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Issue: June 2014

Mangroves in the Wild and in the Home Aquarium (Full Article)

Author: Richard Aspinall


Photographer: Richard Aspinall
Salt-tolerant plants that are as unusual as they are beautiful, mangroves make an interesting and highly functional addition to a brackish or even fully marine aquarium.

   Salt water presents many challenges for plants. For the majority, having their roots surrounded by salt water would mean a rapid death. Not so for mangroves, that collection of woody species that have not only evolved to tolerate salty conditions, but have also adapted over time to thrive across the tropics.

   Mangroves are perhaps the best known of the halophytes (literally “salt lovers”). They are astoundingly successful thanks to a range of remarkable evolutionary developments and survival techniques that have made them common across the tropics and, of course, increasingly seen in home aquaria, where their adaptations, structures, and simple elegance appeal to many aquarists.

   Around 100 species are recognized as being mangroves, though only a few are from the Rhizophoraceae, the family typically regarded as the mangroves. Mangroves are best defined by how they live, rather than by their phylogeny. A better definition might be that mangroves are a grouping of woody plants from several families and genera that grow as trees in saline conditions. Based on this “working” definition, a number of species from genera that contain non-mangroves also fit in this category.

   In my experience, it seems that the most common species aquarists are likely to come across are the red mangrove (Rhizophora mangle) and the black mangrove (Avicennia germinans), and this is fortunate for the sake of this article because the species demonstrate two differing approaches to coping with the saltwater life that I will discuss shortly. Lucky aquarists living close to tropical shores are able to source native species by simply collecting propagules from beaches, assuming local laws and regulations allow.

How Do Mangroves Deal with Salt?

   Mangroves are facultive halophytes; they have evolved mechanisms to cope with drawing their moisture from saline environments and maintaining tissues with a saline content lower than that of the water that surrounds their roots. In some cases, mangroves may experience hypersaline conditions, where evaporation in coastal swamps results in water much saltier than the 35 ppt we are familiar with, and in other cases, flood waters and tides may bring water that is fresh or brackish.

   Mangroves deal with salt in two major ways. The first method is employed by species such as R. mangle, which uses a process analogous to reverse osmosis to exclude salt ions at the roots. This process is not entirely sufficient on its own, and the plants still need to “dump” salt. This is reported to be carried out by translocation of salt ions into leaves that are then shed.

   The second way of coping with “salty roots” is salt secretion, a method that is employed by species of the Avicennia genus for example. The roots of these species partially exclude salt but are not nearly as capable as their cousins. To overcome this, these species excrete salt (visible as small crystals on the leaves) through their leaf pores during transpiration, thus maintaining ionic balance within their cells. This is why species from these genera require misting with water (RO if you have it) to simulate rainfall that will naturally keep the leaves’ surface salt-free and in good health. It is good practice to mist all mangrove leaves regularly to remove dust and salt spray, for cosmetic reasons as well as biological ones.

   Mangroves have also developed adaptations to cope with regular immersion and survival in low-oxygen (anoxic) environments. To overcome these variable and challenging conditions, many species have evolved pneumatophores, which are upward-growing roots that breach the water’s surface for at least some part of the day to allow oxygen to reach the plant’s roots through networks of fine tubules. These “breathing roots” join with the aerial and prop roots to form a dense tangle that traps sediment and plant material.

Vivipary

   Perhaps the most interesting fact about mangroves (Rhizophora species) is their method of reproduction. When aquarists purchase mangroves, they often buy what they may think of as a “seed,” but this is simply not the case. What they are buying is a wonderful thing known as a propagule.

   Propagules form after flowering when the fertilized seed develops into a long structure that, when it has developed sufficiently, falls from the plant. Peter Hogarth, author of The Biology of Mangroves, suggests that this method of distribution is an adaptation to aid local dispersal and not a method that allows mangroves to traverse oceans.

   When exploring a mangrove forest, you see a large number of propagules being washed up very close to their location of origin (and very likely under their parent plants). When pushed by high tides or wave action onto suitable substrate, they will begin to grow. Roots tend to form after 10 days or so and slowly pull the new plant to an upright position.

