This Fast-pulse Xenia is commonly called "Xenia
elongata," and ranges from being a long-stemmed
and dark brown variety in lower light, to lighter bodied
and more compact forms under brighter light, as seen
here. While the validity or accuracy of nomenclature
for Xeniids may be unclear in the hobby, our fascination
and love for these magnificent corals is not. Photo
by Anthony Calfo.
Soft corals of the family Xeniidae, also
known as "Pulse corals," have enjoyed favor and
popularity among aquarists since their introduction to the
hobby. The sensitive nature of shipping specimens, and the
subsequent challenge of procuring healthy colonies, has only
enhanced their allure. Above all, however, it is the living
fireworks of pumping polyps that makes some of these soft
corals so exciting to keep and grow (see a short video here).
The reason(s) why species or individuals pulse their polyps
is not entirely clear, but some possible explanations will
be explored here. The propensity for many in this family towards
pulsatory function is an endearing, if not hypnotic, attribute.
Yet, even the non-pulsing specimens have a distinguished appeal
for their vigor and rhythmic motion with proper water flow.
There is a wide range of appealing Xeniids to be found, from
the common "waving hands" to the rare and most unusual
species not yet established in the hobby. And while the commonplace
and so-called nuisance growth of some colonies might seem
to take a little of the shine off of their allure, there are,
nonetheless, few soft corals so individually beautiful or
fascinating to keep.
The Who's Who of Xeniidae
Let's first address some of the familiar
members of this family, and a couple of others that we hope
to see become established in the hobby. Aquarists will recognize
the genera Xenia and Anthelia as the two most
common (or commonly cited) imports. Xenia (Lamarck
1816) species are hallmark "Pulse corals" that are
usually stalked, and sometimes branching, with polyps restricted
to the cap/crown (capitulum). Xenia polyps may be long,
will often pulse, but are never retractile; their polyps will
contract (shrink), but never actually retract into the cap/crown.
The morphology and color of Xenia species (and those
still unproven specimens that we call by a given species name)
is variable, but popular "types" include: Xenia
elongata (the common, brown "Fast-pulse" Xenia),
Xenia umbellata (white "Pom-pom" Xenia),
and an iridescent blue-green species that strongly resembles
X. elongata (AKA "Silver-tip Xenia").
Hobbyists hold many more varieties in collections, from nappy,
yellow, Red Sea clusters to red-hued Indonesian colonies,
and numerous other members of the genus found in colors ranging
from cream to green and through to dark brown. With perhaps
more than 60 species in this genus, Xenia are widely
distributed from the east coast of Africa through to the central
Pacific. They are generally found in clear, bright, shallow
waters with moderate to strong water flow. Although most Xenia
are not common from turbid or dirty waters, they will colonize
early upon stressed or damaged reef areas resulting from pollution
and natural disaster. A closer look at their physiology reveals
that Xenia have weakly developed structures for organismal
feeding; nutrient uptake of dissolved matter is conducted
in this heavily photosynthetic genus. Target feeding of Xeniids
is not required (if it's even practical or possible) to cultivate
them successfully in aquaria when there is an adequate supply
of nutrients available otherwise (bio-load of fishes and other
One of the attractive white Xeniids known as "pom-pom"
Xenia, or "Xenia umbellata,"
can be adaptable but fares best under moderate to bright illumination
(with slow acclimatization). Photo courtesy of Greg Rothschild.
Once acclimatized, most Xeniids are fast
growing and may even be considered invasive under certain
circumstances. It stands to reason that such successful species
are readily consumed by dedicated corallivores and casual
browsers of cnidarian tissue. Typically "reef-safe"
fishes like tangs will often nibble Xeniids
characters like angelfishes will often make a beeline straight
for a new colony placed in the tank! It's a good idea to establish
Xeniids separately, as in refugia, up to several weeks in
advance for their safety before introducing them to the main
display. Since they do not feed significantly on large solid
matter/plankters, they are quite safe to keep in a (coral)
food-producing refugium with little burden to the functional
benefits of the vessel. In fact, Xeniids are sometimes used
as "animal filters" much like macroalgae with "vegetable
filters" for nutrient export because of their fast growth,
salability at harvest, and for their negligible imposition
on most other desirable life forms in refugia.
Untold numbers of unfamiliar Xeniids enter the trade as incidentals
found on live rock or
with other reef invertebrates. With some basic lab
tools and techniques, you may be able to
identify such guests with help from a good scientific reference
like Fabricius and Alderslade's,
Soft Corals and Sea Fans (2001). Photo courtesy of
Photo courtesy of James Fatherree.
