Coral SOS Signals

Coral SOS Signals Turtle weed (Chlorodesmis fastigiata)
Photo Credit: Dr. Robert Ricker, NOAA/NOS/ORR [Public domain], via Wikimedia Commons

Seaweed-covered corals emit a chemical that entices goby fish to clean them.


Corals’ cleaning call. I’m Bob Hirshon and this is Science Update.

Coral reefs may look like colorful rocks, but they’re surprisingly complex organisms. In fact, they can call in a cleaning crew when they need it, according to Georgia Tech biologists Danielle Dixon and Mark Hay in the journal Science. Hay says a type of seaweed is toxic to coral. But as soon as the seaweed touches it, the coral releases a chemical that attracts goby fish, which eat it off.

So they’re like little barbers that run out, and just eat enough of it so that it doesn’t touch the coral anymore.

The fish were even attracted to water taken from other coral reefs that had been touched by seaweed—yet the seaweed alone didn’t interest them. That suggests the gobies weren’t just snacking, but defending the coral where they make their home. Hay says it’s the first known example of a species calling in a bodyguard with a chemical signal. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

Our ecosystem is full of symbiotic relationships—organisms that both benefit and depend on one another. One of these is the relationship between some types of goby fish and Acropora corals, which, like other corals, is a colony of individual organisms, called polyps, that share common nerve networks and connective tissue. Corals form lush, complex structures, known as coral reefs, that provide shelter to fish like gobies, which also feed on algae and other organisms that collect on the reef.

In most cases, the algal life that collects on coral is harmless as long as it doesn't get too overgrown. But one particular type of algal seaweed, called turtleweed, is highly toxic to coral. When the seaweed touches coral, it releases chemicals that not only damage the coral directly, but also breed disease-causing micro-organisms. 

However, the corals fight back with a chemical of their own. When they're touched by the seaweed, the corals release a signaling chemical that makes two species of goby fish swoop in to clean away the toxic seaweed. One species, the red-headed goby, just removes the seaweed. Another, the broad-barred goby, eats it. When it does, it scores an advantage: a toxic mucus that the broad-barred goby secretes to defend against predators becomes more powerful. 

The researchers tested the relationship in a number of ways to show that the chemical does, in fact, function mainly as a distress signal. When they exposed gobies to samples of the turtleweed alone, they didn't react. But when they squirted them with water taken from areas where turtleweed was damaging coral, they sprang into action. That shows that it's the chemical signal, not the seaweed itself, that the fish follow. So in a way, they're acting like bodyguards for the coral—although they're also defending their own homes.

Now try and answer these questions:

  1. How do the goby fish and Acrapora coral normally depend on each other, day to day?
  2. What happens when turtleweed touches Acrapora coral?
  3. How did the scientists show that this acted as a call for help?
  4. Suppose the gobies chased after samples of turtleweed alone, in addition to water taken from areas where turtleweed attacked coral. What conclusion might the scientists have drawn from that?
You may want to check out the December 7, 2012, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: Why fairy wrens require their babies to show ID, how coral reefs ask for help, and how one microscopic animal patches itself up with borrowed genes. Also: Host Bob Hirshon went down to NASA Headquarters to report on the discovery of water on the planet Mercury.

For more on chemical communication in the wild, see the Science Updates Plant Kin HelpFearless AphidsNot Dead Yet, and Worm Language.

Going Further

For Educators

The blog post The Last Reef explores the global problem of disappearing reefs. The Great Barrier Reef and Discovering the Great Barrier Reef highlight the amazing living structure off the coast of Australia.

For more on chemical communication in the wild, see the Science Updates Plant Kin HelpFearless AphidsNot Dead Yet, and Worm Language.

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