Mangroves in the Home Aquarium.

   I can see two main reasons aquarists wish to grow mangroves. First, they are simply fascinating—a lush, green plant emerging from a tropical marine aquarium just looks impressive and is a subject for conversation. Second, aquarists may choose to grow them because they perform a useful role in the aquarium system. It is likely, of course, that many aquarists will have mangroves to serve both purposes.

   Growing mangroves in the home aquarium is not difficult. They are often bought as propagules or as more established saplings with six months to a few years of growth. Taking mangrove plants from the wild will, no doubt, be restricted in many localities and would at least require the landowner’s permission. More importantly, wild collection may damage natural habitats. As noted, it may be possible to harvest propagules from a local beach if you have easy access and are so inclined.
Fortunately for those of us who are landlocked, mangroves are regularly imported and grown and available online or in stores. Propagules tend to be shipped without pots and will, depending upon their age, have some root growth. The young plants can be raised in beds of substrate (8–10 inches [20–25 cm] is ideal), which can either be fully immersed or kept wet via capillary action. There are specially manufactured pots that, when filled with substrate such as coral sand, will contain the mangrove’s roots. These can be attached to the aquarium glass or hidden with rockwork.

   I recently set up a small tank and created a bommie with mangroves that was moderately successful. I had filled the bommie with live-rock rubble and sand to give the mangroves something to root into, but much of the sand dispersed through the inevitable holes in the rockwork and the roots didn’t form as well as I anticipated.

   In contrast to standard marine aquariums, mangrove systems often have relatively shallow water—at least in relation to the overall height of the aquarium—and very often different depths of substrate. Deeper substrate at the rear of the system can allow for planting of propagules or more advanced specimens, with their prop roots then allowed to grow toward the front of the aquarium and into the water where fish and inverts can be kept. However, such growth will not happen overnight and may well take several years to begin to look “authentic.”

   An increasingly popular use for mangroves is in refugia, and I include in this the plants that many aquarists (like me) grow in pots in their sumps. Personally I grow mine because I like them and, well, because I can, but many aquarists with larger systems and more space will include mangroves in large display refugia.

   Mangroves in refugia will, of course, absorb nutrients (for their own use and that taken up by the bacteria associated with their roots), but the contribution that the trees make to overall maintenance of organically derived nutrients in a marine tank will vary enormously. One small plant will be of little use, while a few dozen will be valuable, depending upon the size and bioload of your system. I would suggest that it’s better, perhaps, to rely on macroalgae, such as Chaetomorpha, for plant-based nutrient export.

   However, refugia are greater than the sum of their parts. The complicated tangle of mangrove roots and the biological interactions taking place around and upon them all serve to provide opportunities for other species, from bacteria to polychaete worms, to prosper. Wherever there is a niche, nature will fill it. This is exactly what one hopes to achieve in a refugium: a complicated series of processes and species interacting with each other, consuming nutrients and, with luck, releasing valuable products, such as crustacean larvae and gametes, back into the water column for consumption by fish and corals.

   A well-stocked refugium, replete with macroalgae and filled with filter feeders and beneficial scavenging invertebrates, can be a joy to behold. If planted with a few mangroves, the attractiveness of the aquarium extends upwards.

   Mangroves are not the most rapid-growing of plants. Their lifestyle requires an energy input over and above that of non-halophytes to stay alive, but once established they may well require pruning to keep them within their allotted space, especially if kept within a sump/cabinet arrangement. Mine seem to respond well to pruning and, given their natural location where they often take often taking the worst of the weather, they are tolerant of being damaged. But like most plants, they are best pruned over a period of months rather than in one “hack.” In this case, the aquarist can employ the techniques of the bonsai master and nip out growing tips to increase ramification (bushiness) and control overall shape.

   Mangroves may well require extra lighting to keep them healthy. When kept in a sump, a standard LED- or T5-based aquarium fixture may be easily provided, but when out of the cabinet, a mangrove may be sharing the light of your main lamps and may cast a shadow onto the aquarium below it. You will need to experiment, but unless you get strong natural daylight, your mangrove is likely to become etiolated (leggy) and lose leaves toward its base.