Anthelia (Lamarck 1816) specimens
are as common, if not more so than Xenia among imports
or rather, the genus Anthelia has been categorically
a better shipper with regard for morbidity and mortality in
this notoriously sensitive family. As such, one can usually
find or order Anthelia successfully from a local merchant
that receives livestock from the Pacific (Indonesia or Fiji,
for example). Members of this genus are handsome in uniform
colors with some variability in morphology. Regrettably, they
lack the dramatic pulsatory habits of their kin, and at best
will only twist or curl pinnules or tentacles in what may
appear to be a sometimes deliberate fashion. Anthelia
are very easy to identify and are unlike any other common
member of this family (excluding the newly erected genus Sansibia
*see below). Polyps
do not rise from stalks, branches, or summits, but rather
grow from an encrusting stoloniferous web or mat (something
like Briareum "Star polyps"). Anthelia
polyps are never retractile and are only slightly contractile.
The natural distribution of this genus is widespread in the
Indo-Pacific and they can occur at greater depth (usually
below 20 meters) than Xenia (generally less than 20
meters). As such, they are quite adaptable to aquarium life
including low or moderate lighting schemes. Once established,
they are characteristically fast growing and suitable for
beginners. Anecdotally, aquarists have noticed occasional
or even seasonal bouts of "self-destruction" when
colonies boom and then suddenly crash and dissolve. At such
times, these colonies usually disburse fragments that often
settle and give rise to new colonies elsewhere. I am not aware
of any concise data that has definitively explained this phenomenon
(aquarium induced or mirroring natural events), and theories
range from the crossed threshold of a nutrient-dependant
critical mass, or stress-induced, to a deliberate reproductive
strategy. By any measure, though, Anthelia are generally
easy to grow and control and can be heartily recommended to
aquarists of all skill levels.
"Giant Anthelia" is one of the hardiest and most
under-rated Xeniids in the hobby. This wonderful
is attractive, weakly aggressive and easily controlled.
When allowed to grow into great patches,
it makes a stunning impression with the action of its swaying
Photos courtesy of James Fatherree.
*Alderslade established a new and
similar looking genus, Sansibia in 2000. Data on this
genus is presently limited, but some pictures of Sansibia
look grossly similar to Xeniid varieties known from the aquarium
trade. Sansibia is noted as having high concentrations
of zooxanthellae and occurring in turbid waters.
Could this be Sansibia? Many odd little soft
corals are acquired as incidental growths on rock and
with other collected invertebrates. Wishful aquarists
like myself retreat to the scientific and hobby literature
to try to find a name for such surprise guests. Identification
by image alone, however, is impractical and unrealistic
for most any coral - to the genus level, let alone species.
I think I can hear my dear friend Eric Borneman weeping
in a corner as I declare that this must be Sansibia
because I just bought a new book with a picture that
looks just like it! And for our next trick, lets rename
all of the Acroporids in our tanks because Charlie Veron
came out with a new book series, shall we? Photo by
Heteroxenia are also observed regularly
in the trade, although they are not readily distinguished
from Xenia by most aquarists. They share gross physical
traits with Xenia and cannot readily be discerned by
most hobbyists from Xeniid kin until colonies mature and form
siphonozooids (the small secondary polyps between larger polyps).
Frankly, since most specimens are collected at immature sizes
or traded as young divisions, few specimens are mature enough
to be distinguished from Xenia in the aquarium hobby.
In their natural habitat, Heteroxenia are found in
calmer, back-reef niches and may occur in muddy or turbid
waters. Their distribution is wide in the Pacific with most
imports hailing from Fiji and Indonesia for the American trade,
and from the Red Sea for Europe. Practical experience in propagating
Heteroxenia by imposed measures has led some to believe
that this genus is less forgiving than others in the family
towards cutting techniques to produce divisions. For such
colonies, it may be better to simply encourage fast growth
and wait for natural division to occur. Slower and gentler
techniques of propagation (e.g., adjustable ties or rubber
bands, as is done with Klyxum Colt corals) is recommended.
Heteroxenia colonies are typically white, cream or
light brown, but their color cannot be used to distinguish
species or identify to genus reliably, of course. For causal
aquarium keeping and conservative farming of the genus (pinching,
constricting or simply waiting for fission) the exact distinctions
between Xenia and Heteroxenia are perhaps not
For many discriminating reef aquarists,
specimens of Cespitularia have been some of the most
sought after corals of any kind. The reasons for their allure
are many, and among Xeniids they have some of the best of
all desirable attributes in the family (re: rarity, color,
visage). They have a "look" (morphology) that is
distinguished and unique. In gross form they resemble stalked
Xenia with a size and structure inclined to grow rather
larger than Xenia. To some they are also reminiscent
of the zooxanthellate Nephtheid "Tree corals." Their
polyps are not restricted to the cap/crown like Xenia,
however, but also grow from the stalks of the colony, although
these polyps tend to be limited, as a specimen matures, to
the upper portions of the colony. Perhaps the most exciting
thing about Cespitularia is their remarkable visage.
