy
mesenteries. Scleractinians can be either solitary, or more commonly, colonial. Scleractinians are the architects of
tropical coral reefs, and about half of the known species are reef builders. The rest live in a variety of habitats from
the tropics to the poles, from shallow to deep water. While reef-dwelling species are mostly photosymbiotic, high
latitude and deep water species lack symbiotic algae.
Examine live scleractininans to see the range of polyp morphologies. Note that some species have large fleshy
polyps that tend to be continuously extended (e.g. Euphyllia), while others have small, thin polyps, or have polyps
that are largely withdrawn into the skeleton during the day (these tend to extend their tentacles at night (e.g.
Caulastrea). While most corals have simple, finger-like or bulbous-tippes tentacles, others have branching tentacles
(some Euphyllia). Note the brownish and greenish colors caused by an abundance of zooxanthellae. Look at the
varied colony shapes formed by corals among the live and skeletal specimens provided, as well as the varied sizes
and arrangements of polyps. Calices are often well defined, circular, with a wall delimiting them from the
surrounding colonial coenenchyme and coenosteum (the skeletal surface underlaying the coenenchyme) (Fig. 1).
New polyps can form either by the subdivision of existing polyps, or arise de novo from the coenosteal surface. Try
to find examples of both kinds of new polyps in the skeletal material. In brain corals, new polyps tend to not lay
down walls to separate them from other polyps in their row and thus remain in a single valley. The multiple polyps
in such valleys are given away by their individual mouths, lying in series. Find the basic skeletal elements on the

coral skeleton: coenosteum, calices, walls, septa (radial elements inside the wall), costae (radial elements outside the
wall, often contiguous with septa), and columella (in center of calices) (Fig. 1).
Fig. 1. Scleractinian skeleton
Order Zoanthidea: Palythoa pickles; live zoanthids in aquarium
Zoanthids are a distinctive group of anemone-like anthozoans that have characteristic, relatively small polyps (3-20
mm diameter), with short, finger-like tentacles restricted to the periphery of the oral disk and arranged in a
distinctive, inner, erect and outer, reflexed cycle. This alteration of tentacles is clearly visible in expanded polyps
and readily identifies live zoanthids (Fig. 2). Zoanthids have a single siphonoglyph, and alternating perfect (large)
and imperfect (small) mesenteries. Polyps tend to be trumpet-shaped, with an expanded oral disk arising from a
narrower column, although some colonial species have the column embedded in coenenchyme. They lack a secreted
skeleton (except in one poorly known genus), but several genera incorporate sand grains in their mesogloea, which
makes them very gritty. Other genera (e.g. Zoanthus) do not do this, and are soft, and slippery. Zoanthids occur
world-wide, but are especially common on coral reefs, where they can carpet large areas through clonal propagation.
They are usually colonial, but a few species are solitary. Palythoa is one of the most common tropical zoanthids,
characterized by a well developed colonial coenenchyme, heavily impregnated with sand. Reef-dwelling zoanthids
always/almost always harbor zooxanthellae.
Fig. 2. Zoanthid polyp (left): note how the tentacles are alternately held up and down, as typical for the order.
Ceriantharian (right), not the two circles of tentacles; labial tentacles are marked with an arrow.
Order Ceriantharia: live and pickled Cerianthiopsis
Cerianthids are tube-dwelling anemones that differ so much from other hexacorallian anemones that some authors
separate them into a different subclass. Unlike any other anthozoan, they live in leathery tubes, woven from
discharged ptychocysts – specialized cnidae unique to these animals. They tend to live in and bury into soft

substrata, and their tubes can extend into the sediment several times the length of the animals, offering ample cover
to withdraw into. And withdraw they can: cerianthids are unique among cnidarian polyps in being able to respond
very quickly to disturbance, withdrawing with lightening speed. Onlike other anemones that have a sphincter muscle
at the base of their oral disk that can close over withdrawn tentacles, ceriantharians lack this and are unable to
withdraw their tentacles. They lack a pedal disk, all their mesenteries are complete, and they possess two tiers of
tentacles: an outer ring of longer tentacles surrounding an inner circle of labial tentacles. Examine the live
Cerianthiopsis americanus provided, observing their habit, and tentacular crown. Gently touch the tentacles to see
their rapid response. Note the elongate, vermiform shape of pickled or live animals that have been taken out of their
tubes. Place a small piece of tube material under the scope to see how they are constructed.
Order Antipatharia: dried or pickled
The Antipatharia, or black corals, are a homogeneous group of colonial anthozoans that secrete an arborescent or
whip-like, scleroproteinaceous skeleton. Although colonies superficially resemble gorgonians (see below), they can
be readily distinguished in lacking the sclerite-filled rind that surrounds the axial skeleton of that group, in having the
simplest polyps among hexacorals, with 6 simple (rather than 8 pinnate) tentacles (Fig. 3), and also by the thorny
protuberances arising from the skeleton in many species. The polyps are obvious in the field, as they cannot
withdraw into the coenenchyme, but are hard to recognize in dried specimens. Black corals range from the tropics to
temperate latitudes, and are especially common in deep water off reefs. The skeleton of large black corals is used to
make jewelry, fetches a high price, and supports a deep diving industry in parts of the tropics.