   I have also read that mangroves are susceptible to low magnesium levels. This isn’t usually an issue in a properly maintained reef tank, but in fish-only systems, the magnesium level may fall below the 1350-ppm mark normally considered acceptable. Symptoms of lack of magnesium are similar to those in terrestrial plants showing the same condition: yellowing and eventual loss of leaves.

Fish and Inverts for the Mangrove Aquarium

   Stocking a mangrove setup is a big subject. In a marine aquarium, it would be worthwhile to simulate natural conditions, perhaps including fish that might best mimic fry (which frequently shelter in mangroves), perhaps a shoal of cardinals such as Zoramia leptacantha. In shallower systems, bottom-dwelling species with little requirement for water depth will be at home. Jawfish would be a possibility, as would various species of goby or blenny.

   It may be tempting to purchase the leaf-mimicking orbicular batfish (Platax orbicularis), which is very attractive as a juvenile and fits the mangrove biotope, but this fish does not acclimate well and I would urge you to leave it in the store (and the store to leave it in the ocean). I have spent time in the wild with fully grown batfish—they are large, intelligent, and attractive creatures that have no place in the home aquarium in my opinion.

   Inverts in the mangrove setup are an easier proposition. For the refugium, an aquarist could choose a number of filter feeders, sponges, and crustaceans, including colonies and groupings of small shrimp that may well fall victim to fish in the display aquarium.
You may choose to keep a very specific biotope that is commonly demonstrated at public aquaria, namely the “upside-down jelly” tank. This involves setting up a system with mangrove roots and sea grass to mimic a natural shallow-water habitat, where Cassiopea jellyfish can settle to expose their undersides, which are filled with photosynthetic algae, to the daylight.

The Ecological Value of Mangroves.

   Mangroves are under serious threat, and while there remains a massive acreage of mangrove forest on the planet, it is shrinking remarkably as coastlines are developed for mariculture and human habitation. This is short-sighted, not only for the forests’ intrinsic value, but also because the mangroves perform a very useful role in protecting and defending coastlines from storms and tsunamis as well as providing shelter for the fry of many species of fish. Mangroves are also superbly capable of absorbing nutrients and sediments from estuaries and coastal systems that would, if they found their way to the reef, cause significant coral degradation.

A Brackish System

   As we’ve discussed, mangroves will thrive in a range of water conditions, from sea water to fresh, and this allows the aquarist the freedom to create a brackish system inspired by an estuarine environment.

   This means aquascaping objects that would not normally be incorporated into a “traditional” marine system, such as root wood, can be used to simulate the densely packed root and branch structures in estuarine habitats. These can be very effective if mixed with mangrove plants grown within deep substrate beds or within pots hidden within the rockwork—or, in this case, the root work.
Fishes for the brackish system are not as colorful as many marine or freshwater species—in turbid environments, colors are not a useful adaptation. However, they can be remarkably interesting and attractive in their own way.

   Scats and monos are commonly kept in brackish systems along with the remarkable archerfish, famed for spitting out water jets to dislodge insects, which fall onto the water’s surface. Replicating this behavior in the home aquarium is possible, with several authors suggesting the use of fake flies. When the archerfish is successful in hitting one of these decoys, it remains glued on a branch and the aquarist drops a real fly onto the water as a reward. To me, this sounds preferable to releasing large numbers of flies close to or around your fish tank, especially if you are using an open-topped system.

   Another interesting group that would thrive in a brackish system are the mudskippers. These fishes require flat areas, onto which they will haul themselves using their powerful pectoral fins. As the name implies, this means mud and other fine sediment in the wild, but I have seen aquarists using sea-washed timber to great effect to provide rafts.
 
Further Reading
Hogarth, P. J. (1999). The Biology of Mangroves. Oxford University Press, Oxford.
Stafford-Deitsch, J. (1996). Mangrove: The Forgotten Habitat. Immel, London.
Tomlinson, P. B. (1986). The Botany of Mangroves. Cambridge University Press, Cambridge.


See the full article on TFH Digital http://www.tfhdigital.com/tfh/june_2014#pg71

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