Corals are fascinating marine invertebrates, often mistaken for plants or rocks due to their appearance. They are actually animals composed of numerous tiny, sac-like individuals called polyps. These polyps typically live together in compact colonies, collectively secreting a hard, external skeleton primarily made of calcium carbonate. This mineral structure forms the framework that defines a coral colony’s shape and supports its growth in the underwater environment.
Different Forms of Coral
Coral species exhibit a remarkable diversity in their physical structures, generally categorized into two main groups: hard and soft corals.
Hard corals are the primary architects of coral reefs, forming rigid structures. Their growth forms include intricate branching patterns, resembling antlers or fingers, large rounded boulder shapes, and expansive flat plates that can extend horizontally across the seafloor.
Some hard corals may also grow in encrusting layers or upright columnar formations. In contrast, soft corals do not produce a rigid calcium carbonate skeleton, giving them a pliable, flowing appearance. These corals often resemble brightly colored plants, feathery fans, whips, or bushy trees swaying with ocean currents.
The Source of Coral Color
The vibrant colors observed in many corals do not originate from the coral polyps themselves, which are largely translucent. Instead, these striking hues are attributed to microscopic algae called zooxanthellae that live within the coral’s tissues.
This arrangement represents a mutualistic symbiotic relationship, where both the coral host and the algae benefit.
The zooxanthellae perform photosynthesis, converting sunlight into energy and producing sugars, glycerol, and amino acids. These organic compounds provide the coral with a substantial portion of its energy requirements for metabolism, growth, and reproduction.
In return, the coral offers the algae a protected habitat and provides carbon dioxide and other compounds necessary for photosynthesis.
Understanding Coral Bleaching
Coral bleaching occurs when corals experience stress, leading to a loss of their vivid coloration. This process involves the coral expelling the symbiotic zooxanthellae from its tissues.
When the algae are expelled, the coral’s translucent polyps reveal the underlying white calcium carbonate skeleton, making the coral appear “bleached.”
Elevated ocean temperatures are a primary cause of bleaching. Other stressors like excessive light exposure or pollution can also contribute to bleaching events.
While a bleached coral is not immediately dead, it has lost its main source of food and becomes more susceptible to disease and mortality if the stressful conditions persist.