-in reality, few reefs are dominated solely by corals, hence the term “coral” reef is often a misnomer.-corals are largest, most showy, obvious, easiest to recognize on many reefs; however, few people realize that red algae generally exceed corals in importance as reef-building organisms.

coral reefs

These coralline red algae are harder than most corals, and this durability enables them to withstand the tremendous power of the huge breakers on oceanic islands.

-within such extreme high-energy systems, coralline algae form an algal ridge that absorbs wave energy and hence protects the more delicate corals, fleshy algae, sponges, & other organisms that inhabit the protected lagoons and back-reef habitats.

-without certain spp. of calcareous coralline red algae, most reefs would not exist.

-therefore, instead of term coral reefs, tropical reefs or biotic reefs is preferred.

-there is no significant fossil record of the evolutionary history of the marine red algae, except for the order Corallinales which extends back as far as the Jurassic.

-cement, holding everything together, is what makes it all work - and the red calcareous red algae are the principal cementing agents of nearly all tropical reefs.-many reefs, especially in the Pacific, are almost entirely formed by calcified plants

also algal nodules(rhodoliths, maerl)

-Red Algae or Rhodophyta is by far the largest & most diversified of tropical reef plants-largely restricted to hard-bottom-predominantly macroscopic in all tropical reefs-equally abundant in Atlantic, Pacific, Indian oceans

The red color is due to the presence of phycoerythrin which reflects red light and absorbs blue light.

- color varies according to the ratio of phycoerythrin to phycocyanin & they may appear green or bluish from the chlorophyll and other masking pigments.

-because blue light penetrates water to a greater depth than light of longer wavelengths, these pigments allow red algae to photo-synthesize & live at somewhat greater depths than most other algae.

-occupy entire range of depths inhabitable by photosynthetic organisms, from high-intertidal to depth as great as 300 M (San Salvador I. Bahamas, greatest depth for known plant life)

Biologically active compounds produced by reef dwelling organisms possess antimicrobial and antiviral activity. These compounds may be important sources for natural product based drugs and medicines

1. Beach2. Reef flat3. Algal Ridge4. Sub-Terrace5. Terrace6. Reef Slope

The reef flat is a rigorous environment. The organisms that live here must be adapted to withstand intense ultraviolet radiation, high salinities, and elevated water temperatures.

Although some areas of the reef flat are barren pavement, pockets of branching corals do occur where there is sufficient water flow to prevent over-heating or desiccation.

Encrusting coralline algae flourish where waves break on the reef, forming an elevated bank known as the algal ridge. As water from the waves returns seaward, its load of suspended sand and gravel scours a series of narrow ridges. The alternating ridges and valleys are known as spurs and grooves.

Corals growing on top of the spurs are stoutly formed and compressed in shape to withstand the extreme force of breaking waves.

On many reefs there is a terrace below the algal ridge. In this zone we often find stout fire corals which are able to withstand the wave surge and strong currents which characterize this area.

Here surgeon fishes and parrotfishes rove over the submarine terrace grazing on benthic algae

As we move down the slope from the submarine terrace, we find increasing coral cover and fishes that prefer areas of rich coral growth, like this regal angelfish

Some outer reefs have flattened terraces that interrupt the gradient of the reef slope. It is here, protected from the wave surge overhead yet still exposed to abundant sunlight, that we find lush gardens of corals with expansive body forms designed to maximize their exposure to sunlight.

Descending the slope, we find corals continue to expand horizontally in shape in order to capture as much sunlight as possible.

Branching forms found in shallow water are largely replaced by plate like forms below a depth of sixty meters.

-reef-building (=hermatypic) corals are of the order Scleractinia in the class Anthozoa of the phylum Cnidaria. ~ 6,000 species -

-contain large quantities of unicellular microalgae called zooxanthellae (dinoflagellates), living symbiotically in the gastrodermis of reef-building corals. -nutrients supplied by the zooxanthellae make it possible for the corals to grow and reproduce quickly enough to create reefs. Zooxanthellae provide the corals with food in the form of photosynthetic products. In turn, the coral provides protection and access to light for the zooxanthellae.

-the role of macroalgae is mainly photosynthetic as well as providing bulk materials, similar to bricks or blocks in construction.

Symbiodinium

Orange = atolls / Red = fringing reefs / Green = barrier reefs

Coral reefs --Corals need warm water, between 18°C & 30°C. If the water is colder, the reefs are poorly developed or nonexistent. Most coral reefs lie between the latitudes of 30 degrees N & S where sea temperatures are warmest.

Corals need access to sunlight (the algae living in their cells need light for photosynthesis) so the water where they grow needs to be shallow and clear. Too much sediment in the water can smother the coral polyps and too much fresh water kills them. That is why coral reefs don't grow close to the mouths of rivers.

Coral reefs are divided into 3 basic types: fringing reefs, barrier reefs and atolls.

 Fringing reefs grow in shallow waters and border the coast closely or are separated from it by a narrow stretch of water. Fringing reefs consist of several zones that are characterized by their depth, the structure of the reef, and its plant and animal communities. These regions include the reef crest (the part of the reef the waves break over), the fore reef (the region of medium energy), and the spur and groove or buttress zone (the region of coral growth which includes rows of corals with sandy canyons or passages between each row)

Barrier reefs are reefs that are separated from land by a lagoon. These reefs grow parallel to the coast and are large and continuous. Barrier reefs also include regions of coral formation that include the zones found in fringing reefs along with patch reefs (small reefs), back reefs (the shoreward side of the reef), as well as bank reefs (reefs that occur on deep bottom irregularities). Coral reefs also include reef flats, the reef crest, and a coral terrace (a slope of sand with isolated coral peaks). These features are followed by another coral terrace and a vertical drop into deeper waters.

Great Barrier Reef in Australia. Belize, C.Am.

- third type of coral reefs are atolls, annular reefs that develop at or near the surface of the sea when islands that are surrounded by reefs subside. Atolls separate a central lagoon and are circular or sub-circular.- two types of atolls: deep sea atolls that rise from deep sea and those found on the continental shelf.- mostly limited to the oceanic basins of the Pacific

Reef morphology -- Reefs are often referred to as the "rainforests of the oceans". Coral reefs host an extraordinary variety of marine plants and animals (perhaps up to 2 million) including 1/4 of all marine fish species. The wide variety of habitats depends a lot on the morphology of the coral reef. Differences in temperatures, light, exposure to waves and tides, currents and the amount of food available result in different habitats and niches.

A) Mud flats close to shore with tide pools - B) Mangroves - C) Seagrass bed - D) Patch reefs on inner reef slope - E) Inner lagoon - F) bottom with mud - G) Pinnacle - H) Outer lagoon - I) Fine sand - K) Acropora corals - L) Algae ridge - M) Sand and rubble - N) Gorgonians and black corals - O) Cave or overhang

Coastal bays: Mangroves often live along the edge of bays and river mouths. The water is often turbid with silty bottoms. Mangroves provide shelter with their root system and are a nursing ground for many species of coral reef fishes.

Lagoons:  Depths my vary from less than a meter to 90m. If there is good circulation of water there may be many patch reefs or just large stands of Acropora corals growing there. There may be seagrass beds or mud flats. Lagoons can trap many fish varieties as tide recedes.

A = coarse sedimentation   B = fine sedimentation   C = daily variation of temperatures   D = number of different species