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Mangroves for the Marine Aquarium


By Anthony Calfo

Mangrove propagules at All Seas Marine

Much has been written about the ecological enormity of importance of mangrove trees in the marine environment. At first glance, we can see that they provide habitat for countless life forms above and below the surface and at the very water's edge. Birds, reptiles, mammals, fishes and invertebrates exploit mangrove communities for food, shelter and reproductive activities. The utilization of these communities as a nursery environment for larval species has extraordinary ramifications far up the web of life. The very structure of these strategically tangled trees is crucial to coastlines for protection from erosion and storms, and in the stabilization sediments from run-off that could otherwise pollute the reef community and subsequently destroy the fish and invertebrates dependant on all. The protection of mangrove habitats is crucial for the survival of coral reef ecosystems and all that depend on it from the fisherman to the fished and down to the living substrate.

Three genera of mangrove are commonly recognized, but aquarists are predominantly interested in the most aquatic variety: the Red Mangrove (Rhizophora mangle). The Black Mangrove (Avicenia germinans) and the White Mangrove (Laguncularia racemosa) are not readily tolerant if at all of full submersion in seawater. 

A summary of the each mangrove tree:

    Red Mangrove (Rhizophora mangle)

can live as submerse, emersed or fully terrestrial (if well-hydrated)

tolerates fresh, brackish or full seawater but cannot be freely moved between gradients

favors fine sand or muddy substrates but can be grown in course substrates or none at all (hydroponics)

is the most temperature sensitive of three genera listed here- requires warmer temperatures (ideally over 60F)

is the most sensitive to pruning. Immature or improper cuts can harm or kill some trees

    Black Mangrove (Avicenia germinans)

can live as emersed of fully terrestrial

tolerates salted air and some salted water, but favors fresh environments

prefers substrates with a decidedly significant soil or muddy component, and tolerates sand

is reasonably tolerant of pruning and tolerates mild frost conditions

    White Mangrove (Laguncularia racemosa)

lives as a fully terrestrial plant

naturally tolerates salted air (coastal) but suffers excess salted water

is very tolerant of pruning

is moderately to very tolerant of occasional frost conditions

For our aquaristic purposes, we address the red mangrove specifically. Indeed, they are the first species likely to be encountered by aquarists. They are the most commonly photographed for their magnificent aerial prop roots (the arched and exposed knobby knees plunging into the coastline and shallows). And they are by far to the most important genera of this family to marine environments.

It is their very elaborate and extensive root system that we must give due regard for in the aquarium. Even a seedling mangrove can develop a formidable root system that can stress or damage glass or acrylic refugiums in as little as three years. We recommend that you pot mangroves in containers and vessels that are as large as possible (to reduce future disturbances of the tree) without making it overly difficult for you to service for transplantation if necessary. Rest assured that growth is so slow (and ultimately managed easily) that these fascinating Angiosperms can be enjoyed perhaps indefinitely in your aquarium system.

Although the collection of mangrove trees is outright forbidden in many areas, the harvest of their abundant seeds (known as propagules) is fairly unrestricted on the whole. Un-sprouted propagules look like long green cigars with a narrowly tapered end (where the leaves and branching canopy will sprout from) and a thickened, blunt end (often tinged brown). The blunt end of a propagule is appropriately weighted by design to increase the likelihood of finding its way into a substrate when cast or carried adrift (if it does not have the good fortune of finding itself plunged into the sand from a straight drop from the parent canopy). Note: un-sprouted seedlings can survive out of water in temperate conditions for up to a year. Propagules can be sprouted in fresh, brackish or saltwater and will do so even fully submerged (although this is not recommended for aesthetics and the cultivation of prop roots if nothing else). One thing is certain, though- you cannot move a mangrove between saline gradients quickly if at all! Aquarists are strongly advised to only seek un-sprouted seedlings. If there are any roots or leaves in evidence on arrival, you simply must be told what salinity the propagules were sprouted in. Failure to abide by this is likely to be fatal for the seedling, evidenced by a shriveled desiccation and demise within weeks (the propagule takes on a wrinkled appearance from the "osmotic shock"). After determining the nature and need of your seed or seedling, a serious planting decision must be made. The matter really boils down to long-term plans versus short-term (or open-ended) residence... and the encouragement of aerial roots versus profuse establishment of capillary roots in a substrate. When mangroves are used markedly for the aesthetic, the encouragement of decorative aerial roots is a large factor. One will find that the cultivation of arched prop roots on mangroves is very easy to encourage, or fail to encourage, despite the lack of tide cycles in the aquarium.

