The growth of a coral is no sphere of the ocean world in which the interaction between animal and environment is so well displayed or more worthy of our understanding than the coral reef. These astonishingly beautiful oases in tropical seas teem with life in an ecosystem that is highly self-reliant.
The Pharaonic structures of the coral world are built by innumerable generations of myriads of tiny polyps and algae. There are two main constructive ingredients – calcium carbonate and time. Man’s hectic intrusion is not in measure with the pace of change in the sea.
The reefs develop as a result of the Growth of a Coral of colonial limestone secreting plants and animals. Yet these are only a small part of the formidable underlying structure. Most of it is made up of rubble and sediments from a great many sources all bound together by cementing algae.
The most familiar kind of coral reef is probably the atoll. The ring-shaped island or group of islets around a tranquil lagoon. In the 19th century, great minds dwelt on its origin and major controversies raged. But we do know that an atoll is born when a volcano erupts and emerges above the surface of warm tropical waters. If the volcano later sinks beneath the surface and if coral has grown up around it as fast as it descends an atoll is formed shaped like a smoke ring.
Just before and during a new moon, a coral polyp sets free hundreds of larvae, called planulae, from a fertilized egg. For five to nine days these larvae half swim with the help of hundreds of tiny hairs that cover their bodies and half drift on the currents of the sea. The larvae from mushroom coral only drift for two or three days, others might drift for more than three weeks.
Almost immediately after the larva is released, the beneficent zooxanthellae algae distribute themselves throughout the larva, giving a brown hue to its translucent body. The larva’s journey is not without its perils. Muddy water can choke it. Protozoa by the hundreds can attack it and digest it. If it survives, the larva thickens and settles onto some smooth surface.
Almost at once, it begins to secrete its limestone skeleton, creating a shelter and cementing itself to its home site. The larva is transformed into a polyp. Its center sprouts into a hollow pedestal crowned by tentacles. Some corals develop no further. They are isolated and are joined to neighboring polyps only by thin appendages at the base.
With the proper environment, however, a single hermatypic coral polyp can be the beginning of a whole new colony of individuals. One way in which coral polyps reproduce is by asexual budding. New polyps bud forth from the cells of this connective tissue and immediately begin to secrete their own stony cups.
Polyps also reproduce asexually by fission, putting forth more tentacles and fleshy partitions until they split from the mouth down, and there are two where there was one. All this time the skeleton of each polyp becomes more rigid and complicated. When a polyp is mature, spermatozoa and eggs begin to develop within its mesentery.
When their maturation has reached a certain point, spermatozoa flow through the mouth of the polyp and float in the sea until they reach other polyps. The other polyps draw them inside with fanning movements of their tentacles. The spermatozoa then fertilize the eggs that have remained attached to the inner linings of the host polyps.
Soon after fertilization, the mesentery re- leases the eggs. Like spermatozoa, they flow through the mouth of the polyp. The larvae develop into hollow spheres of cells perhaps the size of a pinhead. It takes from a few hours to a few days for the pear-shaped larvae to settle and become polyps. Corals reproduce both sexually and asexually.
A cross-section of a polyp with fertilized eggs that will hatch through its mouth demonstrates a stage in sexual reproduction. Budding and fission are asexual phases. Hydroid Like their coral relatives, hydroids reproduce both asexually and sexually. Long tentacles on the polyps capture food. Reproductive polyps break off as free-swimming medusas and settle down away from parent.