Fig. 3. Antipatharian branch showing simple, laterally compressed polyps with 6 tentacles (left); organization of a
gorgonian octocoral (right; from BIODIDAC)
SUBCLASS OCTOCORALLIA
OCTOCORAL MORPHOLOGY- Model of Corallium, whole mount slides, piece of Leptogorgia, various living
octocorals.
We will look at octocoral morphology using gorgonians as a model. Gorgonians are the most common and diverse
octocorals in our regions. They are a polyphyletic assemblage characterized by the possession of an axial tree- or
fan-like skeleton made of scleroprotein and/or CaCO3. Look at the model of an octocoral (Corallium, the precious
coral, a gorgonian with a calcareous axial skeleton) for a quick overview of octocoral organization. Note that polyp
morphology, with 8 pinnate tentacles (pinnae are paired, lateral side branches), 8 mesenteries, 1 siphonoglyph, is
rigidly conserved in this large subclass. In colonial octocorals there are masses of gastrovascular canals (solenia)
that extend from the central gastrovascular cavity, permeate the coenenchyme, and allow direct food sharing among
zooids.
Study the whole mount slide of a gorgonian branch under low magnification to get a closeup view of polyp
organization (Fig. 3). Note the pinnate tentacles. The long, thin, gut-like tube in the center of the polyps represents
the space outlined by the inner margins of the mesenterial filaments that extend lengthwise through the polyp; these
diverge and become curly below the extended polyp within a wide gastrovascular cavity. Note the solenia that
connect these larger cavities and permeate the rind. The large central rod in the slide represents the axial skeleton of
gorgonians. In some slides it has a calcarous core, surrounded by a tough, organic, scleroproteinaceous skeleton,
while other slides are of a species with a strictly scleroproteinaceous skeleton. Note that the rind also has abundant
sclerites (=”spicules”), but these have been removed by acids during the preparation of these slides.

Take a small piece of a branch of Leptogorgia, our common local gorgonian. Note that in disturbed specimens the
polyps can completely withdraw into the coenenchyme. In gorgonians the coenonchyme is usually so loaded with
sclerites that give the rind a gritty toughness. Place a small branch segment into bleach to digest the rind, and thus
expose the sclerites; mount the sclerites on a slide and examine.
Study the expanded polyps of the various undisturbed, living octocorals in the aquaria. Note that the polyps of most
octocorals can readily withdraw into the colonial coenenchyme, thus seeing extended ones is a treat – don’t disturb
these animals. Note also that the polyps in some, or parts of some colonies, are indeed withdrawn, leaving little
more evidence of their presence than a small dimple on the coenenchyme.
OCTOCORAL DIVERSITY
Unlike the relatively well-defined orders within the Hexacorallia, the classification of octocorals is less clear and is
undergoing considerable flux at present. Octocorals have traditionally been classified into three orders:
Coenothecalia (the blue coral, Heliopora), the Pennatulacea (sea pens and sea pansies), and the Alcyonacea, a large,
heterogeneous group representing the rest of the octocorals. In the past people have also subdivided the Alcyonacea
into multiple orders based on their level of colony organization. Recent molecular data suggest that both the
Coenothecalia and Pennatulacea are specialized, relatively young offshoots, and that the Alcyonacea is highly
heterogeneous, with several traditional taxa poly- and paraphyletic. In this lab we will introduce you to some of the
major groups of octocorals, without dwelling on their taxonomic relationships, for this reason we will not use
taxonomic ranks to refer to these groups. Colony and skeletal organization have been the main considerations
shaping alcyonacean taxonomy.
Stolonifera: Carijoa – live & pickles.
Stoloniferans are among the simplest octocorals; polyps arise by budding along stolons and there is little or no
colonial coenenchyme. As a result colonies resemble those of hydroids, the stolons analogous to the hydrocaulus of
hydrozoan colonies. Stoloniferans can have creeping stolons that adhere to the substratum, or can grow into tree-like
forms like Carijoa (these latter are often referred to as telestaceans). Note the simple polyps and connecting stolons.
Stolonifera: Tubipora, the organ-pipe coral.
Tubipora is unique among octocorals in having the sclerites fused into a more-or-less solid skeleton. Each polyp
occupies one of the “organ-pipes” so produced, and thus is largely separate from its neighbors. Like other
stoloniferans, Tubipora lacks a coenenchyme.
“Alcyoniina”: soft corals – pickled Lobophytum, Sarcophyton, Sinularia, live xeniids & nephtheids.
