Reef Ecology

Coral reefs are the most biologically diverse marine eco-systems on earth, rivalled only by the tropical rainforests on land. Corals grow over geologic time and have been in existence about 200 million years. Corals reached their current level of diversity 50 million years ago.

The delicately balanced marine environment of the coral reef relies on the interaction of hard and soft corals, sponges, anemones, snails, rays, crabs, lobsters, turtles, dolphins and other sea life.

A coral reef is composed of thin plates or layers of calcium carbonate secreted over thousands of years by billions of tiny soft bodied animals called coral polyps. It takes years for some corals to grow an inch and they range in size from a pinhead to a foot in length. Each polyp excretes a calcereous exo-skeleton and lives in a symbiotic relationship with a host algae, zooxanthellae, that gives the coral its color. Zooxanthellae takes in carbon dioxide, process is through photosynthesis, and then gives off oxygen as a by-product that is used by the host polyp. Millions of polyps grow on top of the limestone remains of former colonies to create the massive reefs. Yet these tiny animals form the only natural formation visible from outer space.

Corals are divided into two kinds and both are stationary on the ocean bottom. Hard corals such as brain, star, staghorn, elkhorn and pillar corals have rigid exoskeletons, or corallites, that protect their soft delicate bodies. Gorgonians, or soft corals, such as sea fans, sea whips, and sea rods, sway with the currents and lack an exoskeleton.

Recent studies have shown that the health of reefs are directly affected by the health of local mangrove swamps and seagrass beds. Mangroves provide an effective filter for silty and chemical laden run-off, which are trapped by the mangrove roots. Seagrass provides important breeding grounds for many species of plankton and juvenile fish, which populate the reef and provide much of the food for the reef ecosystem.

Coral Reefs around the world face stresses from both natural and human causes. Extensive diving & boat traffic, off-shore pollution, blast & cyanide fishing practices, and global warming all contribute to destruction of the world's reefs. Many tourist sites have learned that their livelyhood depends upon the health of the reefs and have taken positive steps to impliment ecologically friendly tourism. For example, many shoreside communities have banned the use of phosphates in soaps and detergents. Heavily dived areas like the Bay Islands have installed moorings for dive boats, and do not permit vessels to drop anchor near the reef. Marine sancturaries like the Great Barrier Reef in Australia forbids guests from touching the reef or taking souveniers. Fishing is also prohibited in most areas. The island of Grenada has made great strides managing sewage waste from its many beachside resorts, either by digging cisterns or moving out-flow pipes farther off-shore, thus lessening the impact on the reefs.

Following are pictures and descriptions of coral types as well as common diseases that affect them.

Healthy Coral

Boulder Star Coral - (Montastraea annularis)
Colonies grow in irregular mounds. The long thick columns have enlarged dome tops. The surfaces are rough and in plate-like sheets.
Size: 1-10 feet
Color: Green to brown, yellow brown and grey
Habitat: 20-70 feet, they are often the most common coral at this medium depth
Boulder Brain Coral - (Colpophyllia natans)
colonies form large rounded domes. The surface is covered with a convoluted system of ridges and valleys looking very much like a brain. Boulder brain coral is also commonly known as Giant Brain Coral.
Size: 2-7ft
Color: Ridges are brown and valleys are green, tan or whitish
Habitat: 20-80ft., they are often found at reef tops
Elliptical Star Coral - (Dichocoenia stokesii)
Colonies form small rounded heads or domes. Individual coralites are large, about 0.5-0.75 inches and protrude up about 0.25 inches. They are often elliptical or in a Y shape.
Size: 4-15 Inches
Color: Cream to yellow and brown
Habitat: 12-225 feet
Elk Horn Coral - (Acropora palmata)
Colonies form flattened branches that resemble elk horns. The branches orientated parallel to surge direction.
Size: 3-12ft.
Color: Brown to yellow brown
Habitat: 1-55ft., shallow areas where there is constant wave action and wave movement
Finger Coral - (Porites porites)
Colonies form thick, stout brances. During the day, the extended polyps give the coral a fuzzy appearance. Finger coral are often found with bittle stars and sea urchins living among the compact branches.
Size: 1-4ft
Color: Beige to yellow brown, brown, grey and grey with purple overtones
Habitat: 3-160ft
Great Star Coral - (Monastraea cavernosa)
Colonies often form massive boulders or domes. At greater depths, the colonies more often form large sheets. The individual coralite is large and blister-like, often a half inch in size. The great star coral is also known as large star coral.
Size: 2-8 feet
Color: Yellow brown, green, brown and grey
Habitat: 40-100 feet
Lettuce Coral - (Agaricia agaricites)
Colonies form thickly encrusted flat plates with ridges and long valleys. They grow in shingle-like plates.
Size: 4in-3ft
Color: Tan to yellow brown, greyish brown to chocolate brown. Lighter shades may have a bluish or purplish tint
Habitat: 3-240ft, they can also be found in mangroves and back reef areas
Pillar Coral - (Dendrogyra cylindrus)
Colonies form heavy cylindrical spires that grow upward forming large mounds.
Size: 4-10ft.
Color: Light tan to golden brown and chocolate brown
Habitat: 4-65ft., they are found on both flat and sloping bottoms
Common Sea Fan - (Gorgonia ventalina)
Colonies form large fans that grow in a single plane. The fan is a tightly-meshed, interconnected network of branches.
Size: 2-6ft
Color: Purple
Habitat: 3-100ft, they are often found on the seaward side of shallow reefs and slopes where there is gentle and steady water movement
Stag Horn Coral - (Acropora cervicornis)
Colonies form antler-like cylindrical brances. These branches can form dense thickets with only the outer brances surviving.
Size: 1-8ft.
Color: Brown to yellow brown with white tips
Habitat: 10-60ft., Shallow to intermediate depths

