Corals

Ocean Invertebrates in Peril

May 4, 2007 Wesley Rouse

A discussion of the basic characteristics of corals-Taxonomy, Anatomy, Ecology, Food and Feeding, Life Cycle, and Associated Fauna and Flora.

The earth may see its last coral animal within the next 100 years. Impossible? No, at the rate that they are being eradicated, they could all be gone in one century. Learning as much as we can about them might help in saving them. The following focuses on the reef-building organisms, the corals most at risk.

Taxonomy

Corals are in the phylum Cnidaria (formerly Coelenterata) which are invertebrate animals that produce polyps and medusae as a part of their life cycle, are radially symmetrical, have a 3-layered body wall, and produce specialized stinging cells for defense. There are three classes—Hydrozoa, polyp phase dominant (ex. fire corals); Scyphozoa—medusa phase dominant, (ex. jellyfish); and Anthozoa—major polyp stage (or colonies of polyps) without a medusa stage (ex. sea anemones, soft and stony corals). Most of the corals that build reefs are in the class Anthozoa.

Anatomy (see anatomy diagram)

There are two major parts of a coral animal—the living organism and the non-living skeleton. The basic living element is ordinary and found in most cnidarian polyps, a cylindrical tube with tentacles. Genera are found measuring 1 to 3 mm, although a few are larger, and may be pipe-tube-like or flattened. The mouth of the polyp is at the top and is surrounded by a ring (or rings) of tentacles. Most species have stinging cells imbedded in the tentacles. The mouth opens to a gastrovascular cavity where digestion takes place. Waste products are then released out of the mouth opening. The walls of the polyp are 3-layered—ectoderm, endoderm, and mesoglea. Corals also have a unique method of attachment and protection; they produce a non-living calcium carbonate skeleton around the polyp that remains after the animal dies. When millions of these skeletons have been deposited in one place, we call it a coral reef.

Ecology

Many factors are involved in determining where corals are found—temperature, light, water quality, substrate, water depth, circulation patterns, and predation. Most corals are found in tropical waters near the surface where light intensity is high. Furthermore, most need good water circulation and a niche where planktonic organisms are found in great numbers. Because many reef-building corals have a symbiotic relationship with algae, they also need to be in the euphotic zone, near the surface where good light is available. Corals are quite particular to their individual ecological niches, and any change in makeup can kill the polyp easily. As our waters are becoming more and more polluted, more and more of the coral’s ecology is changing, and the species cannot adapt to these changes.

Food and Feeding

Coral animals have three methods of deriving nutrients.

1. They absorb nutrients from the surrounding water directly into the cells.
2. They have a symbiotic relationship with zooxanthellae, minute algal plants bundled into their tissue. This relationship is vital to the life of the coral as depriving the polyp and algae sunlight will invariably kill both the polyp and imbedded algae. Pollution particles in our oceans, particularly near land, can greatly cut the amount of sunlight, thus killing the coral.
3. They catch zooplankton with their tentacles, the major method obtaining food. Some merely push a small bit of water toward the mouth effectively “filtering” the plankton into the mouth. Many species are proficient predators catching small floating plankton on their tentacles. These tentacles are armed with hundreds of stinging cells called nematocysts, which both capture and kill. The food particle is transferred via mucus and/or cilia to the mouth. The food is digested in the gastrovascular cavity.

Life Cycle

Corals reproduce asexually and sexually. In asexual reproduction, called budding and fission, a polyp produces a new “twin” directly beside the existing polyp. Sexual reproduction involves an egg and sperm brooded within the colony producing a larval stage (called a planula). These larvae are then released all at the same time, probably at some phase of the moon, into the surrounding waters. They are usually quite substrate specific, implanting themselves on the reef, beginning a new polyp life form.

Associated Fauna and Flora

Corals reefs are home to hundreds of species from just about every phylum—plants, bacteria, protozoans, worms, shrimp, crabs, fish, clams, molluscs, starfish, sponges, to name a few. Some are parasitic predators, but a surprising number of species are symbionts living together commensally, each gaining some benefit by their relationship. A most distinctive relationship is the zooxanthellae, a minute algae symbiotically living within the polyp tissue.

These are the facts. It will be up to future generations to save or lose these species.

The copyright of the article Corals in Marine Biology & Oceanography is owned by Wesley Rouse. Permission to republish Corals in print or online must be granted by the author in writing.

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