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 invertebrates, etc.).

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… and dubious 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.

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.

*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 Anthony Calfo.

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 too important.

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… avoid laminar water motion.

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 Anthony Calfo.

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.

Xeniid Behavior

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.

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.

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.

Additionally, pulsatility 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.

Eric Borneman

Editorial References:

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: 63-81.

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: 91-144.

Bibliography:

Borneman, Eric, "Aquarium Corals," T.F.H. publications (2001)

Calfo, Anthony, "Book of Coral Propagation, Vol. 1," Reading Trees publication, (2001)

Fabricius and Alderslade, "Soft Corals and Sea Fans," Australian Institute of Marine Science (2001)

Janes, Michael "Effects of Closed Systems on Xeniidae Soft Corals: Form, Composition, and Function" (2003)