I dare not even say "color," because their overall
look is one of translucent and oft-stunning, iridescent quality
- making quite an impression on aquarists! New imports and
stressed individuals will lack bright color or any significant
opalescent quality, but once established under quality lamps
or natural sunlight, they take on a remarkable appearance.
Much of the excitement is due to the tiny calcareous sclerites,
which appear to reflect light and make the coral sparkle or
glitter. Colors range from subtle tan and peach hues with
green tinged polyps to stellar, solid blue and green colonies.
Inspecting Cespitularia in the aquarium at night with
a flashlight reveals a metallic silver appearance. Alas, photographs
capture very little of the ethereal qualities of these corals
and aquarists must see them in the flesh to truly appreciate
them. The few fragments that enter the aquarium trade have
been cited as hailing from Indonesia or East Africa. The natural
range of this genus is very wide, though, throughout the Indo-Pacific
and Red Sea, with specimens recorded in both clear and turbid
waters. They are further observed to favor shallow and wave-protected
environments. In aquaria, provide them with bright light and
moderate, random turbulent or surging water flow
laminar water motion.
This attractive and fast-pulsing Xennid is commonly called
"Silver-tip" or "Blue" Xenia for
its magnificent tendency to brighten with strong blue-green
color when kept under cool colored lamps (10-20k Kelvin).
Photos courtesy of James Fatherree.
Other, much less common Xeniids do appear
in the hobby on occasion. Treasures such as Efflatounaria
may go unnoticed or mistaken for another coral. Without pulsatory
function, some morphs of Efflatounaria bear a gross
resemblance to the common "Colt coral," formerly
Cladiella and Alcyonium and now assigned to
the genus Klyxum. They are unique Xeniids that are
generally "furry-fingered" and branching in form.
Colors range with attractive varieties observed in yellow,
blue-green and some simply brown hued. Savvy aquarists in
aquarium clubs have spotted and actively propagated these
gems. If the aquarist is fortunate enough to come across such
special corals, be sure to actively fragment and share divisions.
Even this tiny fragment of Cespitularia has begun
to show its telltale irridescent glimmer as light is
reflected off of tiny sclerites. With strong VHO blue
or 20k Kelvin Radium lamps, for example, they often
turn a stunning solid blue color - hence the legendary
name "Blue Xenia." They are one of
the most highly sought after of all Xeniids. Photo by
Aquarists have also been tantalized by
magnificent Xeniids, from Australia and elsewhere, called
Sympodium. Images have depicted a most unique color
and morphology in striking, blue-white with blunt stoloniferous
creeping, nubby "fingers" (dense, fully retractile
polyps). Considering how fast-pulse Xenia elongata
evolved from ultra-rarity to being a well established "weed"
in the hobby in a scant decade, we can only hope that other
unique Xeniids like Sympodium, Sansibia, Efflatounaria
and Cespitularia will soon follow suit.
These photos, taken in Menjangen National Park, Northwest
Bali, show Sympodium exposed at low tide.
Photos courtesy of Eric Borneman.
Courtesy of Joseph Weatherson.
As mentioned, not all Xeniids exhibit pulsatory
function. But even the subtle, twisting motion of the tentacles
or the furling and unfurling of pinnules on less active species
is a matter of great fascination for aquarists. Even for the
seemingly inactive species, the simple execution of normal
polyp cycles (expansion and retraction or contraction) is
inevitably a perceived measure of health. As an aside, it
is instead primarily influenced by water flow for many corals
at large. And pulsatory function in performing Xeniids is
not a reliable measure of health, if it is any measure at
all. To be clear, we can all agree that the activity is a
biological expense to the animal. Can we then fairly surmise
that the net benefit gained from the activity exceeds the
debt? And regardless
why do they do it? Most
information we have on the matter is purely anecdotal, although
not wholly insignificant for the sheer number of colonies
kept and observed by aquarists at large. It seems that more
conclusions have been drawn illuminating why Xeniids do not
pulse or fully express their polyps instead.
Some of the most pervasive theories for
this activity have revolved around light theories. Some suggest
that pulsatory function is a means to temper excessive light.
