CORALS

Coral reefs:

• wave-resistant structures notable for their great species richness and topographic complexity

• Great Barrier Reef – 1,950 km long, northeastern Australia

• worlds corals divided into Atlantic and Indo-Pacific biogeographical provinces; probably different in the mid-Miocene

Indo-Pacific different from Atlantic in:

1) higher diversity

2) atolls and rings of island capping submarine volcanoes (rare in Atlantic)

3) extensive development of rich coral population on intertidal flats - poor intertidal development in Atlantic province

4) difference in dominance of species

Formal definition of Coral Reef:

Compacted and cemented assemblages of skeletons and skeletal sediment of sedentary organisms living in warm waters with strong illumination

Physiographic Features:

• reef-building corals - hermatypic corals combined with coralline algae (Order Scleractinia)

• zooxanthellae - endosymbiotic algae

• 25ºN and 25ºS latitudes, 23-25ºC (Florida Keys - 18ºC)

• Astrangia danae (not reef-building) in Long Island Sound - temps as low as 5ºC

• After temperature, light is the next important limiting factor

• Derived from dinoflagellates, zooxanthellae live within the gastrodermal tissues and are essential for rapid calcification

• Reef-building diminishes below 25m and is rare below 75m (Wells, 1957; Goreau, 1959)

• Montastrea and Agaricia can exist; calcification can be cut in half on a cloudy day

Salinity:

• Hermatypic - require high salinity

• However, hypersaline conditions diminish growth

• Persian Gulf reefs develop in salinities greater than 40 ppt

Turbidity:

• High rain runoff – Fiji, north Jamaica and Venezuela results in high particle loading – inhibits coral growth

• Lower coral species richness

•Corals do show differential adaptation for turbidity

• Platygyra and Acropora palmata

•produce large amounts of mucous when sediment is high

•Mucous can remove particles

Wave Energy:

• Acropora palmata, Caribbean - live in reef crest zones, must withstand shock

• Coral can establish damaged structures within a few years from storms

• Hurricanes remove large coral heads (Sammarco, 1971)

Reef Types and Depth Zonation:

2 types:

1) Atolls - horseshoe or ring-shaped arrays of islands, volcanic origin

2) Coastal - border coasts of islands or continents

- Great Barrier , Australia to Eilat, Israel

- Stoddant, 1969 - reef structure complexity

ATOLLS:

• mainly Pacific, a few in Indian Ocean

• Darwin - Subsidence Theory

- confirmed by reef capping – 1400 m at Enewetak Atoll dates

back to Eocene (40-60 mya)

•windward side of reef - Acropora, Pocillopora, Millepora, Heliopora (see fig.)

COASTAL REEFS:

• Parallel shorelines - see fig.

1) back-reef 4) staghorn

2) reef crest 5) break in slope (55-65 m)

3)buttress zone

• Indo-Pacific - share similar feature with that of the Caribbean

- Acropora - wave swept areas

•Order of decreasing exposure:

1) Algal ridge

2) Pocillopora

3) Acropora

4) Faviid - Musiid

5) Porites

Reef Topography - Accretion and Erosion

• At low sea level - erosional terrain may have controlled reef growth

• Although during massive erosion, reef accretion has also occurred during post-glacial rises in sea level.

• Curves of reef growth developed from C-14 dating of coral skeletons in both Atlantic and Pacific show strong concordance with sea level rises

CORAL REEFS

• Mutualisms - mutualistic interactions among spp. are a major determining force in reef community structure

•Pocillopora harbors - an assemblage of symbionts - crabs, shrimp, fishes protect coral; also protect against “crown of thorns” sea star - Acanthaster plani

• other species - cleaning stations

Interspecific competition:

• Lang 1971 - How can solitary corals maintain some space on reefs in the presence of the rapidly growing Acroporids: Acropora spp.

• Scolymia spp. - New Caledonia - within 12 hrs. mesenterial filaments had completely digested competitior

• The most aggressive corals tend to be solitary small corals and occupy minor parts of the reefs (Lang 1972)

• Exception: weakly aggressive and slow growing corals such as Porites and Siderastera tend to be abundant; may be due to high larval recruitment

3 General Mechanisms of Competition:

1) Interspecific digestion

2) Direct overgrowth

3) Shading effects

• East Pacific Panama reefs (low diversity) dominated by Pocilliopora spp.

Large number of coral predators:

• fishes - Arothron

• snails - Jenneria

• Pagurid crabs

•Acanthaster - Triton - Charonia tritonis is predator of Acanthaster

Acanthaster Problem:

• 1960’s in Pacific - Australia, Guam

• Devastation of corals soon followed by recolonization of algae

• Origin of outbursts?

• Higeh densities occur at 1 per 50 m2

• Blasting of channels and passes during WWII with no increase in population

• Triton – possible removal?

