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All Fire corals are calcareous hydrozoans
and belong to the genus, Millepora. The other calcareous
hydrozoans, including the azooxanthellate lace corals, species
of Stylaster and Distichopora, are closely related
to Millepora, but are put into a different taxonomic
group. Being hydrozoans rather than anthozoans, Millepora
are not true stony corals, although their abundance does allow
them to be a major contributor to reef structure and sediments.
Fire corals are found in tropical seas, worldwide, predominantly
on reef crests and in shallow waters subject to high water
movement. For an unknown reason, they are conspicuously absent
from the reefs of Hawaii. Rapid proliferation by fragmentation
allows rapid growth and encrustation of Millepora on
the windward reef crests. Upright branching and vertical plating
growth forms are found in areas removed from severe wave action.
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Millepora
sp. in Ken Gosisnki's aquarium. These corals thrive
in strong light and water flow in the aquarium.. Photo
courtesy of Ken Gosisnki (mustang).
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This
Distichopora sp. resembles Fire coral, and is
related to them. However, it does not have zooanthellae
and lacks the potent sting. Photo by Eric Borneman.
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Unfortunately, the taxonomy of the Fire
corals is in need of revision and is not particularly useful.
There are at least 48 reported species of Millepora
throughout the world. However, because of inter-specific variations
of form and color due to different lighting intensities, water
movement, and other environmental factors, it is not known
how many distinct species are present. In the Caribbean, three
types of Fire corals predominate: a branching or crenelated
form (M. alcicornis), a plate-like or flat-topped form
(M. complanata), and an encrusting or box-like form
(M. squarrosa). Similar morphological types are found
in the Indo-Pacific regions, although the regional differences
are more varied, and more species (both classified and unclassified)
are reported from these oceans. Commonly seen Fire corals
from these regions include encrusting, clavate, blade-like,
upright, and branching calcareous growth patterns. A behavior
common to encrusting Millepora species worldwide is
a tendency to completely encrust living sessile organisms,
particularly gorgonians. The result is a shape that is often
taken to be representative of the Millepora species,
when in fact it is representative of the shape of the encrusted
object. Given the varied growth forms of these hydrozoans,
it may be surprising to learn that the living animals are
so similar.
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A
digitate or branched form of Millepora that very
much resembles the Australian Millepora sp. in
a photo below. However, this is one of the other three
species of Millepora in the Caribbean, M.
alcicornis. Here, it is found in protected waters
of a seagrass covered lagoon at the rhomboid reefs of
Belize. Photo by Eric Borneman.
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Another
Caribbean species of Millepora, M. complanata,
shown here in its plastic-like growth form, beginning
as an encrusting colony and forming upright vertical
fronds. This is often the most common Millepora
in the Caribbean and is found in exposed, as well as
protected, areas. Here it is found on a patch reef near
Portsmouth, Dominica. Photo by Eric Borneman.
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One
of the three Caribbean species, this is the most easily
identified Millepora species, M. squarrosa.
It is named for the squarish indentions on its surface.
Its color is nearly always pinkish, and it tends to
be found in areas where it very much resembles coralline
algae. Upon touching, however, it is very apparent that
it is not coralline algae. Photo by Eric Borneman.
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Anatomy and Behavior
Although they are more closely related
to the typical hydroids that produce jellyfish, the Fire corals
certainly superficially resemble stony corals. They are calcareous,
although their skeleton, unlike that of stony corals, almost
completely encloses the living polyps. Despite the often-textured
pattern of the skeleton, the actual skeletal surface is quite
smooth. Upon close examination, tiny pinholes, or pores, can
be seen scattered across the surface, or coenosteum. Understandably,
Millepora means "many pores." There are three
types of pores dotting the coenosteum: gastropores, dactylopores,
and ampullae.
The gastropores contain the gastrozoids,
or feeding polyps. These polyps are "short and plump,
containing from four to six tentacles that are reduced to
nematocyst knobs (Hyman, 1940)." The gastrozoids are
completely retracted into the skeleton, and rarely emerge.
