Colorless Corals

Coral bleaching, often caused by climate change, now has a new culprit - marine fireworms.

Marine fireworms may be the mosquitoes of the Mediterranean's coral reefs. New research indicates that these coral-feeding worms, famous for their fierce sting, may spread an infectious disease to their prey in the same way that mosquitoes spread malaria. The bacterium behind the disease, Vibrio shiloi, lives within at least one species of Mediterranean fireworm and is transmitted to a stony coral called Oculina patagonica when the fireworm feeds. Once a coral is infected, it suffers from an increasingly common problem known as coral bleaching, which can make entire reefs turn white.

Corals get their color - as well as food and oxygen - from algae that live inside their bodies. However, a number of environmental stresses can cause these colorful animals to expel their algae, exposing their white skeletons and leaving them vulnerable to death. Since coral species tolerate relatively narrow temperature margins, global warming has triggered dangerous levels of bleaching in recent years. Some biologists predict that the current coral bleaching epidemic will kill up to half of the world's reefs this century if the climate change trend continues.

Now that fireworm-borne bacteria have been added to the list of coral bleaching triggers, there is an even greater incentive to cut carbon emissions and put a stop to rising sea temperatures. As researchers recently realized, these bacteria are actually more virulent in warmer waters, serving a double dose of damage to corals affected by global warming.

In the Solomon Islands (south of Papua New Guinea) the coral Coscinarea exesa has shed its symbiotic algae in response to environmental stress. Inset: A healthy coral reef in the Solomons. Photos: Gary W. Williams