• In some cases, you can find higher #’s of coral species with more Acanthaster (Panama, Porter, 1972)

• However, Glynn also observed that Acanthaster selectively grazed on non-branchy species over the dominant Pocillopora

•Anti-predatory devices - very elaborate on reefs

• (Bakus, 1981) - 73% of sponges, coelenterates, echinoderms and ascidians were toxic

• Transition Element Vanadium – 1000 ppm in the tunicate Phallosia

• similar concentration of Arsenic found in Tridacna

• Also, Saponins - Triterpene glycosides

•Gorgonian (Plexaura) have prostaglandin, fish cannot eat it – only fireworms (Hermodice) and some gastropods “The Flamingo tongue” (Cyphoma) have resistance to prostaglandin

Productivity:

• Coral reefs are islands of high production in an open sea of very low primary productivity (Odum & Odum, 1955)

• Very few phytoplanktivores are present on reefs

Coral reef primary productivity:

• 1500g C m-2yr-1 upstream/downstream--2changes

• 3500g all values exceed open ocean productivity

• 2900g

Pacific - El Nino events - can have drastic effects

• warm, nutrient-poor waters to shallow coastal waters in east Pacific

• El Nino causes “bleaching” (also when water column is clean and stable --UV from ozone holes) (Gleason)

• reefs replaced by filamentous algae

• corals affected by diseases

Black-Band Disease:

• cyanobacterium Phormidium corallyticum

• separates coral tissue from underlying CaCO3 skeleton

• coral species differ in susceptibility

• bleaching and Black-Band Disease more common in Atlantic corals

• Florida Keys - both conditions prevail (Lapointe)

Bioerosion:

• Numerous species of animals and plants destroy the skeletal output of reef accretion

• Urchins and grazing fishes bite epibionts and remove coral pieces

• Also, endolithic - boring into substratum (bivalves, sipunculids, polychaetes

• Sponges - Clionidae - found at point of breakage

Biology of Scleractinian Corals:

• secrete skeleton of CaCO3 (aragonite)

• some corals are solitary up to 25-30cm in diameter

• polyp - tentacles, gastrovascular cavities, nematocysts

• many spp. are sequential hermaphrodites - internal fertilization

• planula larvae (which develop in the gastrovascular cavity) are ejected through the mouth

• Asexual budding also allows colony to grow

• planula may be in water column as long as 2 days

• Hermatypic - high rates of calcification and #’s of zooxanthellae in gastroderm

• Ahermatypic -

• Other organisms with high calcification rates - giant clams Hippopus and Tridacna

Growth:

• Acropora - as much as 10cm yr-1; massive hemispherical colonies

• Montastrea annularis - 0.25-0.70 cm yr -1

• Montastrea has different forms - in shallow H2O (10m), the spp. grow massive - hemispherical colonies with the growth vector upward

• platelike growth is favored in deeper H2O (30m)

• favors light capture and avoids rolling when base is bioeroded

• Corals have fewer zooxanthellae in deeper water

• The most simple technique is to measure increments of growth relative to spike on coral head; also, growth bands - cut cross-sections

• Measurement of radioisotope Ca-45 and C-14 permits short-term studies of calcification (<1hr.)

• Ca-45 - estimates 20 mm yr-1 for Porites in Pacific (Goreau, 1959)

Nutrition - Massive Debate:

• Zooxanthellae - taken from one host may not be beneficial to others

• Symbiosis -

• food source

• source of O2

• aid in lipogenesis

• facilitate excretory process

• through absorption of CO2, aid in calcification

Food Source:

• primarily microcarnivores (Young, 1930-31) - C-14 fixed by zooxanthellae found widely throughout tissues (Trench, 1974); photosynthate

• polyp diameter and position correlate with tentacle length

• S = surface area of live tissue

• V = value of shell + tissue

• S/V = good indicator of light-capturing ability

• S/V and polyp diam. inversely correlated - thus, corals with shape well-adapted to zooxanthellae capture have large polyps

• As S/V increases in branching, more light is intercepted - results in a multilayered morphology, as in Acropora palmata

• allows S to be 3x surface area of bottom substrate

• S/V and polyp diameter are hyperbolically inversely correlated; thus, corals with a shape well-adapted for zooxanthellae capture have large polyps (Montastrea)

• DOM also an important source of food

• C-14 glucose taken up by Fungia perhaps through mesentarial filaments

Lipogenesis:

• Pocillopora elevates lipid synthesis

• 300% in the light relative to dark

• zooxanthellae very important!

• Zooxanthellae convert acetate to lipids

• polyunsaturated fatty acids less common in corals

• may indicate lipogenesis by animal instead of zooxanthellae

Excretion:

• P and N reduced with zooxanthellae present

Calcification:

• zooxanthellae play major role; cloudy day = calcification reduced

• inhibition of enzyme carbonic anhydrase decreases calcification