If seen at all, they tend to form a white fuzzy film over
the coral surface. The gastrozoids are connected beneath the
skeleton in a network or canal system (secreta) connected
by plates, called coenosarcs which allow nutrient movement
within the colony. That nutrient is provided predominantly
from the capture of small planktonic animals. The coenosarcs
and coenosteum are areas of Fire coral heavily laden with
zooxanthellae. While these symbiotic algae may be found elsewhere
on the coral surface and within the living polyps, the concentration
around these areas greatly enhances the ability of the polyps
to meet their energy requirements in a most efficient manner
since this is the area most constantly exposed to sunlight.
Approximately 75% of the daily carbon required by Millepora
is provided by its symbionts.
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The
surface of a Millepora skeleton, lacking the
corallites of stony corals. The pores that are visible
are mainly those housing the hair-like feeding polyps,
the dactylozooids. Cyclosystems are not present on this
specimen. Photo by Eric Borneman.
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The most often visible structures of Fire
corals are the short, thin, hollow, potent stinging tentacles
of the mouthless dactylozoids. Dactylozoids, looking like
fine transparent hairs, are very important functionally; these
nematocyst-laden tentacles provide for both the corals' defense
and their primary method of food capture. They are also a
primary means of aggressive takeover of territory occupied
by other species, along with the overshadowing and crowding
out of nearby competitors through rapid growth. Although sometimes
randomly scattered across the surface, there is often a pattern
of five to nine dactylopores that surround each gastropore.
This arrangement is called a cyclosystem, and it serves to
increase the efficiency of nutrient transfer by allowing the
surrounding feeding tentacles to flex and bring prey to the
central gastrozooid. The dactylozoids are equipped with three
types of nematocysts: stenoles, isorhizas and the unique macrobasic
mastigophores, found only in Millepora species. Because
of this limited distribution, macrobasic mastigophores are
the definitive characteristic used in their taxonomic classification.
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The
many varied forms of Millepora sp., shown here
as skeletons. Top left, M. complanata;
top right, M. squarrosa; bottom left,
M. dichotoma; bottom right, M. alcicornis.
Photo by Eric Borneman.
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The identification of Fire corals by color
is not always easy. They tend to look like dead corals, since
there is no visible tissue, fleshy polyps, or mucus. Furthermore,
there are no defined cups (corallites) as found on the true
stony corals (scleractinians). However, all species tend to
have a characteristic white edge or tip that serves to warn
the unwary that these corals are not to be touched. Blue coral
(Heliopora coerulea), related hydrocorals such as the
Lace corals (Stylaster spp. and Distichopora
spp.), and several other true stony corals may have similar
white edges or smooth surfaces. Some common colors seen in
Fire corals are cream, brown, green, yellow, and purple, but
mustard brown is by far the most common variation.
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A
bladed growth form of Millepora sp. in an atoll
lagoon at 20m depth, Sulawesi, Indonesia. Here, large
fields of isolated colonies were the predominant coral
on the sandy sloping bottom. This area receives tremendously
fast water flow during tidal changes. Photo by Eric
Borneman.
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For the most part, reproduction of Fire
corals takes place sexually. Asexual reproduction by budding
results in small medusae with four or five nematocyst knobs
being released by the colony from the third pore type, the
ampullae. The medusae die within hours of free life, but only
after sex cells are formed and released. The union of sperm
and egg results in free swimming planulae, which soon attach
to a substrate to form new colonies. Fragmentation of the
main colony is another possible method of asexual reproduction,
and most imported specimens arrive as pieces broken off a
larger colony.
Captive Care
Fire corals have gained popularity in reef
aquariums, although they are not commonly offered and many
aquarists who have them, find that they spread quickly and
can be difficult to control. Intense light and high current
are necessary for maximum growth rate and survival. The hair-like
tentacles will extend night and day to feed on plankton, but
can be completely retracted into the skeleton if needed. In
fact, waving an object over a colony will often cause complete
retraction of all stinging hairs, and will thus eliminate
any potential for the dreaded "burn" of Fire coral.
Given their capacity to sting, it may be surprising to observe
how many fish and shrimp take refuge among the recesses and
branches of Fire coral. Hawkfish, because of their "skinless"
pectoral fins, are often found perched high atop Fire coral
colonies with utter disregard for the potentially dangerous
nematocysts. For humans, however, the sting can be quite painful
to sensitive skin. Yet, touching these corals with the hand
or fingers usually does not cause any burning sensation, since
the nematocysts cannot penetrate thicker skin.
