GEOLOGICAL SCIENCES 100
CORAL REEFS MINI-COURSE
KYGER C LOHMANN

 
5.   The Ecology of Building the Reef 

The Reef:  A balance between CONSTRUCTION vs. DESTRUCTION

 Constructive Components of Reef

1. Foundation:   Massive head corals form the solid and stable foundation on which the reef system can florish.  A prinicipal requirement for eary reef is the ability to maintain a strong subtrate or base on which subsequent growth can develop.  In many case, after major storm events (hurricanes), the remainder of the reef may comprise only this solid foundation.
2. Framework Components:  Much like the steel girder system of a skyscraper, the framework of the reef allows for rapid upward growth of the reef into shallower water.  A common example of the type of coral that fills this role is the Elkhorn coral (A. palmata).  Note that the shape that this coral develops is in direct reponse to the nature of the energy across the reef.  (See later notes.)

3. Encrusting Components:  Upon this framework, corals, sponges, hydrozoans and bryozoan coat the surfaces.  If one were to peer into the crack and crevices amonst the framework corals, a new and diverse world of organisms occuply this niche (or space) with the environment.  A consequence of the encrusters is to add additional strength to the reef structure.  As such, as we move up into the highest energy parts of the reefs we find that this region is dominated by encrusting growth forms.
    Although technically not encrusting, the hydrozoan Millipora (firecoral) forms complex honeycomb like structures which are capable of withstanding extreme energies associated with the rim or crest of the reef.  At this site, breaking ocean waves produce the highest energies associated with the reef system.

4. Bafflers and Binders:  In class I used the example of a motor cycle rider.  If one examines the hair of this fast driving, wild individual after riding down the road without his/her helmet, bits and pieces of bugs, debris and cigarette butts are intertwined amongs the strands of hair.  In the same fashion, flexible seafans, grasses, seawhips, and algae locally slow the speed of moving currents above the reef.  As these water slow in speed, material carried in suspension settle out upon the reef surface.  This sedimentation leads to the upward accretion or growth of the reef system.
 
 

Destruction of the Reef
The construction of the reef system is continuously counterbalanced by destructive processes.  These include:
1) physical processes ; and 2) Biological processes (Bioerosion).

Physical Processes:  This destruction is related to the interaction of energy possessed by waves, tides and currents with the framework of the reef system.  Note that as one examines the distribution of energy across the reef (from deep forereef to shallow reef crest) the energy of wave systems progressively increases.  Thus, the impact of waves across this transition results in increased breakage and abrasion of the structures developed by corals and other reef organisms.

Biological Processes:  The reef is a community of organisms, all of which interact in a complex trophic (or feeding) structure.  Some organisms can be major destructors of the reef simply as a result of their efforts to obtain food.  For example, in addition to living tissues of the corals, algae and organic debris litter the reef surface and sediment.  Other animals actively use this material as foodstuff for their existence.  One can subdivide the feeding style of these organism into several modes of feeding.

 Munchers and Crunchers....Some animals, particularly the parrot fish and some starfish (crown of thorns) actively prey upon the living coral polyps.  For example, it is common to hear the crunching of the strong parrot fish beaks as they clip the growing tips off of living coral branches.  Similarly, the crown of thorns (starfish) inverts its stomach around a branch of coral, secreting digestive fluids until the coral tissue is dissolved, and then absorbing this 'coral broth' directly into through its stomach tissues.  In the normal reef setting, the growth rate of corals greatly exceeds the rate at which these organisms can destroy the reef.  However, when the ecologic balances are upset and populations of these organisms increase beyond normal limits, their destructive actions can lead to the devastation of broad areas of the reef surface.

Grazers:  Given the vast amount of algae that infest most surfaces of the reef, it is not surprising to have animals graze upon these fields of photosynthetic foodstuffs as part of their ecology.  Perhaps most significant of these animals are the sea urchins, sea cucumbers, etc. (Echinodermata).  In particular, the sea urchin possess a series of five teeth that serve as small scrapers and scourers that can actively break up the surface of the coral reef skeleton.  It is common to find the stomach contents of these urchins to be filled with fragment of the coral skeleton.  As a result, this type of feeding can be quite destructive to the reef.

Borers:  This is a unique life style whereby organism actively drill or bore into the dense skeletons fo the coral reef.  On example of such animals is the boring clam, Lithophaga which means rock eater. If one were to cut a trench across the reef.....the dead and discarded skeletons of the reef corals are commonly riddled by numerous channels and tubes which were formed by these clams boring through the skeleton in search of food.  Overall, this does not actively destroy the living reef structure and therefore is more of a passive process.

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