Bioerosion: bane or boon to coral reefs

SALT LAKE CITY -- One of the most recognizable characters on kids' television is a marine creature that works in the Krusty Krab diner and is buddies with a squid, a crab, a starfish and a psychotic plankton. SpongeBob's real-life counterparts occupy the lowest animal rung on the Linnaean ladder, lacking nerves, muscles, internal organs and any appendage capable of flipping burgers.

But they do display remarkable biodiversity, encompassing about 5,000 species. One genus bores into coral reefs, rich tropical marine ecosystems that are in dramatic decline around the world, according to scientists gathered at the University of Utah in Salt Lake City for the biennial International Bioerosion Workshop.

Bioerosion, a field where geology and biology intersect, studies the erosion, or traces, caused by mollusks, sponges, urchins, crustaceans, fish and other organisms that bore, cling and eat hard marine substrates.

"Some clams have a coarse sculpture on their shell, like ribbing. They rotate and use their shell like a drill bit," said U. paleontologist, Tony Ekdale, who helped organize the conference. "Others will secrete a chemical that dissolves stone or coral. In most cases they are doing it to create a place to live."

One penetrating topic of debate focuses on the role of bioeroding organisms on the health of coral reefs. Do they enhance coral ecosystems, or speed their demise by weakening reefs' structural integrity?

"Some of these corals are like Swiss cheese (and) susceptible to being destroyed in a storm," Ekdale said. "In the case of a clam living inside the dead portion of a coral skeleton, that clam can serve as a food source of a parrotfish. So you have a double whammy with the fish breaking up the coral to get at the clam. In that sense bioerosion is a destructive process. In an ecological sense, however, it promotes biodiversity."

German scientist Christine Schonberg has devoted her research to straightening out sponge taxonomy and fleshing out the coral-adhering genus Siphonodicyon. By going through old museum collections, she has identified nine new species that cling to coral to thrive in a dangerous world.

"They evade predation. They are protected in the substrate and evade environmental changes," Schonberg said. "They don't die and bleach the way coral does. In a way it's bad because they continue to erode the hard substrate."

Coral reefs, built from calcium carbonate secretions, are under assault from destructive fishing practices, nitrate runoff from agriculture and oceanic warming and acidification.

One of Ekdale's graduate students, Michelle Mary, studies 125,000-year-old, Pleistocene-era corals in the Florida Keys where fossilized coral limestone is quarried as a building material. Mary took advantage of quarry walls to examine bore holes left by bivalves in ancient star and brain corals.

She found that the smaller coral heads were more heavily bored than the larger heads, leading her to speculate that shellfish penetrated the coral after it was already dead. This process would spread pulverized sediments from old bore holes to create habitat for other organisms, ultimately promoting reef biodiversity, she said.

E-mail Brian Maffly at bmaffly(at)sltrib.com.