Page 38 - Coral Reef Teachers Guide

Coral Forest Teacher’s G u i d e What and Where are the Coral Reefs?

The Mid Reef: Na-

ture’s Wonder

The Mid Reef or alternately referred to as plat-
form reef is characterized by a primarily radial
growth pattern. A platform reef may or may
not lie behind a barrier reef and may under-
go elongation if established on a sandbank.
These reefs reside in 30 to 50 feet of water and
are home to massive sea life and a wide bio-
sphere. These reefs begin by growing towards
the surface of the ocean, eventually spreading
out sideways. Reefs grow actively outward as
well as upward, especially in the stable con-
ditions of a continental shelf. Any given reef,
having depth and temperature fixed by its lo-
cation, will have its shape determined by the
direction and force of the water currents that
bring food and by the shape of the base on
which it grows. Where the forces of growth are
equal in all directions, radial expansion re-
sults in platform like reefs. With further radial
growth, lagoonal platform reefs develop. The
shape of an elongated platform reef may be de-
termined by the orientation of rising and fall-
ing tidal currents. Platform reefs range from
small clumps of coral to huge, sprawling reefs. Some mature adult corals are hermaphro-
They are widespread throughout the coastal ditic; others are exclusively male or female.
zone of Meso-American Barrier Reef and nu- A few species change sex as they grow.
merous Caribbean islands. The Mid Reef is Internally fertilized eggs develop in the pol-
found in deeper waters than the patch reefs yp for a period ranging from days to weeks.
of near-shore environments at depths 20-60 Subsequent development produces a tiny
feet. Bank reefs are significantly larger than larva, known as a planula. Externally fer-
patch reefs and are common dive and snorkel tilized eggs develop during synchronized
destinations and have high species diversity, spawning. Polyps release eggs and sperm
meaning many kinds of animals and plants into the water en masse, simultaneously.
live on and around this type of reef. Bank reefs Eggs disperse over a large area. The timing
have something special called spur and groove of spawning depends on time of year, water
patterns. The spur and groove formation is temperature, and tidal and lunar cycles.
made up of low ridges of corals (spurs) sep- Spawning is most successful when there is
arated by sandy bottom channels (grooves). little variation between high and low tide.
The most commonly found corals are elkhorn, The less water movement, the better the
staghorn, seafans, sea whips and brain corals. chance for fertilization. Ideal timing occurs
Corals reproduce both sexually and asexual- in the Spring. Release of eggs or planula
ly. An individual polyp uses both reproductive usually occurs at night, and is sometimes
modes within its lifetime. Corals reproduce in phase with the lunar cycle (three to six
sexually by either internal or external fertiliza- days after a full moon). The period from re-
tion. The reproductive cells are found on lease to settlement lasts only a few days,
the mesentery membranes that radiate in- but some planulae can survive afloat for
ward from the layer of tissue that lines the several weeks. They are vulnerable to pre-
stomach cavity. dation and environmental conditions.

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