Study the diverse soft corals provided: live in our aquaria, and pickled up front. Soft corals are octocorals that
develop a large, fleshy coenenchyme (colonial tissue between polyps). Polyps in many species can withdraw entirely
into the coenenchyme. In addition to housing the polyps, the coenenchyme is loaded with sclerites, and solenia
permeate it to provide nutrition throughout colony. The coenenchyme allows soft corals to grow in a variety of
shapes: mats, massive colonies, to varied arborescent forms. Many, but not all soft corals have specialized
siphonozooids (Fig. 4), reduced polyps that lack tentacles, but have an efficient siphonoglyph; these inflate the
colony by drawing water into the coelenteron. When polyps retract, siphonozoods are recognizable by their smaller
opening than those of retracted gastrozooids, clearly visible on the surface of pickled Lobophytum or Sarcophyton.
The siphonozooids also are the reproductive zooids in those soft corals and gorgonians that have them, except sea
pens. Study the variation in growth form of pickled and live soft corals. Note also that the body of many species (all
the pickled exemplars, for example) are packed solid with sclerites, and some of these sclerites are so large that they
are easily discernible by the naked eye. Soft corals are abundant and locally dominant on Indo-Pacific reefs, from
where all the pickled as well as live forms we have came from, but are effectively absent from Atlantic reefs. The so
called Atlantic “soft corals” are gorgonians. Many reef-associated soft corals, including all those in the lab, are
zooxanthellate. New research suggests that soft corals are not a monophyletic group.

Fig. 4. Surface of soft coral with siphonozooids (Sarcophyton; left); the sea pensie Renilla (right)
Gorgonians (Scleraxonia, Holaxonia & Calcaxonia) – diverse pickles, live Leptogorgia and others.
Study the diversity of pickled gorgonians and examine live Leptogorgia. Gorgonians are arborescent (tree-like)
octocorals with an axial skeleton of gorgonin (a scleroprotein) and/or CaCO3. Gorgonians are diverse and
traditionally divided into three orders (see above), largely based on the nature of their axial skeleton, although new
research is showing that their relationships to each other and to other octocorals are more complex. Gorgonians have
a well-developed coenenchyme, like soft corals; they are effectively soft corals on a stick. As in soft corals, polyps
can generally withdraw into the coenenchyme, and they share nutrients across abundant solenia (the model you
studied earlier is of Corallum, a gorgonian with a calcitic axial skeleton). The rind of gorgonians is usually thinner
than that of soft corals and so full of sclerites as to be quite gritty. Study the diversity of growth forms among
pickled gorgonians. Take a small piece of rind from some, place in a syracuse dish, cover with bleach, then wash
with water, mount the sclerites on a slide, cover with cover slip, and examine them. Can you tell different species
apart based on their sclerites?
Coenothecalia: Heliopora – dry skeleton & photos of live colonies
Heliopora coerulea, the only definite species of this group, is the only octocoral that is a “real” coral in the sense
that it possesses a solid, CaCO3 skeleton (Tubipora’s skeleton is but weakly fused sclerites). The skeleton is built of
crystalline aragonite; the blue color derives from iron salts. Note the fine texture of the skeleton and the small
openings from which the small, typical, octopolyps emerge, and into which they can withdraw. Polyps may be
extended or withdrawn during the day. Check the photos of live Heliopora, to see the whitish polyps. Heliopora is
zooxanthellate, and is abundant on Indo-Pacific reefs, its skeleton is a major component in many reef limestones. It
is a “living fossil”: the genus has persisted largely unchanged since the Cretaceous, the age of dinosaurs.
Pennatulacea: sea pens – pickled Renilla, Virgularia, Ptilosarcus, live Renilla if we are lucky
Sea pens are the most complex of octocorals, they are a distinctive, readily recognizeable clade. Their
distinctiveness have led to their treatment as a distinct order in the past, but recent work suggests that they are related
to one particular family of gorgonians (Ellisellidae)! Sea pens have three types of zooids: the primary zooid that
forms the axis of the colony, usually with a support rod running through it (absent in the aberrant Renilla) and a rootlike
base that is anchored within the sediment. Other zooids are often organized in leaf-like branches that commonly
project laterally from the colony axis (though again not in Renilla – which has them all on a single leafy surface).
These are large, typical gastrozooids, and much smaller siphonozooids that draw water into the vast, colonial
gastrovascular cavity. The gastrovascular cavity permeats the colony, and includes the large cavity running through
the axis of the primary zooid. Sea pens live on soft bottoms, anchored into sand or mud with the root-like basal
extension of the primary polyp. Sea pens are common in a variety of environment from tropical reefs to cold waters,
and are notable as perhaps the most diverse cnidarian group in the deep sea. Study the local sea pansy (Renilla) up
close, and examine the other (more typical) genera on demonstration (Fig. 4).