How white band disease sweeps through a reef

Photo by mattk1979 via Flickr.

Living in panama and working on the reefs I would watch white band dis­ease sweep through the pop­u­la­tion,” said assis­tant pro­fessor of earth and envi­ron­mental sci­ence Steve Vollmer. He pur­sued post-​​doctoral research at the Smith­sonian Trop­ical Research Insti­tute on Barro Col­orado Islan.

The dis­ease he’s talking about is one of 22 that affect coral reefs. All of these have cropped up in the last three decades and many seem to be linked of rising ocean tem­per­a­tures and devel­op­ment, said Vollmer. WBD in par­tic­ular “is iden­ti­fied by a pro­gressing band of dying white tissue.”

When he started asking what was known about the dis­ease, he found the answer to be “not much.” Vollmer has since ded­i­cated his research to its study and in the last few years has made sig­nif­i­cant progress.

The pathogen is still unknown, although we have a good idea,” he said. One of the first and most impor­tant exper­i­ments Vollmer’s lab car­ried out was to deter­mine whether the white band pathogen was a bac­teria or a virus. They did this by sep­a­rating what they call “the slurry of pesti­lence and disease”—a solu­tion of dis­eased tissue—into two frac­tions. One con­tained only bac­teria while the other con­tained viruses and small microbes. When they tried to inoc­u­late healthy corals with the two frac­tions, only the bac­te­rial frac­tion caused the dis­ease. While a spe­cific pathogen has yet to be pin­pointed, Vibrio and Rick­ettsiales bac­teria are poten­tial culprits.

In a paper recently released in the journal Sci­en­tific Reports, Vollmer and his team, including grad stu­dents Sarah Gignoux-​​Wolfsohn and Chris Marks, explored the trans­mis­sion mech­a­nisms of the pathogen—regardless of its identity.

First they looked at two dif­ferent snail species (Coral­lio­phila abbre­viata and C. caribaea) that are fond of nib­bling on the a coral type affected by WBD: Acro­pora cer­vi­cornis, also known as staghorn coral. For two weeks prior to the exper­i­ment, they lim­ited the snails’ diets to only healthy corals or only sick corals…or they starved them alto­gether. Then they let the snails go to it with healthy corals.

The C. abbre­viata species man­aged to infect the healthy coral regard­less of what they’d been eating for the last two weeks—even snails that only ate healthy coral caused WBD in the exper­i­mental indi­vid­uals. This sug­gested to the team that C.abbreviata also acts as a reser­voir for the pathogen, car­rying it around for weeks at a time without betraying any of its own symp­toms. C. caribaea on the other hand never trans­mitted WBD to healthy corals. The team sus­pects there are two pos­si­bil­i­ties for this dis­crep­ancy: either C. caribaea just didn’t eat enough coral to transmit the dis­ease (they appar­ently have smaller apetites than their abbre­viata bud­dies) or they are for some reason inhos­pitable to the pathogen.

In a second exper­i­ment, Vollmer’s team looked at the pathogen’s ability to travel through the water column to infect dis­tant indi­vid­uals. They added dis­eased coral bits to a closed-​​circuit aquarium that either con­tained healthy intact coral, or injured coral. Trans­mis­sion occurred in the case of the latter, but not the former, sug­gesting that water only acts as a trans­mis­sion vector when it comes to injured coral.

This work could have mean­ingful impacts on how the dis­ease is man­aged and con­trolled. Corals are foun­da­tional species for marine envi­ron­ments, said Vollmer. “Reefs are essen­tial habitat for fish and economies depend on them for tourism.” As water tem­per­a­tures con­tinue to rise and as human devel­op­ment con­tinues to creep across the globe, under­standing dis­ease mech­a­nisms is critical.