 

Reef Diseases

White Pox Disease

White Pox Disease is a newly discovered disease that appears as blotches all over the coral, from base to tip. In its advanced stages the living tissue of the coral is reduced 50-80%. At some locations, it has decimated 50-80% of elkhorn corals (acropora palmata), the major reef-building coral for shallow reefs. Elkhorns only inhabit western Atlantic and Caribbean waters. It was first observed at Key West-area reefs in the summer of 1996.

Sea Fan Browning & Fungus

1995 it was observed that many purple sea fans in South Florida Reefs were turning brown. The filter feeders appear to be "stuffed" with particulate matter. Some have lesions that create gaping holes while others were infected with a fungus that causes deep purple spots on the sea fan. The fungus attacks the fragile gorgonians such as aspergillus, which are typically found in soil. Sea fans in Puerto Rico, the Bahamas and Jamaica are also turning brown.

Blackband Disease

Blackband disease has attacked boulder corals like the lesser and greater star corals for many years, but an acceleration of diseased coralheads began in the late 1980's at many Florida Keys reefs. The leading edge of the disease is a cyanobacteria. Healthy coral growth appears to mushroom up and around the dead area over time. Harold Hudson of the Florida Keys National Marine Sanctuary developed a method of preventing its spread by vacuuming the disease off corals and applying protective clay to the affected tissue. Since the disease is high in phosphates, many coastal areas have introduced a phosphate ban on soaps and detergents.

Yellowband or Yellow Blotch Disease

Yellowband or Yellow Blotch Disease is characterized by wide, irregularly shaped bands or blotches on the leading edge of the disease, which progressively consumes live coral tissue, leaving behind only white substrata. It usually affects boulder corals (montastria anularis). Very little is known about the disease.

White Plague Type II Disease

White Plague Type II is a fast growing new disease attacking Caribbean coral reefs. It usually affects small star corals, dichocoenia stokesii as well as Pillar Corals, Boulder Corals, and Star Corals. It has been documented that it destroys tissue at an alarming rate. This may occur several times during the course of a season. This disease been reported in the Bahamas and upper Florida Keys.

www.reefrelief.org

Coral Bleaching

Coral bleaching is the whitening of coral colonies due to the loss of symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white calcium carbonate skeletons of the coral colony. Corals naturally lose less than 0.1% of their zooxanthellae during processes of regulation and replacement (Brown and Ogden, 1993). However, adverse changes in a coral's environment can cause an increase in the number of zooxanthellae lost. There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures.

 

Sponges

Sponges, or Porifera are a diverse group of sometimes common types, with about 5000 species known across the world. Sponges are primarily marine, but around 150 species live in fresh water. Sponges have cellular-level organization, meaning that that their cells are specialized so that different cells perform different functions, but similar cells are not organized into tissues and bodies are a sort of loose aggregation of different kinds of cells. This is the simplest kind of cellular organization found among parazoans. Fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian (600 to 540 Million Years Ago). The approximately 5,000 living sponge species are classified in the phylum Porifera, which is composed of three distinct groups, the Hexactinellida (glass sponges), the Demospongia, and the Calcarea (calcareous sponges).

The hexactinellids, or glass sponges, number roughly 500 species which are mostly found in deeper waters, 200 to 2,000 meters. It is interesting to note that while other sponges possess the ability to contract, hexactinellids do not. Moreover, hexactinellids possess a unique system for rapidly conducting electrical impulses across their bodies, allowing them to react quickly to external stimuli.

The Damospongia is by far the most diverse sponge group. Greater than 90% of the 5,000 known living sponge species are Damosponges. As their great number of species would suggest, demosponges are found in many different environments, from warm high-energy intertidal settings to quiet cold abyssal depths. All of the known freshwater poriferans are demosponges. Demosponges take on a variety of growth forms from encrusting sheets living beneath stones to branching stalks upright in the water column.

