Pavona of the Desert by Eric Borneman September 1997 Aquarium.Net
Pavona of the Desert
(or... What Up with Those Polyps?) ...and some interesting findings on growth patterns
By Eric Borneman
family : Agariciidae genus: Pavona common species : cactus, decussata, varians, venosa , and perhaps 49 others common names : cactus, leaf, lettuce, plate, fronded, potato chip reef friendliness : 6 (1-10)
I like Pavona .
Sounds like a children's story. <g>
They are often hailed as the "beginners SPS" corals. This is not without good reason, as Pavona are really quite hardy in the aquarium and are quite beautiful and unique.
The genus Pavona consists of a large number of corals which are quite diverse in terms of their growth patterns and morphologic variations. The name comes from the Latin word, Pavo, meaning peacock. The skeletal pattern may, in some regards, be patterned like peacock feathers. They are a member of the large family, Agariciidae, which also contains the genera Pachyseris , Leptoseris , and the Caribbean Agaricia , among others. Each of these genera are composed of truly fascinating corals. It is all I can do to not talk about all of them with equal enthusiasm. However, Pavona is the topic du jour, so off I go into the fascinating world of agaricids.
These corals are not common in the areas many SPS corals are found...that is, they are more abundant in lagoons and protected areas where their propensity to form plate-like formations is enhanced. The average lighting where the majority of these corals are found is about 40% of surface level irradiance. Pavona species commonly found available within the reef aquarium hobby are normally leafy or plate-like (foliaceous) species, although submassive, columnar and encrusting types may also be seen. Pavona species can be found throughout the Indo-Pacific region, including Australia, the Red Sea, the Eastern coast of Africa, and even the Western coast of Central and South America.
I have given Pavona a fairly low reef friendly rating. This is for a number of interrelated reasons. First, almost all specimens to the trade are broken off large mother colonies. Secondly, while common, few are abundant species. Third, areas like lagoons are often the first areas affected by landbased activities, agricultural runoff, etc. Fourth, it is not a tremendously fast growing coral. However, Pavona is quite disease resistant and can recover from such breakage easily. Furthermore, such breakage is part and parcel of natural forces and can be thought to be relatively non-invasive. Finally, the overall success of Pavona species in captivity has made them an easily fragmented and successfully captive-bred coral. While it is certainly to everyone's advantage to support captive propagation as these type corals are inherently more adaptable to aquarium conditions and are non-destructive to natural reef systems, wild harvested Pavona is still a beautiful, unique and welcome addition to reef aquariums. Its availability is still comparatively limited, and should not be any immediate threat to collection areas at this time.
In terms of its skeleton, the crowded Pavona corallites are a bit odd. They have poorly defined walls consisting of mere indentations formed by thickened septo-costae. These are interconnected between corallites, spanning the walls and hillocks from center to center of the corallites, giving the surface of the coral a linked or star-shaped appearance. They have a columella rising from the center and the unique polyps arise from both sides of the leafy plates. Thus, they are termed bifacial. Calices are round to oval, with sometimes raised hillocks occurring between them. These are termed collines. Individual colonies of Pavona form huge stands of bold frilly colonies. They are most commonly seen in shades of brown, green, pink, gray, and yellow, although variations may occur. Contrasting shades of brown seem to be the most common in the aquarium trade. Occasionally, the margins of these corals may be somewhat paler.
In the aquarium, Pavona are quite hardy and resistant to disease. The members of other family members are subject to white band and black band diseases in nature, and Pachyseris (at least) is subject to Rapid Tissue Necrosis. I am unaware of Pavona reported with either of these diseases in the wild, although black band has been induced in studies and it seems likely that they can occur. In my experience, Pavona has proven quite resistant to RTN in the aquarium. Although they should be acclimated properly, they can tolerate a somewhat broad range of lighting conditions. According to Riddle, they require 160 -1,340 mE m s of light in the wild. Therefore, most well illuminated reef tanks will be able to provide such levels throughout most of the tank. Nonetheless, these corals are almost exclusively phototrophic, depending almost entirely on their zooxanthellae to meet their daily energy budget. In fact, I have found that these corals bleach rather quickly if left unexposed to light or if exposed to heavy sedimentation for even a relatively brief period. This tends to confirm their heavy dependence on their zooxanthellae. Feeding in these corals is somewhat limited to more heterotrophic species. This attribute is very common to this family. Pachyseris , in fact, has no polyps at all and depends entirely on photosynthesis and passive (possibly ciliate) transport of organic material into its tissue. Normal dietary input to this coral consists of protozoans, bacterioplankton, and microphytoplankton.