Because there are no visible living polyps,
it can be difficult to ascertain the health of a specimen
in a tank. Other than through growth and a visually maintained
robust color without bleaching or algal overgrowth, little
else may be apparent to ascertain the corals' health. Color
changes may be related to improper light intensity or rapid
change in light or water conditions. Because they possess
zooxanthellae, proper acclimation is as important with these
hydrozoans as with any stony coral.
Well equipped to capture prey, these corals
will accept foods offered to them. However, prey sources are
generally very small because of the tiny openings into which
food must pass to reach the polyps. In this regard, they are
not more or less difficult to care for than most scleractinians.
In terms of predators and disease, Fire corals are very resistant
to attack. Some fireworms (Hermadice carunculata) have
been reported to graze on Millepora, although they
prefer other corals. Other potential predators of Millepora
include specialist nudibranchs of the genus Phyllidia, and
filefishes from the genera Aluterus and Cantherhines.
It Burns, It Burns!!!!
Although it can be quite painful, a sting
from Fire coral is rarely dangerous unless accompanied by
an allergic reaction or anaphylactic shock. In fact, the most
serious effects seen after extensive stings are possible nausea
and vomiting for two to three hours afterwards. The sting
caused by these animals is a result of the injection of a
water-soluble, heat affected, proteinaceous toxin. The discharged
nematocysts cause small welts on the skin with red lesions
around the raised areas. Swelling, blisters, and pus-filled
encystations may occur soon after being stung. However, all
symptoms generally disappear after 24 hours. If stung, treatment
consists of a breakdown of the protein by soaking the affected
area in hot water, swabbing the welts with vinegar, or applying
a paste of meat tenderizer. After initial treatment, topical
anesthetics may be applied to ease the burning sensation.
It may also ease suffering to repeatedly issue forth numerous
expletives in a loud voice.
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A
digitate, or branched form of Millepora sp. on
a protected shallow reef flat on the Great Barrier Reef,
Australia. This particular species had an extremely
strong sting, as I discovered from the pain of an exposed
ankle accidentally brushing against a colony. Photo
by Eric Borneman.
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Several people have contacted me after
having had itching or welts remaining on the skin for up to
several weeks after being stung. If the venom is not quickly
inactivated, it acts much as poison ivy does, causing a Type
IV delayed hypersensitivity reaction. The rash can generally
be treated with topical antihistamines like cortisone or Benadryl.
Other complications that may arise are likely related to any
cuts or scratches that may accompany the sting. As with all
surfaces underwater, and especially corals, bacteria abound
on them. Numerous species that are known human pathogens or
capable of causing infection exist on these corals, and areas
that become reddened, fail to heal, or red streaks visible
under the skin are among a few symptoms that would indicate
infection. In such cases, medical attention is imperative.
I am not a physician, and offer the advice above based on
many direct experiences with fire coral stings. Anyone should
seek proper medical advice after marine injuries, and not
depend on the treatment advice of untrained persons.
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This
Millepora sp., originally acquired from Larry
Jackson, shows the encrusting nature of the coral. The
original fragment was on a small square block seen in
the lower center of the colony. It has since encrusted
an entire rock and several smaller coral fragments.
I keep it on a separate rock to help prevent it from
spreading further, although it is not likely this will
work in its current position. Photo by Eric Borneman.
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Conclusion
Fire corals are a very natural addition
to any reef tank, and will thrive if given proper conditions
for their success and growth. The biggest problem with Millepora
in the aquarium is the quick growth and ability of the corals
to overgrow and encrust surfaces, with the subsequent difficulty
of controlling or halting such growth and the loss of specimens
which have been encrusted. Even if a careless brush with these
corals can result in some unpleasantness, there is no real
risk to most other tank inhabitants, such as fishes. In fact,
the shelter for many creatures afforded by the colonies provides
them with a natural safe haven. Because of their prolific
nature and abundance in the wild, it seems almost inappropriate
not to have a small colony as part of an indoor reef crest.
They are often dominant hermatypes, easily propagated by fragmentation,
and abundant so that little to no impact on wild populations
occurs from wild collection. I am a big fan of Millepora
in aquariums, just so long as my surfaces remain far away
from its surfaces!
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