Hard corals and soft corals are marine invertebratesanimals. They belong to the phylumCnidaria, sharing a family tree with jellyfish (life cycle: polyp stage + medusa stage= free-swimming umbrella) and anemones (no medusa stage, stationary, remain polyp-shaped animal). They do not have a vertebral column (spine or backbone). They form colonies of many identical individual polyps.
Hard corals (HC) extract calcium carbonate from seawater to build a rigid, rock-hard external skeleton. The hard coral polyp lives inside a tiny limestone cup called a corallite. If a predator approaches, the polyp retracts inside this hard cup for safety. As generations of polyps die and new ones grow on top, they form massive coral reefs.
Tentacles: arranged in multiples of six (hexacorals), mostly with stinging cells (nematocysts) against predators.
Zooxanthellae
Nutrition:
H.C. are mostly carnivorous (zooplankton). Polyps extend their tentacles, usually at night, to capture tiny copepods and larval crustaceans. They use stinging cells (nematocysts) to immobilize and bring prey to their mouths.
They live with symbiotic algae called Zooxanthellae (= photosynthetic dinoflagellates) inside their tissues. (Dinoflagellata life cycle: 1-mitosis, 2-sexual reproduction, 3-planozygote, 4-hypnozygote, 5-planomeiocyte). Zooxanthellae give corals its beautiful colours and provide them with sugars and other nutrients through photosynthesis, providing them with up to 70-90% of its energy needs in clear, sunlit waters, not deeper than 60m. Additional nutrients are dissolved organic materials in the sea. HC rely lesser on zooplankton.
Soft Corals
Soft corals (SC) are held together by a flexible, jelly-like core called mesoglea and a wood-like protein rod. To maintain their shape, they rely on internal sclerites – microscopic, spiky calcium carbonate needles embedded in their tissues, allowing them to gracefully sway back and forth with ocean currents. As they cannot retreat into a protective shell, they rely on chemical warfare, producing highly toxic terpenes that make them taste terrible to reef fish and protect them from encroaching algae.
DinoflagellataDinoflagellata life cycle
Tentacles: arranged in groups of eight (octocorals), often with tiny, feather-like side branches called pinnules.
Nutrition:
SC show greater diversity in feeding strategies. Their diet includes zooplankton (copepods and larval crustaceans), phytoplankton (microscopic algae), bacteria and microbes, organic particles (detritus), dissolved organic matter. They also have symbiotic relationship with zooxanthellae, providing them with sugars and other nutrients by photosynthesis. However, SC rely more on filter feeding.
HC + SC are made of colonies of polyps.
Polyp
Coral Life Cycle
A polyp is an animal of very simple structure, a living fossil that has not changed for half a billion years. They are cylindrical in shape. The outer layer is the ectoderm, the inner layer the endoderm with the mesoglea in between. With the tentacles at the upper end of the mouth polyps sense and captures food such as copepods and fish larvae (newly hatched fish). Polyps reproduce asexually by budding (cell division). In many cases the buds do not separate from the parent but form colonies growing to great size containing vast number of individuals. The reef-building corals are polyp-colonies, strengthened by the formation of a firm skeleton.
Polyp
Polyp
Zooplankton / Copepod
Zooplankton / Copepod
Zooplankton / Copepod
Lophelia
Deep Water Corals
Other corals such as Lophelia do not rely on zooxanthellae and can live in much deeper water as deep as 3,000m.
They are cold-water corals that does not contain zooxanthellae (symbiotic dinoflagellates). Their diet include zooplankton (Copepods, Krill, small Crustaceans and Larvae).
Future of Coral Reefs
75% of the world’s corals are threatened due to overfishing, coastal development, pollution, thermal stress, ocean acidification, crown-of-thorns starfish, and introduced invasive species. Aquatic life has been put under a substantial amount of stress because of the pollutants caused by land-based agriculture. Particularly, exposure to the insecticides and the fungicides have played a major part in polyp retraction and biomass decrease. Corals are increasingly at risk of bleaching where polyps expel the zooxanthellae in response to stress such as high-water temperature or toxins.
Triton’s trumpet is one of the few animals to feed on the crown-of-thorns starfish. Occasional plagues of this large and destructive starfish have killed extensive areas of corals. The triton is tearing the starfish to pieces with its file-like radula.
Plankton / Carbon Sink
Plankton also play important roles in the earth’s carbon cycle. When phytoplankton die, some of the carbon they take in through photosynthesis sinks to the ocean depths, where it is sequestered from the atmosphere. This process, called the biological pump, makes the ocean the earth’s largest carbon sink.
Some sea anemones (ortiguillas) are edible. They are typically battered and deep-fried. Cooking removes the risk from their stinging cells, and they are known for having a flavor reminiscent of the sea .
