to- nitrogen ratio declines, because more nitrogen
is added by the decomposer community. Small fish
and invertebrates feed on the leaf fragments, now
rich in both carbon and nitrogen. Thus the energy
captured by the mangrove leaf is slowly released in
little pulses, supporting a diverse animal and microbial
community. Mangroves are “nurseries” for many fish
and invertebrate species that will later enter the coral
reef or pelagic food web.
In many areas of the tropics, mangrove forests are
being removed, and the essential function of coastal
mangrove forests in supporting marine diversity is
being destroyed. Mangrove forests are being replaced
by such operations as shrimp aquaculture, a change
that eliminates the energy pulses that mangroves
otherwise supply.
Mangroves are woody plants tolerant of high
internal concentrations of salt. These plants respond
to the physiological challenges of high temperature
and concentrated salt exposure with a variety of
adaptations. For example, some have salt glands on
their leaves that effectively remove excess sodium
and chloride. Others accumulate salt in special areas
in their leaves, and still others filter salt from the
root system. Mangroves are also tolerant of soils low
in oxygen. The thick, odorous, muddy substrate that
anchors them is virtually anaerobic, or devoid of any
gaseous oxygen. But some mangroves have roots that
extend above the soil, modified to obtain oxygen from
air. Mangrove leaves are similar to leaves of many rain
forest tree species in that they are simple, unlobed, and
very thick, with a heavy waxy cuticle— characteristics
that aid in the storing of water and prevention of excess
water loss through transpiration. The plants’ major
control of gas exchange, as in leaves of all land plants,
is through stomatal openings on the leaves.
About 34 mangrove species occur globally in tropics
and subtropics. Southeast Asia has the greatest diversity
of mangrove species, whereas the Neotropics have
the least, with only eight species, some of which are
relatively uncommon. Some mangroves are adapted
to colonize shallow sand flats, trapping sediment and
gradually building a dense, muddy, organic soil. As
mangroves are far more salt tolerant than other tree
species, they tend to line tropical coasts, and their
abundance extends inland along tidal rivers. Though
subject to disturbance (especially from hurricanes and
monsoons), they typically rebound, though recovery is
often protracted. Mangroves range in form from short
and shrub- like to trees from 10 to 20 m (approx. 33– 66
ft) in height, occasionally taller.Types of Mangroves
Red Mangrove (Rhizophora mangle; plates 12- 64– 66)
is an abundant species that can grow as a bushy shrub,
a stunted tree, or a full 20 m (66 ft) tall tree. It has
reddish bark and numerous aerial prop roots, some of
which are firmly anchored to the substrate and some of
which grow downward toward it. Prop roots provide
a firm anchor for the plant. The broadly spreading
roots help assure stability against winds, tides, and
shifting sands. Prop roots contain openings, called
lenticels, important in transporting air to the oxygen-
starved deep roots. Leaves are oval and thick, dark
green above, yellowish below. Flowers are pale yellow
with four petals. Fruits are reddish brown and produce
elongate green seeds that actually germinate while still
attached to the parent plant. Seedlings resemble green
pods and are about the length of a pencil. They drop
from the plant and initially float horizontally in the
sea, becoming flotsam in the tropical ocean. Seedlings
eventually absorb sufficient water that they reorient
vertically. Should the tide carry the vertically floating
seedling to a shallow area, once it touches substrate
it will anchor and begin to put out roots. Dispersal is
effective, and Red Mangrove has populated coasts of all
tropical seas around the world.
Black Mangrove (Avicennia germinans) is abundant
throughout the Neotropics, often forming pure stands
in anoxic substrate. Black Mangroves tend to grow inPlate 12- 66. Red Mangrove propagules attach to sediment,
and the plant begins to grow quickly, forming prop roots.
Photo by John Kricher.230 chapter 12 cruising the rivers to the sea