Others believe, on the contrary, that it is a means of improving
exposure on the limited surface area of slender tentacles
and pinnules, in contrast to their better-exposed and pigmented
base, stalk and branches. It's been reported that the tentacles
and pinnules of Xeniids on average can have ~ 100X less zooxanthellae
than other tissues on the animal (Janes, 2003). Some hobbyists
have interpreted this, erroneously perhaps, in support of
a correlation between light and pulsatory function. Xeniids
are also some of the most successful cnidarians in symbiosis
with zooxanthellae and seem to derive the overwhelming majority
of their "nutrition" from the products of photosynthesis
(thriving in controlled culture systems without feeding of
any solid matter). That is to say, they do not feed like "hungrier"
corals, lacking developed digestive structures to do so.
One of the most amazing things about most Xeniids is their remarkable range of reproductive strategies.
A new colony can be formed from fragments as small as a single
pinnule! Infected Xenia with necrotic
stalks and captitulums ("crowns") can still be salvaged
by snipping off tentacles, and even the feathery
pinnules, to start new colonies elsewhere. Photo courtesy
of Amy Larsan (Tippytoex).
Still, the optimal cultivation of zooxanthellae
supported by the pulsing of polyps does seem at least plausible
to some folks, even if untrue. We could also consider and
compare the high density of zooxanthellae in Xeniid tissues
overall with that of other familiar corals; they share similar
densities with the likes of Poritids and Faviids, and they
have greater densities than the Pocilloporids and most Acroporids!
This is certainly very telling about their strong autotrophic
tendencies. Their response to photoperiods reflects this nature
in kind. They are highly adaptable to a wide range of light.
You will notice that Xeniids placed at depth or under weak
illumination will often stretch to spread out their tissues
and subsequently their zooxanthellae for better opportunity
to catch dim light. Conversely, over-illuminated Xeniids (barring
actual light shock or photoinhibition) will contract early
in the day to shield their tissues. In such cases, it is tempting
to say that pulsatory function has stopped directly because
of excess light, but the truth here may be that the cessation
is merely a symptom of multiplicity - a more complex dynamic
at hand. But pulsatory cessation is only one response by the
coral to an excess of light and is not directly correlative,
nor will it necessarily happen every time or to the same extent.
This is underscored by the fact that Xeniids often pulse,
and also cease to pulse, during nocturnal periods of time.
For some colonies, such activities are equally conducted both
day and night. To laymen like myself, it begs the question
why a Xeniid would endure the biological expense of pulsing
at night if it is driven by light (which I do not believe
it is, personally)? I certainly do not know the answer to
this question but hope at least to spur contemplation of the
phenomenon in others with these lines of exploration and anecdotes
of aquarium husbandry.
Other influences, to varying degrees, on
Xeniid health and polyp activities, have been recorded in
the annals of reef husbandry: control of water temperatures
and water quality (oxygen, pH and buffering ability). Temperature
is a very straightforward issue with this family; they are
more sensitive to high water temperatures than most common
corals: a reality all too tragic and "fragrantly"
familiar to importers forced to contend with rotting masses
of mishandled Xeniids. Although they may tolerate a slow climb
from comfortable tropical temperatures in the 70's F to the
low 80's F, a sudden spike of more than 3 or 4 degrees F,
particularly into the mid 80's or higher, can often prove
to be fatal. There are several serious aspects to this. The
first and most obvious concern is the decrease in dissolved
oxygen at higher temps. Beyond stress to the system and other
animals at large, corals suffer by the thickening of the anoxic
microlayer that surrounds their body, by virtue of the nature
of fluid dynamics (a relationship that is underestimated too
commonly in reef aquaria with poor water flow). A coral can
"suffocate" from such increases in the anoxic microlayer
of water that surrounds them. The most common example of this
is illustrated by the poor rates of survival for this family
in shipping. In shipping bags, with no water movement aside
from the rough handling of boxes in transit, the dynamic of
decreasing oxygen levels and an increasing microlayer around
the coral is amplified. The stress causes mucus to build and
the mucus affords the proliferation of bacteria. The bacteria
at first may not necessarily be pathogenic, but rather become
so as they proliferate and mucus continues to increase. Note:
when a sick, injured or stressed Xeniid succumbs to an infection,
it is often fast progressing and highly infectious to other
healthy Xeniids in the system and some other corals too. These
afflictions are sometimes nicknamed a "meltdown"
or "brown jelly" infection. This suffrage is mitigated
by the fact that Xeniids have so very little skeletal mass
or tissue by weight. Thus, a seemingly minor stress or injury
can quickly become morbid or even fatal for the lack of dense
and resistant tissues. The spread of an infection can be fast
and thorough in aquaria. Hobbyists foolish enough to add fresh
Xeniids without a proper quarantine have often suffered severe
losses in their systems for the transgression and underestimating
the highly infectious potential of newly acquired specimens.
