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Coral Structure and Function I Maritime coastal - greenish - particulate Caribbean - blue - clear • BLUE – water reflects blue of the sky – water refracts sunlight (more blue light) – no interference from green plants • CLEAR – little particulate matter – few phytoplankton in the water • less dissolved organic carbon • PHYTOPLANKTON – microscopic algae - flourish in colder ocean waters – live in upper 60m - the PHOTIC ZONE – give local Maritime waters their colour • as you descend through water column – lose more and more light – reds go first (lower energy) – gives a bluish cast to everything • much more pronounced locally than in the Caribbean – we have far more photosynthetic organisms in the water – absorb the light (red & blue ) for photosynthesis - leaves green • So- the blue colour & clear water of tropics due to few photosynthetic organisms in tropical waters • Tropical waters are still very PRODUCTIVE • bottom of food chain events – primary production – production of organic material from inorganic • trophic pyramids/webs - find plants at the bottom – use SUNLIGHT energy to fix CO2 into organic molecules – Primary Production • plants consumed by primary consumers etc. • less total biomass as you go up the pyramid • increase size of organism as you go up the pyramid • eximine coral reefs ecosytem: • “how does this flourishing ecosystem survive with so few producers - the plants ” ? • clear water, few phytoplankton ??? • In the reef system primary production is mostly BENTHIC (bottom) • Open ocean (or local Maritime), primary production is mostly PELAGIC (water column) • Much of the productivity from corals • Cnidaria - from the Latin “nettle” – a plant • have often been mistaken for plants – attached to a substrate – do not wander about – same colour as many marine plants – same branched nature and growth habit • were originally classified as plants • by the naturalist John Ray (1627-1705) • In 1723, Jean Peyssonel decided they were animals • naturalist John Ellis 1776 • a microscope modified for aquatic work • found the animal polyps on many reef organisms • then considered to be animals for a while - with no plant component • improvements in microscopy confirmed their animal nature, with polyps filtering out plankton with their tentacles • subsequent studies showed that the reef is composed of many organisms, as well as the Cnidarians • The Royal Society Coral Reef Expedition 1896-1898 • Funafuti Atoll (Ellice Islands - Tuvalu) • The Royal Society Coral Reef Expedition 1896-1898 • Funafuti Atoll (Ellice Islands - Tuvalu) • The Royal Society Coral Reef Expedition 1896-1898 • Funafuti Atoll • analysis of cores - • The Royal Society Coral Reef Expedition 1896-1898 • Funafuti Atoll • analysis of cores - mostly: 1. Calcareous red algae 2. Calcareous green algae (Halimeda) 3. Foraminifera (20-40m protists, porous CaCO3 shell) 4. Corals • Top 18m of the core was 80-90% Halimeda • Calcareous red algae • Calcareous green algae (Halimeda) Foraminifera • Corals • 20C - new understanding of trophic pyramids, attention turned to reef productivity – – – – very productive (produce lots of biomass) lots of life lots of diversity productivity couldn’t be due just to the calcareous green and red algae • so where were the primary producers ?? • Extensive examination of atolls (Eniwetak – Marshall Islands) • lots of encrusting algae on the surface of corals, but also ... • examine corals in more detail • true nature of the Cnidarians • algae growing inside the cells of the coral polyp • These algae - ZOOXANTHELLAE • enough algae inside the coral polyp to account for massive primary production • their presence explained the plant-like growth habit of the Cnidarian – to increase surface area for light absorption • Also explained the colours of the corals • 1950s - Tom & Gene Odum • suggested the coral polyp and the alga were in some sort of mutualistic relationship – the polyp itself is a miniature ecosytem – the two organisms exchange nutrients and other benefits – “Trophic Structure and Productivity of a Windward Coral Reef Community on Eniwetok Atoll” 1955 • Corals are predacious animals - suspension feeders • two main methods of prey capture – nematocysts – mucus • extend tentacles - mostly at night – zooplankton are most plentiful (move up from deeper waters) • whole surface of the coral becomes a trap for plankton • paralyze prey – sting with NEMATOCYSTS • trap prey – sticky MUCUS on tentacles • tentacles produce WAVE-LIKE action sweeping the mucus and prey into the mouth • down the pharynx (gullet) to the gastrovascular cavity for digestion • tentacles produce WAVE-LIKE action sweeping the mucus and prey into the mouth • down the pharynx (gullet) to the gastrovascular cavity for digestion • prey digested, mucus recycled, solid, undigestible material (eg silt) ejected • Tentacles mostly retracted during the day – help corals avoid predation – protect from UV • Corals also get some nutrients from seawater – – – – dissolved amino acids glucose inorganics not usually much, except in locally polluted areas • structure of the polyps and skeleton of the coral is a simple combination • Most scleractinian (stony) corals – colonies of polyps – linked by common gastrovascular system (coenosarc) • polyp made up of two cell layers – outer epidermis (or ectoderm) – inner gastrodermis (endoderm) • non-tissue layer between gastrodermis and epidermis = mesoglea – made of collagen & mucopolysaccharides • non-tissue layer between gastrodermis and epidermis = mesoglea – made of collagen & mucopolysaccharides • "lower layer" of epidermis = calicoblastic epidermis – secretes the calcareous external skeleton • "upper layer" of epidermis is in contact with seawater • The corallite is the part of the skeleton deposited by one polyp • The skeletal wall around each polyp is called the theca • The coral structure also includes calcareous plate-like structure known as septa • One of the epidermal cell types is the cnidocyte – contains organelles called nematocysts – discharge toxic barbed threads – capture zooplankton prey • gastroderm cells line the body cavity – capable of phagocytosis (food particles) – contain the intracellular algae – extend into tentacles • zooxanthellae not in direct contact with the cytoplasm of the coral gastroderm cell • zooxanthellae reside inside a vacuole – the symbiosome (animal origin) • Much of the food needed by the polyp comes from the SYMBIONT • Many corals have different growth forms - can vary with local environment - light, depth etc. • Local environment affects distribution of the zooxanthellae • Zooxanthellae: – ZOO - animal – XANTHE - gold-coloured • single-celled alga, with 2 flagellae – a dinoflagellate • spherical, 8 - 12um dia • Most dinoflagellates are free-living – unusual group of algae – feeding modes ranging from photosynthetic autotrophy to heterotroph • Many dinoflagellate produce toxins – e.g. ciguatoxin causes ciguatera "fish poisoining” • Other toxic dinoflagellates responsible for algal blooms – e.g. red tides (Gymnodinium) – paralytic shellfish poisoining (Alexandrium) • dinoflagellates – chlorophylls a and c – lack chlorophyll b – characteristic dinoflagellate pigments diadinoxanthin and peridinin • ~ 3 x 106 cells/cm2 • coloured tinge to the coral • brown to yellow brown