Part 2—Hard Corals
Coral reefs are one of the most diverse and beautiful natural environments on earth. The world’s first coral reefs occurred about 500 million years ago, and the first close relatives of modern corals developed in southern Europe about 230 million years ago. Coral reefs are extraordinary living geological structures. They are highly diverse and productive, but grow best in oceanic waters with very low levels of nutrients. Coral reefs can form only in relatively clear, shallow, warm waters, where water temperatures range from 18-33° C.
What makes a coral reef?
Corals belong to the group of animals (Cnidarians) that also include anemones, jellyfish, bluebottles and hydroids. This group of animals is quite simple, with a nervous system but no brain. The basic structure shared by all corals is the coral polyp. This is the basic building block of a coral colony. The polyp is a small tube-like structure, with a central space opening via a mouth. The mouth of the polyp is surrounded by six (or a multiple of six) tentacles. A coral polyp resembles a small anemone. A coral colony is made up of thousands of tiny little animals. A coral reef is therefore made up of millions of these animals.
Why are some corals hard?
In hard corals, the polyp sits in a small limestone cup (corallite) that it secretes and that protects the soft polyp tissue. The limestone cup is in fact made by coral polyps extracting calcium from seawater. When the polyp grows upwards and divides into two, each new polyp secretes a new corallite. Thus as the coral grows upward and outwards, the coral tissue lifts itself up and lays down new skeleton, forming something similar to tree rings. In most coral species, the new polyps remain joined together as they grow, and food can be moved between the polyps through the coral colony
How do corals eat if they cannot move around?
Inside a corals polyps clear outer tissues live microscopic algae called zooxanthellae. This is called a symbiotic relationship, where both parties benefit from their partnership. The algae transform sunlight and use the polyp’s waste to make sugars through Photosynthesis. The coral helps itself to some of the sugars enabling it to grow and the algae gain protection inside the coral polyp’s tissues from predators. The sugars can provide up to 98% of the polyp’s dietary requirements. This is why corals are found in shallow water, as they require sunlight to survive. The zooxanthellae give the corals their brilliant colours of pinks, greens, yellows, and browns.
How do corals reproduce?
Corals reproduce in two ways: asexually and sexually. Some corals divide to form new individuals. This is known as asexual reproduction. Sexual reproduction takes place as mass spawning event, where polyps release millions of eggs and sperm, all at the same time and on the same night. This timing is linked to moon cycles. Polyps are either male or female or both male and female. After the eggs and sperm are released, they float to the surface. The fertilised eggs that escape predation by other animals hatch into larvae and drift with the plankton and currents. The tiny percent that survive and settle on the reef then begin new coral colonies.
Why are we told not to touch corals?
Although corals look like strong sturdy rocks, they are in fact very fragile creatures. The actual layer of ‘live tissue’ on a coral colony is very thin; therefore touching with hands, or knocking with fins can scratch this layer of tissue off and cause scarring. Most corals have special mucus they cover themselves with. Studies say that this mucus helps the coral colony combat infections by containing antibacterial agents, protects the coral from harmful UV sunrays by containing agents similar to what we use as sunscreen, it stops them from drying out and keeps the coral moist when exposed at low tide, and can also help the coral shed sediment. The mucus can be wiped off through contact. This therefore puts the coral at risk of being exposed to all the above factors. If a coral is injured, death is not instantaneous however slowly infections take hold, and the coral generally dies within 3-5 months.
Identification of Hard Corals
Coral Identification can be very hard as there are over 700 species of coral. Common names for corals refer to their colony shape. These include massive or boulder like, staghorn or branching, plating or tabular, encrusting, columnar, and foliose (leaf or vase like). Below are some photos of a few of the life forms that you will observe around the reefs in the Mamanucas. This is just to get you started on your snorkel.
Image: Acropora ‘Staghorn’ Branching Coral
Image: Brain Massive Coral
Image: Cabbage Foliose Coral
Image: Branching Coral
Image: Acropora Tabular Coral
Image: Acropora Digitate Coral
This is part 2 of 4 in the Common Reef Creatures series.
- Part 1: Common Reef Creatures—Invertebrates
- Part 2: Common Reef Creatures—Hard Corals
- Part 3: Common Reef Creatures—Reef Fish
- Part 4: Common Reef Creatures—Reef Fish Continued