The term mangrove spans about 55 species in 20 genera belonging to 16 families. Mangroves are terrestrial plants that inhabit coastal areas in the tropics and subtropics and have a particular love for saltwater. Mangroves require an abiotic environment with stable salinity and low wave energy. With roots stretching up and into the water, and leaves branching up, they are hard to miss. Inhabiting such a unique niche comes with many challenges such as competition, excessive salinity, submergence, desiccation, and habitat destruction. (Parida and Jha 2010).
[Snorkeling through the mangroves in Bimini, the Bahamas. Photo By Baylie Fadool ]
Habitat destruction is a very real threat in Bimini, the Bahamas: in 2014 mangroves were removed from North Bimini for construction, and removal continues to be a threat. The loss of habitat impacts many species that depend on mangroves. Lemon sharks are one of the many species utilizing mangroves as a nursery area and for predatory protection. The Bimini Shark Lab PIT Project captures every newborn lemon shark in two mangrove nursery areas in Bimini, and tags them with a PIT tag. With about a 96% capture rate by night 4 out of 6 (Gruber et al. 2001), we can clearly see declines in the number of newborn lemon sharks during years of high levels of construction. Today, mangroves remain an important ecological community throughout Bimini.
[In 2014, mangroves were removed from North Bimini. This is seen in the right of this photo where sand has been dredged where mangroves once set their roots.]
In Bimini, there are four species of mangrove: red (Rhizophora mangle), black (Avicennia germinans), white (Laguncularia racemosa), and buttonwood (Conocarpus erectus). Mangroves in Bimini are located in a saltwater environment with limited freshwater access. In addition, we know that mangroves have the capability to grow in freshwater. This brings up an interesting concept: are mangroves participating only in salt exclusion, or are there adaptations for salt dependency? Some mangrove species have demonstrated a dependency on saltwater for long term survival. For example, R. mangle has been found to have low survival rates when in an environment without access to saltwater (Kanai and Sakai 2020).
Mangrove Life History and Reproduction
As shown in the diagram below, the mangrove life cycle begins when a mature propagule is released from an adult mangrove plant. A propagule is the structure released from a parent plant that can disperse and become a new plant. While the propagule is attached to the parent tree, they are building up the nutrients that they need to go off and grow into their own plant (Mangrove Action Project). Interestingly, embryonic development continues while the propagule is dispersing within water, and they have the capacity to survive up to a year in the water (Mangrove Action Project). This strategy of reproduction is called viviparous reproduction. While this diagram represents the typical life cycle of a mangrove, there are slight differences between specific species. There are differences found in the length of propagule, length of dispersal, survival success of the propagule, and even the degree of viviparity (“Reproductive Strategies of Mangroves, nhmi.org). In the context of salinity, different species of mangrove respond to saltwater in a variety of methods including salt exclusion, salt secretion, and salt accumulation .
[Photo By Mathew A. Vanderklift, Christopher Doropoulos, Daniel Gorman, Inês Leal, Antoine J. P. Minne, John Statton, Andrew D. L. Steven and Thomas Wernberg - Extracted from this Commons file, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=110786394]
Saltwater is saltwater due to its concentration of NaCl (sodium chloride) which separates into Na+ and Cl- ions. Salt tolerance is defined by how a plant deals with each of these ions.
How do they deal with salt?
Physical adaptations to a life in saltwater include shallow roots, prop roots and pneumatophores ( aerial roots to take in more oxygen when in anoxic soils), and tough leaves. Physiological responses are needed for tidal inundation, salinity and temperature fluctuation, and anoxic soil.
Plants in high salinity environments, such as mangroves, are able to live in these conditions by excluding salts. That being said, the mechanisms of this defense differ by mangrove species. Mangroves are able to utilize salt glands in their leaves to excrete salt from their system, filtrate salt out of their system via specialized cells, and there are even those species that accumulate salt. Accumulating and compartmentalizing ions (Na+ and Cl-) into vacuoles (a structure within each plant cell that regulates turgor pressure in plants) of the cells allows the plant to avoid the negative effects of salt in their system (Parida and Jha 2010).
Another adaptation to counter balance the amount of salt in their system is to accumulate compatible solutes. These are solutes that work well within the cellular metabolism of the plant and can be accumulated to maintain osmotic balance, which is the balance of salt and water (Parida and Jha 2010).
Salt Tolerance and Dependence in Mangroves
I have read papers and heard professors mention mangroves as both facultative and obligate halophytes. A halophyte is a salt tolerant plant that is found growing in an area of high salinity. Classifying a halophyte as obligate or facultative is dependent on their salt tolerance (Parida and Jha 2010). An obligate halophyte would be unable to survive in an environment without saltwater whereas a facultative halophyte would. Wang et al. 2011 concluded that mangroves are obligate halophytes. Their research found that mangroves can spend little time in freshwater and will ultimately not survive if they remain in an exclusively freshwater environment. Mangroves were shown to absorb Na+ and Cl- and some of the enzymes within the plant were shown to be stimulated by salinity concentrations. This does not mean that each enzyme was stimulated by high salt concentration but instead that salt is required to some capacity for certain enzymes to do their job (Wang et al. 2011).
Parida and Jha 2010 states that mangroves are facultative halophytes, yet acknowledges that there are authors that consider mangroves obligate halophytes. There are mangrove species that grow best at certain salt concentrations and require salt for optimum growth. That being said, there is a difference in salt tolerance depending on the species of mangrove, which leads to difficulty in classifying mangroves as a whole as obligate or facultative. The presence or absence of excess NaCl impacts different species in different ways (Parida and Jha 2010).
[Juvenile lemon shark swimming through the roots of the mangroves, which they use for
protection against predators. Photo by Baylie Fadool]
Mangroves are important because they provide valuable habitat and nursery areas for countless fishes, elasmobranchs, and invertebrates. Their structure and location allows for an area protected from predators and extreme weather conditions where marine organisms can live and grow. The Shark Lab has studied mangrove habitats for decades and know that juvenile lemon sharks grow up in this habitat due to prey source (i.e., yellowfin mojarra) and the safety that the mangroves provide. Unfortunately, habitat destruction in Bimini has led to a loss of this valuable habitat. Mangroves also increase stability of coastal soils and thus reduce erosion and provide a buffer against storms.
Further, mangroves are carbon sinks; this means that they can capture and store carbon dioxide. In addition, mangroves are a biofilter and will take up nutrients and pollutants from the water, thus improving water quality.
Gruber, S. H., de Marignac, J. R., & Hoenig, J. M. (2001). Survival of juvenile lemon sharks at Bimini, Bahamas, estimated by mark–depletion experiments. Transactions of the American Fisheries Society, 130(3), 376-384.
Hiromi Kanai, Atsushi Sakai, An obligate-halophytic mangrove, Rhizophora mucronate, does not require Na+ for the uptake of nutrient ions in their roots, Aquatic Botany, Volume 169, 2021, 103328, ISSN 0304-3770, https://doi.org/10.1016/j.aquabot.2020.103328.
Mangrove Action Project. https://mangroveactionproject.org/
Parida, A.K., Jha, B. Salt tolerance mechanisms in mangroves: a review. Trees 24, 199–217 (2010). https://doi.org/10.1007/s00468-010-0417-x
Reproductive Strategies of Mangroves. nhmi.org.
Wang, W., Yan, Z., You, S. et al. Mangroves: obligate or facultative halophytes? A review. Trees 25, 953–963 (2011). https://doi.org/10.1007/s00468-011-0570-x