‘The Fire Coral’ Pt. 1
A feeding Balanus sp. barnacle encrusted by Millepora alcicornis
Music, Video, and Aquarium
2010 Coral Morphologic

Millepora alcicornis, or fire coral, is not actually a true coral, but a hydrocoral. Hydrocorals are colonies of hydroids that secrete a shared limestone skeleton, making them more closely related to jellyfish than true corals. Here in Florida, fire coral is extremely abundant on our reefs where they serve as the underwater equivalent of a sunburn to unsuspecting divers. Skin contact with fire coral will result in immediate burning pain, followed by an itchy welt that can last for several days.

Fire coral is frequently found encrusting over neighboring corals, starting from the bottom and slowly killing the coral until the colony is completely encased in living limestone. Because fire coral contains symbiotic zooxanthellae (like most tropical stony corals), they are capable of fast growth rates that help build a coral reef. Upon close inspection of fire coral, the stinging polyps can be seen as needle-like projections. At even closer magnification, grape-like bunches of stinging nematocysts can been seen protruding along the polyps’ length. These polyps are retractable, and when an edible food particle is captured, it can be drawn back towards one of the many mouths that dot the surface of the colony. In the video we see a colony of barnacle shells (Balanus sp.) that have been encrusted by fire coral. Unlike the corals though, the barnacle can continue to live beneath the veneer of fire coral.

Barnacles are most commonly found living in the inter-tidal zone where they live periodic lifestyles of low tide rest and high tide activity. When immersed in water, the barnacle feeds with legs specialized for feeding called cirri. The cirri are covered with comb-like filaments that rake the water for passing plankton. If a particle is caught in the cirri, it is drawn back to the animal’s mouth and eaten. When barnacle larvae settle out of the floating plankton themselves, they permanently affix themselves to a life-long location. Barnacles have a special ‘cement gland’ under their bodies that produces an impressive proteinaceous adhesive that holds the animal firmly, in spite of the heaviest of waves. A series of calcareous plates (commonly six) form a turret that protects their soft bodily tissues from predators. Despite their simple appearance, barnacles are in fact crustaceans, like shrimp, crabs, and lobsters.