Coral, often mistaken for plants or rocks, are marine invertebrates that form colonies of individual polyps. These tiny animals provide habitat for sea life and contribute to ocean health. They also support coastal protection and biodiversity. Corals are classified within the phylum Cnidaria, a group that also includes jellyfish and sea anemones.
Hard Corals
Hard corals, also known as stony corals or Scleractinians, are the primary architects of coral reefs. These organisms secrete a rigid external skeleton composed of calcium carbonate in the form of aragonite crystals. Over generations, the accumulation of these stony skeletons forms the foundational structure of coral reefs, which can extend for many kilometers. Each hard coral polyp, having tentacles in multiples of six, builds its own cup-shaped depression within this shared skeleton.
Hard corals have a symbiotic relationship with microscopic algae called zooxanthellae, living within their tissues. These algae perform photosynthesis, converting sunlight into energy, and transfer organic material to the coral host. This energy supply allows hard corals to grow and deposit calcium carbonate at accelerated rates, particularly in the clear, shallow waters where sunlight can penetrate. Growth rates vary, with some branching species growing up to 10 centimeters per year, while massive corals grow slower, from 0.3 to 2 centimeters annually.
Soft Corals
In contrast to their stony relatives, soft corals do not produce a rigid external calcium carbonate skeleton. Instead, they possess a flexible, often plant-like appearance, swaying with ocean currents. Their support comes from internal structures, such as proteinaceous or spiny elements called sclerites. Some soft corals, like gorgonians, have a protein-based core made of gorgonin, contributing to their flexibility.
Soft coral polyps have eight tentacles, distinguishing them from hard corals. While they do not build massive reef structures, soft corals contribute to the complexity and biodiversity of reef ecosystems by providing shelter and feeding grounds for marine organisms. They also play a role in coastal protection by forming structures that break wave energy.
Key Distinctions
The fundamental difference between hard and soft corals lies in their skeletal structure and overall form. Hard corals construct a dense, external skeleton of calcium carbonate, resulting in a stony, immobile structure that forms the backbone of coral reefs. This rigid framework provides the foundation for diverse marine habitats. Conversely, soft corals lack this solid external skeleton, relying on flexible internal elements for support, which allows them to bend and flow with water movement.
Another key distinction is their primary ecological function in reef building. Hard corals are the main reef builders, creating the large, permanent structures that define coral reefs. Their growth patterns include massive, branching, or plating forms. Soft corals, while present in reef ecosystems and adding to biodiversity, do not contribute to reef formation in the same manner. They exhibit diverse forms like fans, whips, and tree-like structures, and their polyps have eight tentacles, unlike the six or multiples of six found in hard corals.