In this week's blog post, Sanibel Sea School's marine science educator Shannon Stainken tells us about one of her favorite plants:
What is a mangrove and what makes them interesting, compared to other trees?
Mangroves are a collection of trees and shrubs that grow along coastlines in the tropics. Their range is similar to that of coral reefs. Mangroves are salt tolerant plants, but they do not require salt for growth.
They're incredibly tough trees that can survive despite daily flooding from the oceans. Even if it were freshwater, the flooding alone would be enough to drown most plants. For humans, mangroves bear the brunt of ocean-born storms and prevent erosion. But perhaps most interesting of all is that they are viviparous, meaning they give live birth!
How many types of mangroves are there in the world? How many can we find on Sanibel?
This is a tricky question because mangroves are classified based on ecology rather than evolutionary relatedness. There is still some debate regarding whether or not certain trees are true mangroves. Before you can come to a number, you first you have to decide what to include. There are between 50-60 true mangrove species belonging to 15-16 families, depending on which source you consult.
On Sanibel, we have three species of true mangroves and one associated species. Red, white, and black mangroves inhabit our coastlines, with buttonwood trees often intermixed.
How can I identify the local species?
You can distinguish between red, black, and white mangroves by their leaves, specialized root structures, and propagules.
Red mangroves have large, waxy, elliptical-shaped leaves. They have prop roots that extend out from the trunk before hitting the water. The largest of our three mangroves, their propagules are long, cylindrical, and green. They almost look like enlarged green beans!
Black mangroves have a similar leaf shape to red, but they’re almost white underneath from excreting salt. If you stumble across a black mangrove, be sure to lick a leaf – they taste great! They also have pneumatophores; meaning their roots slip beneath the surface and then shoot projections back up out of the ground. One tree can have up to 1,000 pneumatophores! Black mangrove propagules are small, bean-like, and flattened.
White mangroves have lenticels (raised pores) on their trunk to bring oxygen down to their roots. Their leaves are more circular than red or black mangroves and have a small indentation at the top. They also have two bolt-like notches at the base of their leaves called extrafloral nectaries. They secrete sugars to attract ants to the leaves to protect them from other herbivorous insects. The propagules are flattened, lens-shaped and approximately 2 cm in length. They’re originally pea-green, shifting to brown within days after ripening and falling from the tree.
At Sanibel Sea School, we like to use the following poem to remember the three species:
Red, red, pointy head
Black, black, salty back
White, white, bolts on tight
What role do mangroves play in the larger ecosystem?
Mangroves play so many important roles. For one, they are ecosystem engineers! Mangroves have been known to create entire islands, which is one reason we see species zonation in mangals (mangrove forests). Red mangrove propagules will take root first, and once their prop roots start collecting more and more sediment, black mangroves can come settle, then eventually white mangroves too. Another role they play is as a nursery habitat. Many commercially and recreationally important fish will spend the first stages of their life hiding in mangrove roots until they’re large enough to head offshore. Lastly, they prevent coastal erosion and protect us from hurricanes.
Which animals depend on mangroves for habitat, food, or other services?
So many! Common fish that remain in mangroves for their entire lives are gobies, spotted sea trout, and pipefish. Other fish that spend some part of their life in the mangroves are groupers, snapper jacks, schoolmasters, sheepsheads, grunts, and gray snappers. Tarpon, snook, bonefish, redfish, rays, and sharks like to hunt in the mangroves.
Herons, egrets, bitterns, spoonbills, limpkins, and ibis are among the wading birds that visit mangroves in search of food. Ducks, grebes, loons, cormorants, and gallinules have been observed in the mangrove habitats of South Florida too. Some of these waterfowl are year-round residents, while others appear during migration or as winter visitors. Birds of prey include the southern bald eagle, osprey, and peregrine falcon.
In addition, a wide variety of invertebrates, reptiles, amphibians, and mammals rely on mangroves for at least part of their lives. Manatees feed on their fallen leaves, mangrove tree crabs live on the roots and feed on red mangrove leaves, fiddler crabs burrow in their mud, and the list goes on.
How do mangroves reproduce?
Mangroves have one of the most unique reproductive strategies in the plant world. They disperse via water with varying degrees of vivipary or embryonic development, rather than producing dormant resting seeds like most flowering plants. Red mangroves are truly viviparous: a sprout will grow and receive nourishment from the parent tree. The sprout will grow through the seed coat and break the fruit wall all while still attached. When ready, they break off and drop onto the ground or in water. Black mangroves are cryptoviviparous: the embryo emerges from the seed coat, but remains in the fruit before separation from the parent plant occurs. White mangroves are semiviviparous: germination occurs during dispersal.
Do mangroves have any medicinal properties, or other everyday uses for humans?
Yes, a number of mangroves and associates contain substances which show antiviral, antibacterial or antifungal properties. Leaf and bark extracts have been known to treat Newcastle disease, vaccinia, and hepatitis B viruses. Some can even treat skin disorders and sores including leprosy. Headaches and indigestion have also been treated by ash or bark infusions.
Red mangroves have edible fruits, but they’re quite bitter apparently. You can dry their leaves to make tea, too. The bark can be used for making natural dyes; they give red, olive, brown, or slate colors depending on the soaking agent used. Black mangrove stems and leaves can be combined with a smaller amount of rubber vine (Rhabdadenia biflora) and cultivated tobacco (Nicotiana tabacum) to make a soothing remedy for stingray wounds. In some cultures, the bark and twigs are boiled and ingested to promote childbirth.
What threats do mangroves face, if any?
Mangroves once covered three quarters of the world’s tropical coastlines, with Southeast Asia hosting the greatest diversity. Now, half the world’s mangroves have already been cleared or destroyed – and those that remain are under threat. Mangrove forests are sometimes seen as smelly or unproductive, so they’re cleared for agricultural land, urban development, or other infrastructure. Dams and irrigation can also change a watershed, leading to changes in salinity that can be bad news for mangroves. Pollution and climate change are additional threats.
Is it possible to grow mangroves at home?
Yes, you can grow mangroves at home! You first have to soak the propagules for at least 24 hours in tap water. Fill a pot with pebbles, and a mixture of potting soil and sand. Next add salt water or freshwater and push the propagule into the soil. Water it often and make sure it gets plenty of sunlight, then after a few months of growth the seedling can be planted outside.
Is there anything else you want to share about mangroves?
Sanibel Sea School loves mangroves! We sell Mang Gear shirts in our shop – the company plants a mangrove for every shirt purchased. Also, one of our very own counselors in training is beginning to work on her senior capstone project, which is projected to take about 2 years. Her project is going to be centered on mangrove restoration. It will be a collaborative effort between Sanibel Sea School, Coastal Keepers, the City of Sanibel, and MANG. The goal is to grow mangrove propagules into sprouts, determine a restoration site on Sanibel, then plant them!