in the Caribbean
by Dave Wilhelm, MSEI


An estimated 60 to 70% of the earth's coastline in tropical regions is lined with mangroves. Their geographical distribution is roughly the same as corals. There are as many as 40 different mangrove species worldwide with 8 species occurring in the Atlantic and 4 common to the Caribbean basin.

Mangrove communities and coral reefs are linked by more than geography. Mangroves provide two major functions contributing to the health and diversity of coral reefs; filtering terrestrial run-off and trapping sediments, and serving as a nursery for many of the reefs inhabitants.

Mangroves are salt-tolerant woody trees. They draw up saltwater by maintaining a lower hydrostatic pressure in their above-water cells (osmosis). However, excessive salt can be toxic. Mangroves exclude a great deal of salt at or near their roots, but still have a salinity level approximately 10 times higher than most other plants. Black Mangroves excrete excess salt via salt glands whereas Red Mangroves secrete salt by cuticular transpiration. Waxy leaves reduce the loss of water and are often covered with excluded salt crystals.

The Red Mangrove is pollinated by the wind, while the other species are pollinated by animals vectors (most commonly, bees).

Mangrove roots are adapted to anchor the trees firmly in the soft mud and to ensure that sufficient amounts of oxygen reach the base of the tree. The above ground component of the root system is porous and ventilates the buried portion for respiration. New prop roots grow from branches that overhang the water, thereby extending growth seaward. It is the densely woven maze of prop roots that acts as a filter and traps sediments, building and extending the coastline.

Mangroves specialize in land reclamation, gradually moving coastlines seaward and even creating islands. Red Mangrove (Rhizophora mangle) seedlings (propagules) grow to 1 foot before dropping. The waxy fruit end repels water and the pointed end absorbs water, thus it floats pointing down. It can survive for a year, even sprouting leaves and roots. When it touches bottom, it takes root and in 1 year will be 1 meter tall. In 3 years, it will have a maze of prop roots. In 5 years, a diminutive forest will be created.

The prop roots (Rhizophores) trap sediment and dropped leaves (1 acre of Red Mangroves = 3 tons of leaves per year). Bacteria and mold decompose the leaves and help form a rich, densely packed sediment layer. This mud is then invaded by algae (Hard and Soft Fan algae, Penicillus, Caulerpa, Halimeda) which helps stabilize the sediment. The bacteria in the upper few centimeters use up all the oxygen and the layer below becomes anoxic and smells like rotten eggs.

As the mud builds up, the Black Mangrove (Avicennia germinans) is able to colonize because it can live in shallower water and in anoxic sediment by sending up air-breathing rootlets (Pneumatophores).

In mature mangrove swamps, the White Mangrove (Laguncularia racemosa) and finally the Gray Mangrove or Buttonwood (Conocarpus erectus) form a mixed forest several meters back from the waters edge. Mangrove Ferns (Acrosticum aureum) commonly line the edge, as they can live in fresh or salt water environments. Saltwort (Batis sp.) and Glasswort (Salicornia sp.) will form an underbrush among the White and Gray Mangroves, stabilizing the dryer sandy soil and creating land.

A mangrove forest's most important function is as a nursery for innumerable fish and crustacean larvae which feed on the rich organic detritus. An estimated 75% of American Pink Shrimp (commercial shrimp) undergo larval development in the mangrove swamp. Many bird species such as Egrets, Ibis and Anhingas benefit from this abundant food source.

Living above the waterline on the prop roots of the Red Mangrove are commonly found the Mangrove Periwinkle (Littorina angulifera) and the Mangrove Crab (Aratus pisonii).

On the prop roots in the intertidal zone, barnacles are most common in zone 2 (between High High and Low High tides), zone 3 (between Low High and High Low) is dominated by oysters and in zone 4 (between High Low and Low Low), mussels are the predominant species.

In the upper part of zone 2 will be one of two species of Star Barnacles (Chthamalus rhizophorae in the Antilles & C. bisinuatus in muddy areas) and, living among them, Nerite snails (Neritina clenchi & N. piratica). Just below, lives the common zone 2 indicator species, the Ivory Barnacle (Balanus eburneus).

The presence of the Mangrove Oyster (Crassostrea rhizophorae) and/or the Flat Tree Oyster (Isognomon alatus) marks zone 3. There are a number of oyster drills and other snails that prey upon the oysters. Among them are the West Indian Murex (Chicoreus brevifrons) and the West Indian Crown Conch (Melongena melongena). Also found are two dominant red algae, the blackish purple Sea Moss (Bostrychia sp.) which resembles miniature pine trees and Caloglossa leprieurii which looks like transparent purple macaroni.

Zone 4 is dominated by the mussels. Depending on locale, they can be one of several species; the Scorched Mussel (Hormomya exusta), the larger Hooked Mussel (Ischadium recurvum) or the yellowish Tulip Mussel (Modiolus americanus). Usually associated with them is the Pale Anemone (Aiptasia pallida) which grows in large colonies.

In the subtidal region of the prop roots live a myriad of organisms. At least 25 different species of tunicates (solitary, colonial and compound) can be found here. Various sponges reside here as well, the bright red Fire Sponge (Tedania ignis) being most common. Fanworms, both Sabellids (parchment-like tubes) and Serpulids (calcareous tubes), often make a home on the roots. Look for the Black-spotted Fanworm (Sabella melanostigma) and the Christmas-tree Worm (Spirobranchus gigantea). In addition to the Pale Anemone mentioned above, the Stinging Mangrove Anemone (Bunodeopsis antilliensis) is usually apparent. Green filamentous algae, such as Chaetomorpha, give the root a hairy, disheveled appearance. Occasionally you can see groups of juvenile Spiny Lobsters (Panulirus argus) or other crustaceans hiding among the prop roots.

Channels between the mangroves form the deep sublittoral region. This is usually the area that serves as the nursery for many reef fish. The mangrove swamp provides two main things to help increase their chances for survival past the larval stage that the coral reef does not; an abundant source of nutrients for food and a maze of prop roots nearby for protection from predators. A number of species mature to adulthood in the mangroves before venturing out to the reef.

Another common resident in these channels is the Upside-down Jellyfish (Cassiopeia xamachana) which gently pulsates on the muddy bottom with it's mouth facing upward.

Although mangroves are finely adapted to their stressful environment, they expend so much energy in coping with their highly saline habitat that they have little energy left for defense. Fortunately, the tannin in its bark makes its leaves unpalatable, and the harsh habitat eliminates most competition. So the mangroves have existed for millennia.

Mankind, however, is regularly bulldozing mangrove swamps and smothering them with mud to build condominiums and hotels. They are also poisoned, polluted (oil spills being extremely devastating), harvested for their tannic acid, and burned as firewood.

When the huge mangrove swamps are gone, the effect on the entire marine ecosystem will be catastrophic. Let's make sure that does not happen!

Entire contents copyright 1993, 1998 Dave Wilhelm, Marine Science Education Institute
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