Date post: | 25-Dec-2015 |
Category: |
Documents |
Upload: | kory-obrien |
View: | 216 times |
Download: | 1 times |
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