In the absence of tidal cycles, red mangrove propagules can be trained to develop noble aerial roots by beginning life tied gently with flexible gardener's tape (available at a landscape or garden center) to a thin plastic pipe or rod. Be sure to use flexible tape as a rigid tie otherwise will cut into the plant as it grows. An un-sprouted seedling can then be tethered at a depth where only the lower 1/3 of the propagule (the thick, blunt discolored end) is initially submerged in water. Roots will sprout, incidentally, before leaves will. As roots begin to grow and develop, the body of the plant is slowly moved upwards on the stake. In this teasing manner, strong roots will grow thickened and extensively to support the weight of the body above the water. It will take many months before the body of the propagule can be liberated from the water with and arched and anchored root system. The plant will likely need to be rooted in the future, but only after satisfactory roots have developed above the water surface.

If instead you simply stick a propagule into a bed of sand like a dart, root development will occur fast and profusely. However, prop roots are unlikely if possible here at all without a replication of tides and the exposure of some roots to air. Red mangroves will grow in a wide range of substrates but prefer fine sand and muddy sediments ideally. Fertilizing the substrate may be helpful but unnecessary or dangerous in average aquariums where levels of dissolved organics are typically (and nutritively, in this case) high. One exception to the matter may be elemental magnesium; mangroves have been implicated in aquariums as depleting Mg to the point where it skews the balance of minerals. Trace elements may be supplemented deliberately if tested for and monitored. Or, one may simply rely on regular partial water changes for this and contributions to overall water quality. Like most vegetable filters and refugiums, a mangrove basin (aquarium, inline bucket, etc) should be fed raw overflowing water from the display for opportunities to exploit dissolved and particulate matter. It would be counter-intuitive to feed mangroves clean-filtered water given their natural habitat and abilities.

Lighting is a simple matter with mangroves. They are quite adaptable to a wide range of light but prefer bright illumination. Expensive reef aquarium fixtures are not necessary however. Common plant bulbs from the local hardware store are quite fine. Many aquarists have grown fine mangroves under incandescent (including mercury vapor and metal halide) plant-growth spectrum floodlights or spotlights. Fluorescents lamps are found in useful spectrums but lack intensity in all but the closest applications with mangroves.

Pruning red mangroves is a sensitive matter and rather a moot point for most with consideration for their categorical slow growth in captivity. If you must trim your tree, be sure to resist and pruning until after the axial tip has branched. Damage to the lead tip before splitting can be fatal for young specimens While we are on the topic of growth as well, it should be interesting to note that irrigation of the leaves is nearly as much of a limiting factor of growth in Rhizophora as light and nutrients! Misting the leaves daily (or at least several times weekly) with purified water, as mineral rich water imparts clogging minerals, helps to rinse away the salt crystals exported through the leaves of this marine plant- a fascinating adaptation! With other aspects of good mangrove husbandry in order, a lack of leaf irrigation will still significantly reduce growth.

It is surprising to hear aquarists debate the efficacy of mangroves in the marine aquarium as vehicles for nutrient export when you can weigh their functional abilities clearly against their growth, which is dreadfully slow. In fact, their naturally sluggish growth is recognized by numerous governments on native coastlines where legislation controls or forbids pruning. At large, even occasional storm damage can be devastating. En masse, in wild habitats, they are outstanding vehicles for nutrient export- fixing nutrients in their enormous and collective biomass. In the aquarium however, you do not have a forest of 20 or 30 feet tall mangroves... you don't even have one that big! The scrappy little seedlings that you do have, instead, demonstrate leaf growth concurrent with leaf drop at times. The proof is in the pudding, as they say: they are weak nutrient export mechanisms in the aquarium because they do not produce stable or harvestable mass quickly. If you are looking for a vegetable filter, there is a long list of algae (and even plants like some sea grasses) that can provide greater harvestable mass for nutrient export. Mangroves are simply marvelous to look at and a pleasure to include for minor aesthetic and biotic advantages in the home reef ecosystem.

Summary of Husbandry for the Red Mangrove in the aquarium:

Acquire un-sprouted seedlings (propagules) whenever possible

Discover and match the salinity of seedlings purchased if already sprouted

Fine sand and muddy sediments are preferable to course media for rooting

There is no such thing as "too bright" when lighting acclimated mangroves

Leaves much be irrigated several times weekly or better to shed exuded salt crystals

There are many aesthetic and mildly utilitarian applications for mangroves in aquaria with few notable disadvantages. One of the best and most natural ways to enjoy this rare flowering marine plant is in an upstream, fishless, deep sand bed refugium. Beyond significant natural nitrate reduction (NNR), you will enjoy the copious production of natural plankton and epiphytic matter in a mock mangrove microcosm. As a display suggestion (if we compromise on the fishless refugium) lets consider a presentation of cardinalfishes living in a magnificent commensal display with the remarkably agile Diadema urchin. You are limited almost only by your imagination in applications of mangrove trees in the captive marine aquarium.

Bigger PIX: The images in this table are linked to large (desktop size) copies. Click on "framed" images to go to the larger size.

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