Memebers of the group Calcarea are the only sponges that possess spicules composed of calcium carbonate. They are predominantly found in shallow waters, though at least one species is known from a depth of 4,000 meters.

Sponges are characterized by the possession of a feeding system unique among animals. Poriferans don't have mouths; instead, they have tiny pores in their outer walls through which water is drawn. Cells in the sponge walls filter goodies from the water as the water is pumped through the body and out other larger openings. The flow of water through the sponge is unidirectional, driven by the beating of flagella which line the surface of chambers connected by a series of canals. Sponge cells perform a variety of bodily functions and appear to be more independent of each other than are the cells of other animals.

Other characteristics of sponges include a system of pores (also called ostia) and canals, through which water passes. Water movement is driven by the beating of flagellae, which are located on specialized cells called choanocytes (collar cells). Sponges are either radially symmetrical or asymmetrical. They are supported by a skeleton made up of the protein collagen and spicules, which may be calcareous or siliceous, depending on the group of sponges examined. Skeletal elements, choanocytes, and other cells are imbedded in a gelatinous matrix called mesohyl or mesoglea. Sponges capture food (detritus particles, plankton, bacteria) that is brought close by water currents created by the choanocytes. Food items are taken into individual cells by phagocytosis, and digestion occurs within individual cells.

Reproduction by sponges is by both sexual and asexual means. Asexual reproduction is by means of external buds. Some species also form internal buds, called gemmules, which can survive extremely unfavorable conditions that cause the rest of the sponge to die. Sexual reproduction takes place in the mesohyl. Male gametes are released into the water by a sponge and taken into the pore systems of its neighbors in the same way as food items. Spermatozoa are "captured" by collar cells, which then lose their collars and transform into specialized, amoeba-like cells that carry the spermatozoa to the eggs. Some sponges are monoecious; others are dioecious. In most sponges for which developmental patterns are known, the fertilized egg develops into a blastula, which is released into the water (in some species, release takes place right after fertilization; in others, it is delayed and some development takes place within the parent). The larvae may settle directly and transform into adult sponges, or they may be planktonic for a time. Adult sponges are always sessile.

Sponges fall into three main groups according to how their bodies are organized. The simplest sponges are the asconoid sponges. These are shaped like a simple tube perforated by pores. The open internal part of the tube is called the spongocoel; it contains the collar cells. There is a single opening to the outside, the osculum. The next-most complicated group is the syconoids. These tend to be larger than asconoids. They also have a tubular body with a single osculum, but their body wall is thicker and the pores that penetrate it are longer, forming a system of simple canals. These canals are lined by collar cells, the flagellae of which move water from the outside, into the spongocoel and out the osculum. The third category of body organization is leuconoid. These are the largest and most complex sponges. These sponges are made up of masses of tissue penetrated by numerous canals. Canals lead to numerous small chambers lined with flagellated cells. Water moves through the canals, into these chambers, and out via a central canal and osculum.

Sponges are found in virtually all aquatic habitats, although they are most common and diverse in the marine environment. Many species contain toxic substances, probably to discourage predators. Certain other marine animals take advantage of this characteristic of sponges by placing adult sponges on their bodies, where the sponges attach and grow. The chemicals also probably play a role in competition among sponges and other organisms, as they are released by sponges to insure themselves space in the marine ecosystem. Some of these chemicals have been found to have beneficial pharmaceutical effects for humans, including compounds with respiratory, cardiovascular, gastrointestinal, anti-inflammatory, antitumor, and antibiotic activities. Sponges also provide a home for a number of small marine plants, which live in and around their pore systems. Symbiotic relationships with bacteria and algae have also been reported, in which the sponge provides its symbiont with support and protection and the symbiont provides the sponge with food. Some sponges (boring sponges) excavate the surface of corals and molluscs, sometimes causing significant degradation of reefs and death of the mollusc. The corals or molluscs are not eaten; rather, the sponge is probably seeking protection for itself by sinking into the hard structures it erodes. Even this process has some beneficial effects, however, in that it is an important part of the process by which calcium is recycled.

Corals and Sponges commonly seen at Roatan Dive Sites:

Azure vase sponges
Orange elephant ear sponges  
Red and pink rope sponges  
Barrel Sponges  
Black corals
Brain Coral
 
Elk Horn Coral
Gorgonians
Lettuce coral
Pillar coral
Sea fans
 
   

Web Links to Explore:

www.reefball.org

http://www.nwis.bc.ca/spars/ - Artificial Reef Building

http://www.actwin.com/fish/species/fish.msql - Index of fishes and Marine invertebrates (photos)

www.coral.org

http://pcrf.org - Planetary Coral Reef Foundation

http://www.wcmc.org.uk/data/database/reefbase.html - Reefbase

http://www.cofc.edu/~coral/coral.htm - Coral reef ecology

http://www.uvi.edu/coral.reefer/index.html Coral Reef introduction

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Drew Thompson ©1999-2002

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