Tag Archives: Acropora

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Corals call for fish aid when attacked by Seaweed

Corals under attack by toxic seaweed do what anyone might when threatened: they call for help.

Results reported this week in the journal Science show that threatened corals send signals to fish “bodyguards” that quickly respond to chomp the destructive algae, which can kill the coral if not removed.

Scientists Danielle Dixson and Mark Hay at the Georgia Institute of Technology found evidence that these fish respond to chemical signals from the coral in a matter of minutes.

The fish are gobies and they spend their lives in the crevices of specific corals, receiving protection from their own predators while removing threats to the corals.

The symbiotic relationship between fish, and the coral on which they live, is the first known example of one species chemically signalling a consumer species to remove competitors.

“This species of coral is recruiting inch-long bodyguards,” said Mark Hay, a biologist at Georgia Tech and co-author of the paper. “There is a careful and nuanced dance of odors that makes all this happen.

“The fish have evolved to cue on the odor released into the water by the coral, and they very quickly take care of the problem.”

The importance of large herbivorous fish to maintaining the health of coral reefs has been known for some time. They control the growth of seaweeds that damage coral.

But Georgia Tech scientist Danielle Dixson, also a co-author of the paper, suspected that the role of the gobies might be more complicated.

To study that relationship, she and Hay set up a series of experiments to observe how the fish would respond when the coral that shelters them was threatened.

They studied Acropora nasuta, a species in a genus of coral important to reef ecosystems because it grows rapidly and provides much of the structure for reefs.

To threaten the coral, they moved filaments of Chlorodesmis fastigiata, a species of seaweed that is particularly chemically toxic to corals, into contact with the coral.

Within a few minutes of the seaweed touching the coral, two species of gobies–Gobidon histrio and Paragobidon enchinocephalus–moved toward the site of contact and began neatly trimming away the offending seaweed.

The research was part of a long-term study of chemical signaling on Fiji Island coral reefs. It was aimed at understanding these threatened ecosystems and discovering chemicals that may be useful as pharmaceuticals.

“These little fish would come out and mow the seaweed off so it didn’t touch the coral,” said Hay.

“This takes place very rapidly, which means it must be very important to both the coral and the fish. The coral releases a chemical and the fish respond right away.”

In corals occupied by the gobies, the amount of offending seaweed declined 30 percent over a three-day period, and the amount of damage to the coral declined by 70 to 80 percent.

Control corals that had no gobies living with them had no change in the amount of toxic seaweed and were badly damaged by it.

The researchers learned that one species – Gobion histrio – actually eats the noxious seaweed, while the other fish apparently bites it off without eating it. In the former, consuming the toxic seaweed makes the fish less attractive to predators.

Further Reading:
Corals Chemically Cue Mutualistic Fishes to Remove Competing Seaweeds, Danielle L. Dixson & Mark E. Hay, Science 9 November 2012: Vol. 338 no. 6108 pp. 804-807. DOI: 10.1126/science.1225748
National Science Foundation

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Mapping Coral Disease Clusters in the Caribbean

In the last 30 years, more than 90 percent of the reef-building coral in the Caribbean has disappeared because of a disease of unknown origin.

Now, scientists from the University of Florida have used a GIS (geographic information system) to show the whereabouts of the clusters of diseased coral. Their findings, published in the journal PLoS One, may help researchers derive better hypotheses to determine what contributes to coral disintegration.

Microbiologists and toxicologists often run laboratory tests on small samples of Acropora species of coral to determine the factors that contribute to white-band disease, known as WBD. It’s visually identified as a white band moving from the base of the coral up, killing the coral tissue as it goes, leaving only the exposed coral skeleton behind.

Laboratory results spur a range of theories of causation — anything from opportunistic pathogens to specific bacterial infections. Other scientists suggest that WBD is not the result of an outside agent, such as bacteria, but rather a stress response from the coral in reaction to changes in the marine environment, such as ocean pollution and rising ocean temperatures due to climate change.

This study searches for patterns in the incidence of the disease.

The researchers found the locations of significant disease clusters, information scientists can then use to narrow where they should take samples for further laboratory tests. This is the first of several studies established by the researchers exploring which types of spatial analysis are the most appropriate for various types of coral data from the Caribbean.

For thousands of years, Acropora was the predominant coral in the Caribbean, but more than three decades of disease have destroyed the species ability to survive, forcing marine life out of their coral habitats, which exposes them to attack by predators.

Acropora corals are distinctive – no other corals resemble their complex branching structure, which provides ideal habitat for many coral reef organisms. In terms of size, Acropora corals range from a dinner plate to a family car. Acropora species appear to be among the fastest growning corals, with upwward growth between 10 and 20 cm per year. Acropora is usually branched, except when young, but the size and shape of the branches vay according to the species concerned.

GIS systems are increasingly being used to assess coral reefs.

Further reading:
Lentz JA, Blackburn JK, Curtis AJ, 2011 Evaluating Patterns of a White-Band Disease (WBD) Outbreak in Acropora palmata Using Spatial Analysis: A Comparison of Transect and Colony Clustering. PLoS ONE 6(7): e21830. doi:10.1371/journal.pone.0021830
Geographical Information Systems
A low-cost procedure for automatic seafloor mapping, with particular reference to coral reef conservation in developing nations, Trond-Inge Kvernevik, Mohd Zambri Mohd Akhir and Jill Studholme