Corals vs. snails on a tropical reef

The warm, shallow waters surrounding the Bahamas' Exuma Islands are home to a variety of corals, including soft corals also known as gorgonians. In this view are three types of gorgonians: sea fans (just right of center), sea whips (far right and far left), and sea plumes (in front of the large lavender sea fan.)

Kristen Whalen, a graduate student in the MIT/WHOI Joint Program in Oceanography, collected gorgonian corals near the Perry Institute of Marine Science in the Exumas, Bahamas, and brought them back to the laboratory to analyze toxic chemical compounds they produce as a defense against animals eating them. Photo by Ann Tarrant, WHOI.

An apparently peaceful scene (called "Rainbow Reef" for the variety of colorful reef organisms there, including orange encrusting sponges) belies the life-and-death struggle going on between soft corals—vulnerable, stationary, and edible—and mobile, voracious, predatory snails that can decimate the corals.

Beautiful and damaging, "flamingo tongue" snails (Cyphoma gibbosum,) their orange spotted mantle tissue covering their shells, graze on the individual polyps (visible as tan dots on the coral branches) of a soft coral called the black sea rod (Plexaura homomalla), and leave white translucent egg masses behind as they consume the polyps. The corals' defense is to produce chemical weapons — toxic compounds that make them distasteful. The snail, though, possesses counter-weapons: biochemical mechanisms that detoxify the coral compounds.

In a closer view, the coral polyps, the snails' mouths on the coral branch, and the scars the snails leave behind are clearly visible.

The snail Cyphoma dining on one of its favorite meals, a purple gorgonian with fur-like brown tentacles named Biareum asbestinum, known locally as "dead man's fingers."

Not deterred by the coral's chemical defense compounds, Cyphoma completely strips the coral of its polyps, leaving bare skeleton behind and visible in this close-up photo.

Locked in combat on the reef, soft corals, over evolutionary time, developed poisonous compounds to deter consumers, while the marine snail Cyphoma evolved a "defensome"—a collection of genes and proteins that detoxify the compounds, and continues to graze on corals. Will there be a next step in the arms race?