Perhaps most distinctive are the polyps of Pavona . Although Veron reports that only the polyps of P. explanulata are extended during the day, this is not true in captivity. As is found most often, the reverse if often true in our aquariums and Pavona positively bristles with polyps during the day. My personal colonies are expanded day and night. At such times, it is apparent where this coral gets its common name of Cactus. Tiny tapered needle-like polyps blanket the living surface of the coral, imparting a most fuzzy and spiny appearance. Even if offered tiny particulate food, the tiny polyps can not be visibly observed to feed. Perhaps more astounding are the sweeper tentacles. I hate to use the word "cute," when describing the coral equivalent of long range ballistics, but what else can one say? Thin transparent sweeper tentacles form regularly on Pavona , and in surprising density. These threads are remarkably long in comparison to the polyp size, although still rarely longer than an inch or two in total length. They are especially prevalent when Pavona is subjected to moderate to strong water flow, and they usually from along the margins of the coral. Still, I have seen patches of sweepers forming for no apparent reason in the center of a leafy plate, swaying.....cutely.... and horizontally in the current. It is notable that sweeper tentacles in most corals, while classified as an "aggressive" structure, are normally used in a defensive manner. That is, to prevent the encroachment of neighboring corals onto "home turf." Pavona is no exception to the coral gangland. In fact, I have noticed an interesting phenomena. A year ago, I fragmented and glued a small branch of Montipora digitata very close to a small piece of Pavona decussata . Over time, both coral encrusted a firm base, and began spreading over the substrate while they both sent up their vertical growths. The zone between the bases narrowed over time, and soon they were in close contact. As expected, the Pavona began to overwhelm the usually subservient Montipora and the classic "war zone" between adjacent colonies became apparent. A thin 1/4 to 1/2 inch barren zone existed between the encrusting bases and the Pavona has continued its advance toward the Montipora . Yet, no sweeper tentacles have ever been observed in this area. However, the vertical fronds of the Pavona regularly sport sweepers that sway in the current at its upper edge. Interesting. Another interesting observation is the occasional appearance of small bubble-like herniations of the coral tissue on the surface. I am entirely unsure of the significance of these areas, but they do not some to be detrimental. In fact, they seem to appear when the coral is looking particularly glorious. I would really appreciate hearing from anyone who has knowledge of the significance of these small almost spherical tissue areas that look like buds ready to detach from the coral, but never quite release.
Perhaps even more interesting is the effect that water movement seems to have on the growth pattern of at least some foliaceous Pavona colonies. These corals, as previously mentioned, are most prevalent on protected reef slopes and lagoon-type areas. In other words, in areas protected from strong surge and wave action. The growth pattern of leafy plates should make this apparent. Interestingly, the bifacial nature of Pavona indicates several things. First, that the coral is highly dependent on maximal surface to volume ratios that can meet its energy needs. Deeper water corals like some Agaricia , Turbinaria , Leptoseris , and others which are noted for forming leafy plates are often unifacial, exposing as much horizontal surface to capture the available light. To grow vertically would lessen the surface area available to direct sunlight. So Pavona must form these patterns to adapt to ambient conditions. Secondly, vertical plates must be relatively susceptible to breakage in times of increased hydrodynamic drag, as severe water movement would cause the plates to break. It was no surprise, then, that when I placed a specimen of Pavona cactus in my tank, that I was especially careful to place it so that the currents would effectively move most of the polyps on the broad plates. This is no easy task, since a well washed plate side shields the "other side" from our typically laminar water flows. Thus, the edge of the Pavona was placed so as to receive the majority of powerhead flow. Over time, I have noted what would appear to be a growth pattern that has been determined almost exclusively to the principles of hydrodynamics.
Although Pavona grows quickly in size compared to many massive corals, it does not have the blinding growth of some of the other SPS corals. Yet, its growth is methodical and pronounced. Over time, the colony has pout forth several new fronds, and enlarged its plates noticeably. Without exception, all the primary growth has been in a direct line with the main water flow. This has served to reduce hydrodynamic drag on the coral, allowed for maximal flow of water over the polyps, and therefore the highest rates of calcification. It has also grown more on the trailing flow side of the colony than on the leading edge where water flow has been the greatest. This would correspond exactly to what ambient conditions might be in its natural habitat. It also corresponds to growth which contributes to the success of the colony as a whole. Once again, water flow is shown to be of paramount importance in coral growth and success.
However, there is a hitch to my observation. In nature, this same species forms elaborate frilly colony with plates that grow off in all directions. They do not consist of the few sided plates that our small specimens consist of when we purchase them. Therefore, what is the adaptive significance of such growth forms? In theory, the many sided, frilly growth forms of Pavona colonies serve a similar purpose. In sheltered areas, water flow is often minimal. And yet, good flow is important to rid the coral of waste, and to bring it nutrients. If one looks at the hydrodynamics of water flowing over folded multi-sided formations, this growth would favor turbulence...in effect, creating a maximal amount of random mixing currents within a colony where such currents do not exist by force alone. The bifacial nature of the colonies allows for maximal light benefits. The colonies, as a whole, are compact and sweeper tentacles develop from the margins of the plates, effectively warding off any encroachment from the outside perimeter. The colony is protected, isolated and grows to best succeed in the niche environment in which it is found. Fascinating. Thus, we see that nature has provided for the evolution, yet again, of an organism highly developed to maximize its survival in a coral jungle.
Until next month,
Eric Borneman (EricHugo@aol.com)
Riddle, Dana. 1995. The Captive Reef. Energy Savers Unlimited, Inc., Harbor City, Ca. pp. 29, 184-7.
Veron, J.E.N. 1986. Corals of Australia and the Indo-Pacific. University of Hawaii Press, Honolulu: 317-61.
Wood, Dr. Elizabeth M. 1983. Corals of the World. T.F.H. Publications, Ltd., Neptune City, N.J., pp. 90-1.
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