Tiny polyps save corals from predators and disease

December 22, 2017

In a new study published today in Proceedings of the Royal Society, the scientists show how tiny hydrozoans, polyps smaller than one millimeter and commonly found in dense colonies on the surface of hard corals, may play a role in keeping corals safe and healthy.

Tibor Navracsics, Commissioner for Education, Culture, Youth and Sports, responsible for the Joint Research Centre said: "EU scientists are actively contributing to solve global problems. This is a good example of the scientific excellence that can help us to protect our environment and support the policy making."

Tiny polyps protect corals from being eaten by fish and snails

The researchers from the JRC, Marhe Center of Milan-Bicocca University and the King Abdullah University of Science and Technology researched almost 2500 coral colonies in Maldivian and Saudi Arabian reefs, searching for signs of predation, temperature induced stress, and disease. For each colony, they also looked for the presence or absence of symbiotic hydrozoans and concluded that corals living in association with hydrozoans are much less prone to be attacked by coral-eating fish and gastropods, such as snails, than hydrozoan-free corals.

Predators are likely deterred by the venomous organelle ejected by hydrozoans' nematocysts, similar to those found in medusas' tentacles. Obviously, an individual hydrozoan polyp of less than one millimeter cannot cope with fish with a body mass billions of times its own. Yet when hydrozoans reach very high densities on coral surface, sometimes more than 50 individuals per square centimeter, it creates a sort of a continuous, stinging carpet that can discourage fish from foraging.

This effect is even more pronounced for coral eating gastropods, which are exposed to hydrozoans' nematocysts longer and more intensely as they crawl. This finding is important for coral conservation because coral eating gastropods are regarded as a major threat to reef persistence as a consequence of warming water temperatures.

The authors also found that the presence of symbiotic hydrozoans goes hand in hand with better health of corals. This could be either a secondary effect as wounds caused by predators are more easily infected by pathogens, and/or the result of "friendly" hydrozoans feeding on pathogenic protozoans trying to invade coral hosts.

Unfortunately, even hydrozoans cannot protect corals against mortality directly associated to increasing temperature (i.e. coral bleaching), as demonstrated by the massive coral die out recorded last year in the Great Barrier Reef. Yet, by contributing to the overall health of corals, they are likely playing an important role in making reefs more resilient, possibly shifting the critical threshold between reversible and irreversible disturbances. If this is the case, this would mean that an almost invisible and largely overlooked microscopic organism could be buying time for half a billion people relying on coral reef ecosystems for their livelihoods.
-end-


European Commission Joint Research Centre

Related Coral Articles from Brightsurf:

The cement for coral reefs
Coral reefs are hotspots of biodiversity. As they can withstand heavy storms, they offer many species a safe home.

Cauliflower coral genome sequenced
A newly sequenced coral genome offers tools to understand environmental adaptation.

Shedding light on coral reefs
New research published in the journal Coral Reefs generates the largest characterization of coral reef spectral data to date.

Nutrients make coral bleaching worse
Nutrients can aggravate the already negative effects of climate change on corals to trigger mass coral bleaching.

Surprising coral spawning features revealed
When stony corals have their renowned mass spawning events, in sync with the moon's cycle, colonies simultaneously release an underwater 'cloud' of sperm and eggs for fertilization.

Collaboration is key to rebuilding coral reefs
The most successful and cost-effective ways to restore coral reefs have been identified by an international group of scientists, after analyzing restoration projects in Latin America.

Stress testing 'coral in a box'
Save the corals: Mobile rapid test to assess coral thermotolerance developed in an international collaboration with the University of Konstanz

Can coral reefs 'have it all'?
A new study outlines how strategic placement of no-fishing marine reserves can help coral reef fish communities thrive.

Coral reefs 'weathering' the pressure of globalization
More information about the effects human activities have on Southeast Asian coral reefs has been revealed, with researchers looking at how large-scale global pressures, combined with the El NiƱo Southern Oscillation (ENSO) climate pattern, can detrimentally impact these delicate marine ecosystems.

Coral reefs: Centuries of human impact
In her AAAS talk, ASU researcher Katie Cramer outlines the evidence of the long-ago human footprints that set the stage for the recent coral reef die-offs we are witnessing today.

Read More: Coral News and Coral Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.