There is also the common belief that Xeniid
polyp pulsatility is influenced by stable and properly elevated
pH and alkalinity/mineral hardness of the water. It may simply
be that, like the lighting theories described above, the cessation
of pulsatory activity is complex and not directly correlative:
a function of multiplicity, merely influenced in part by depressions
in pH or mineral hardness, for example. Nonetheless, aquarists
have observed with consistency time and time again that established
and vigorously pulsing Xeniid colonies will often cease pulsing
suddenly and en masse when a certain threshold for
pH or alkalinity is crossed. I personally have observed the
phenomenon many dozens of times over the course of a decade
with large colonies of Xenia elongata in my propagation
facilities. With digital pH meters on growout systems, I could
watch entire pools with hundreds of mature colonies abruptly
stop pulsing when the pH dropped below ~ 8.3, as per the calibration
of my instrument(s). When I would dose calcium with caustic
calcium hydroxide later, thereby raising the pH, they would
resume pulsatory function promptly. Now even if this phenomenon
proves not to be directly correlative, a proper alkalinity
of 8-12 dKH and a stable pH in the range of 8.3-8.6 would
seem to be more conducive to the health of captive reef invertebrates
living in an already compromised environment overall. And
the sensitivity of this family of corals demands stable water
quality beyond issues of specific polyps expressions addressed
here. Nonetheless, it is interesting food for thought and
fodder for more disciplined aquatic scientists to consider
and explore with hopes of answering these questions for us.
At any rate, enjoy those lovely pulse corals.
Photo courtesy of Graham Gregorich.
Science Editor's Note:
Dr. Yehuda Benayahu
is among those with a great deal of knowledge on the Xeniids.
He began a talk to an audience of aquarists with the statement,
"Please don't ask me why they pulse. I don't know."
The point is that no answer to pulsatility has been conclusively
demonstrated. Anthony mentions a number of points that are
indeed anecdotal to the behavior. I would like to expound
on a few points.
As mentioned in the
article, the Xeniids have dramatically reduced feeding apparatus.
In particular, they almost totally lack the ability to capture
prey or particulates, and their mesenteries are reduced to
the point where intercoelenteric digestion is rudimentary
to non-existent. However, they are capable of dissolved nutrient
uptake directly across the epidermal tissue surface. Pulsatility
has been suggested to be related to this ability. In strongly
coordinated pulsing, the contraction movement is much stronger
than the movement of the relaxation extension, and this results
in a net efflux of water through the colony. In other words,
water is drawn from around the colony, through the colony,
and outwards from the center of the colony. This has been
hypothesized to be related to the facilitation of dissolved
nutrient uptake. It also correlates well with anecdotal observations
of many Xeniids that display a coordinated and strong pulsing
in nutrient poor tanks and a cessation of pulsing in high
nutrient tanks. Of course, there are exceptions, as Anthony
mentions in the article.
has been found to be affected by a number of other factors.
This is a coordinated neuromuscular response, and the pulsing
can vary from single pinnule bending or flexing, to isolated
uncoordinated pulsing, to rhythmic, coordinated, forceful,
colony-wide pulsing. The behavior requires ATP, a cellular
energy source, and without adequate energy, pulsing cannot
occur or may occur in a less vigorous manner. Furthermore,
the effects of various agents on pulsing has been demonstrated
rather comprehensively in Red Sea Xeniids almost fifty years
ago by H.A.F. Gohar in laboratory experiments in Ghardaqa,
Egypt. He used a variety or stimuli, including electricity,
drugs, temperature, and chemicals to determine their effect
on pulsatility. He found that some stimulated and some inhibited
pulsatility, as might be expected from a neuromuscular response.
Interestingly, in light of Anthony's discussion of temperature,
is that coordinated pulsing took place in a range of temperatures,
with the extremes of the temperature treatments (both hot
and cold) causing inhibition or cessation.
Likewise, I think
many of the anecdotal observations in various tanks relate
to any number of these type factors. Pulsatility is not determined
or controlled by one factor, but can be affected by many factors,
some of which may or may not be the case in individual aquariums.
Gohar, H. A. F. and
H. M. Roushdy (1956). "The neuromuscular system of the
Xeniidae (Alcyonaria). I. Histological." Publications
of the Marine Biological Station Ghardaqa (Red Sea) 10:
Gohar, H. A. F. and
H. M. Roushdy (1959). "On the physiology of the neuromuscular
system of Heteroxenia (Alcyonaria)." Publications
of the Marine Biological Station Ghardaqa (Red Sea) 10: