Soft corals form a diverse group of colonial cnidarians within the order Malacalcyonacea, characterized by the absence of a rigid calcareous skeleton, which gives them their notable flexibility and fleshy appearance. Unlike hard corals (Scleractinia), their structure is supported by an internal skeleton composed of microscopic calcium carbonate elements called sclerites, which provide both support and protection. These organisms possess polyps with eight pinnate tentacles (octocoral symmetry), a diagnostic feature of the group. Approximately 2,400 species of soft corals have been described, distributed across all oceans, from shallow tropical waters to mesophotic and abyssal regions.
Morphologically, soft corals display significant variability in shape and size. They can form lobed masses, fans, fingers, bushes, or sheets, depending on the species and environmental conditions, ranging from just a few centimeters to over a meter in height. Their colors are equally diverse, including shades of brown, yellow, orange, red, purple, and even fluorescent hues, partly due to fluorescent proteins and, in photosymbiotic species, the presence of zooxanthellae. Their flexibility allows them to sway with currents, facilitating food capture, gas exchange, and sediment removal.
In terms of feeding, most soft corals are mixotrophic. Species hosting zooxanthellae obtain a significant portion of their energy through photosynthesis, explaining their abundance in clear, well-lit waters. However, they also complement their nutrition by capturing plankton and dissolved organic matter using their pinnate tentacles armed with cnidocytes. In deep or light-deprived environments, species lack symbionts and rely exclusively on heterotrophic feeding, adapting to low-productivity conditions through slower metabolic rates.
Soft corals play a crucial role in marine ecosystems by providing shelter and substrate for numerous invertebrates and fish, contributing to the structural complexity of reefs. Additionally, they produce a wide array of bioactive compounds, many of which are studied for their pharmacological potential, including anti-inflammatory and antitumor properties. Although generally less affected by bleaching than hard corals, they still face threats from climate change, ocean acidification, and habitat degradation, highlighting the importance of their conservation.