Black Mangroves Mini Report
Genus: AvicenniaSpecies: germinans
Avicennia germinans, Linnaeus, 1764 (Lamiales, Verbenacae) is known by different names, salt bush and honey mangrove for example, but the most commonly used title is the black mangrove. Until as late as the 1960’s this species was considered of too low productivity to be an essential part of the environment in which it grows. Because of this belief, no efforts were made to stem the harvesting of A. germinans for various purposes. Tanners saw the species as a source of tannin, which is present in its dark-colored bark, from which the name ‘black mangrove’ arose. Some used its wood for shipbuilding because of its tendency to resist termite attacks. Still others would gather its nectar when it bloomed to make honey. A. germinans populations were drastically reduced, however efforts to reinstate them have been made more recently after the mangrove’s part in the ecology of estuaries was discovered. (Rogers, 1905)
Avicennia germinans is found in tropical and subtropical environments. They grow in dense patches along the high tide mark of estuaries and brackish swamps and mud flats. These areas are protected from large wind and wave action, which are detrimental to the newly fallen seeds trying to take root. A. germinans is viviparous, so they start as seeds that germinate while still on the plant. This allows them to be ready to take root as soon as they drop to the ground and are washed up on shore. From there it only takes up to five days to be firmly established if the seeds are in an area of relatively low water action. The seeds are shaped like oversized lima beans, and their flat characteristic causes them to be washed farther up shore than the red mangrove seeds. So this seed shape adaptation helps determine zonation. Avicennia germinansAvicennia germinans can be found moving into deserted areas once inhabited by the red mangroves that were killed off in a frost, acting as a pioneer species. Avicennia germinans stands are usually found closer to the shoreline than the white mangroves, but behind the red mangroves; although both species’ visible roots extend in a vast network around them so the two often look intermingled. They can grow to be anywhere between 7-25 meters. They tend to become more shrub-like towards the northern borders of habitation because they do not tolerate cold well. can oftentimes tolerate more saline environments than the red mangrove.
The major roles that Avicennia germinans plays in estuary habitats can be put into four main categories. The first is how A. germinans traps silt and debris in its pneumatophores, or roots that grow up from the ground to above the highest tide. In this way, A. germinans builds up the shoreline and can even venture further out towards the water, although they will have to start growing more inland if the water activity increases and wipes out the tentative, newly built shoreline. There are certain types of filamentous algae that live on and beneath the mangrove roots. These help trap the debris and form a nutrient-rich mat on the floor beneath the mangrove roots.
The second also has to do with all the debris trapped by Avicennia germinans’A. germinans such as shrimp, gastropods, types of bacteria, crabs, and bivalves. These organisms in turn are the link between autotrophs and heterotrophs. If A. germinans did not provide both shelter and nutrients (in the form of detritus) for these varied organisms, none of the other species that refuge in estuaries would have food, and would die out. pneumatohores. Leaves from the mangroves themselves, silt, and other dead plant and animal material are trapped and form a thick mat of decaying matter called detritus. Detritus forms the very base of the food chain in the estuary ecosystem, providing the main source of energy for many benthic organisms that live in the shade of
The mangroves, growing along the edge of the shoreline, are advantageously placed to filter runoff from the surrounding land flowing into the water. To a certain extent, Avicennia germinans can prevent many of the excess fertilizers and other harmful agents from reaching the estuary. If all this nitrogen-rich waste were to make its way into the water, eutrophication would occur and the surrounding environment would rapidly decrease; many species dying out.
Perhaps one of the most important contributions Avicennia germinans gives its ecosystem, and what makes it a keystone species, is the habitat it provides for many smaller animals and plants. There are millions of such species that depend on A. germinans for either food, shelter, a nursery for their young, or places to lay their eggs (Sheridan and Hays, 2003). Fish such as snook, barracuda, and many reef fish stay in the mangroves until they become adults. Then they leave, but will return to leave their young in the protective roots of the mangroves. Some marine animals spend their whole lives in and on the mangroves. Barnacles, gastropods such as the coffee bean snail and the queen conch, bivalves like clams and oysters, hermit crabs, spider crabs, and many more species all depend on A. germinans for both shelter and provision of food.
Avicennia germinans is a “facultative halophyte”, which means it “takes advantage of the lack of competition” for space and nutrients right along the salty shores of estuaries and swamps (Dawes, 1981). This means that the mangroves don’t need the salty environments to grow, but they can tolerate it. In order to do this, A. germinans has evolved several adaptations that allow them to flourish where few other plants can. The first, and most obvious, is the roots of A. germinans. They don’t have well-developed taproots because there is no oxygen in the substratum beneath them. Instead, they have developed pneumatophores: roots that stick up from the ground to above the high tide mark and provide the plant with necessary oxygen. The cortex of these pneumatophores is modified to where it has large spaces specifically for gas exchange, this being the primary function of this type of root. It is called aerenchyma. Avicennia germinans also has anchoring roots that descend into the anaerobic substratum. These develop a corky outer layer and function only in stabilizing the plant. The third type of root is horizontal and fibrous, extending into the floor just under the pneumatophores, and functions in the absorption of nutrients. (Dawes, 1981)
The leaves themselves have adaptations as well. “Black mangrove roots can filter 90 percent of the salt from sea water,” but the leaves take care of the rest (Schongalla, 2002). They have hydathodes, or vein ends in the epidermis, that function in the excretion of salt from the leaves. The salt can be seen as white crystals on the tops of the leaves. In an environment where freshwater isn’t available, this adaptation is crucial. All of the stomata are located on the underside of the leaves, and are sunken. This prevents excess water loss due to evaporation. The leaves also have a layer of 1 to 3 cells under the epidermis without chloroplasts that can store large amounts of water. (Dawes 1981) Overall, Avicennia germinans is aptly suited to thrive in the brackish waters of estuaries and not only provides for itself, but is an indispensable part of millions of other marine species that make their homes in and among A. germinans.
Dawes, C. J. Marine Botany. Wiley-Interscience Publication, New York, 1981, pg. 180-190.
Rogers, J. The Tree Book. Doubleday, Page and Company, New York, 1905, pg. 78-79.
Schongalla, M. Salt management in Rhizophora mangle and Avicennia germinans. Marine Ecology Articles, 2002.
Sheridan, P. & Hays, C. Are mangroves nursery habitat for transient fishes and decapods. BioOne, 2003. pg. 449-458.