In the Fall 2020 semester, I went on a field trip to the mangroves of Key Largo, Florida for a Tropical Ecology class. That was one of my favorite field trips to date--we took a boat out into a mangrove lagoon and snorkeled at two different sites! Besides the mangroves there, I also saw vibrantly colorful sponges resting on the soil or attached to mangrove roots, green-tinted jellyfish and sea slugs basking in the sun's rays for energy, schools of minnows passing by, and large snook and barracuda prowling for their next meal.
What is a mangrove? Put simply, a mangrove is a tree or shrub that lives along any of the tropical or subtropical coastlines of the world. They all have in common the ability to live in waterlogged and highly salty environments that most other plants cannot tolerate. Mangroves are often referred to as one of the forests of the sea. That's because the mangrove plants of those regions provide the nutrients and shelter necessary for a massive abundance of life. Most of the visible life there is small, but that's because mangroves are nurseries where thousands of oceanic organisms spend their juvenile years--in other words, where they are born or grow up! They also serve as rookeries (nesting areas) for many species of birds. The mangroves themselves are the primary source of nutrients in these ecosystems--some organisms eat mangroves directly, but more commonly, the great amounts of leaves and wood they drop each year are recycled by small organisms such as bacteria or invertebrates. Those nutrients and organisms are then consumed by increasingly larger organisms, and the process spreads outwards to form the food chain of a thriving ecosystem!
Left by vishwasnavadak, licensed use via Unsplash https://unsplash.com/@vishwasnavadak
Right by Florida Memory, Public domain, via Wikimedia Commons
Mangroves reduce flood, storm, and erosion damage, the effect of which is not only notable in the local natural environments but also for the human dwellings inland. They bind and build up soil, and mangrove roots also filter nutrients and pollutants, boosting the water quality of their regions and ultimately where the passing water ends up in the surrounding oceans.
Mangroves are well-adapted to their often or constantly waterlogged and salty environments. Many have prop roots that act like stilts, holding their main bodies and leaves above the water. They have other roots that grow upwards, called pneumatophores, that are used to oxygen from the air since the water they live in is oxygen-poor (meaning lacking the desirable amounts of oxygen). They will largely prevent salts from entering their bodies (salt exclusion) or secrete salt from their leaves (try licking such a salt-extruding mangrove leaf sometime--I have, and it's pretty salty)! Some mangroves are so well-adapted to salty environments that they can grow in salinities (salt levels) of up to 75 parts per thousand or ppt, about two times the salinity of ocean water!
A salt-extruding mangrove leaf. Photo by Ulf Mehlig, CC BY-SA 2.5 https://creativecommons.org/licenses/by-sa/2.5 via Wikimedia Commons
Each mangrove species is particularly adapted to a certain part of its coastline. In Florida, common mangrove species include the red, white, and black mangrove. The red mangrove can sit directly in sea water, which gives it an advantage against other species in that area which they dominate. The white mangrove can tolerate lower salinity as a result of rainfall and so lives further inland. Finally, the black mangrove is even more adapted to life further inland and so resides there. These unique regions of domination by different species are referred to in ecology as zonation. Zonation is the result of succession, the competition between different species where one species wins over, or "succeeds", another species and dominates an area.
Photo by Phil's 1stPix. https://www.flickr.com/photos/1stpix_diecast_dioramas/ Creative Commons use, https://creativecommons.org/licenses/by-nc-sa/2.0/
Here's an idea of what life under the mangrove roots looks like!
Curtis et al. (2006), CC BY 2.5 https://creativecommons.org/licenses/by/2.5 via Wikimedia Commons
Sea slugs from the Clarki genus can be found in some mangroves (we saw quite a few on our field trip)! These sea slugs steal and use chloroplasts: that's as if you or I ate lettuce and used its components to get energy from the sun.
The upside-down jellyfish depicted below have a symbiotic relationship with algae, which get energy from the sun and give excess energy to the jellies. These same kinds of creatures were abundant at my mangrove trip!
Photographer: Bastique, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/ via Wikimedia Commons
Photo by Caroline Rogers, U.S. Geological Survey. Public domain.
Sponges and corals can often be found growing on mangrove prop roots! Snails, worms, anemones, barnacles, and oysters will also cling onto these roots.
Photo by Ellen26, Licensed Use via Pixabay https://pixabay.com/users/ellen26-2011087/
Even raccoons will roam mangroves for food.
Photo by Bob Care in Florida Keys National Marine Sanctuary, Public domain, via Wikimedia Commons.
Sometimes you'll find larger predator fish, like this snook or a barracuda, in mangroves.
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