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Freshly Baked Science


Corals: Upside Down Jellyfish with an Impressive Arsenal for Hunting

by Hagen Gegner

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5th January 2019

Corals are known to be threatened by the effects of climate change such as ocean warming, endangering their existence globally in a human influenced time – the Anthropocene. However, scaling down and zooming into their coastal alien world these fragile coral animals suddenly become fascinating predators with impressive molecular weapons. But what is a coral other than a stone-like colourful underwater feature?

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Figure 1. A coral reef full of life in Indonesia. This incredible reef structure is made by thousands of tiny polyps that collectively are referred to as a coral colony. Photo credit: Hagen Gegner.

What we see as “a” coral is actually the collective effort of hundreds to thousands of tiny polyps, upside - down jellyfish, that is creating an ever-growing calcium skeleton: a coral reef. These individual coral polyps are clones and genetically identical to each other within one coral. They live in symbiosis with algae that reside inside their cells, making the coral a holobiont, an ecological unit comprised of more than one organism (1). By this definition, you, as a host for billions of bacteria and viruses, are a holobiont, too.

Besides this complex living arrangement, coral polyps are incredibly basic in structure and made up of merely two cell layers. The outer cell layer is called the ectoderm and the inner cell layer the endoderm – that’s it. To deliver the energy demanded by building this impressive calcium skeleton that eventually becomes the complex reef structure, corals have two options: They can receive energy from their algal symbionts that live inside the endoderm, or they start feeding on their own. While the former is their number one choice that attracts a growing body of research to uncover this mysterious symbiosis with algae and the issues arising with climate change (coral bleaching), the latter is equally fascinating and surprisingly vicious.

Corals are often depicted as fragile animals threatened by ocean warming (2), while this indeed is true when on the hunt, these sessile Jurassic champions show a rather aggressive hunting technique. Magnifying one single polyp, we can identify the most prominent feature visible - the tentacles that emerge around the polyp’s mouth. These tentacles are used to grab food and simply put it into the mouth where it then gets digested. But how does a polyp “grab” food without a thumb and how can they catch anything that moves? The answer is with specialized cells.

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Figure 2. A magnified coral polyp of a staghorn coral. The polyp resembles an upside - down jellyfish. In the middle of it, you can see the mouth that is surrounded by tentacles. The cells of corals are translucent, the colour that is visible in the tentacles comes from their algal symbionts that serve as energy source for the whole polyp. Photo credit: Hagen Gegner

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The polyp´s tentacles are full of cnidocytes, explosive cells that define the phylum of the corals and jellyfish: Cnidaria. These cnidocytes consist of tiny capsules that retain coiled up cell structures that can be shot at targets when released. There are several sub-groups of cnidocytes with specialized tasks, e.g. nematocytes. Nematocytes are cells used for hunting that can shoot a syringe-like filament able to penetrate the skin of prey (zooplankton – small drifting animals) and inject a venom. This, of course, is not limited to prey but works also on naïve divers who had the urge to touch a fire coral or got in contact with a jellyfish.


This powerful molecular spring mechanism inside the nematocytes explodes within a split of a second (700 nanoseconds) releasing these capsuled syringes (3). Other cnidocytes shoot harpoons with glue instead of venom to attach prey to the tentacle or shoot a filament that coils around prey to immobilize it further. All these cells are contained inside the ectoderm of the tentacles multiple times and can only be fired once. Who would have thought that these fragile animals carry such a molecular arsenal?


The next time when you are on holiday and get stung by a jellyfish or accidentally touch a coral, instead of being angry at them, firstly, get some vinegar to sooth your sting and then show off your newly acquired knowledge of one of the most powerful molecular spring mechanisms that just harpooned you. Remember, the admiration and fascination of nature peaks when we understand the underpinnings of it, although we got painfully stung by it.



1. Bosch, T. C. G., & Miller, D. J. (2016). The holobiont imperative: Perspectives from early emerging animals. The Holobiont Imperative: Perspectives from Early Emerging Animals. https://doi.org/10.1007/978-3-7091-1896-2

2. Hughes, T. P., Kerry, J. T., Álvarez-Noriega, M., Álvarez-Romero, J. G., Anderson, K. D., Baird, A. H., … Wilson, S. K. (2017). Global warming and recurrent mass bleaching of corals. Nature, 543(7645), 373–377. https://doi.org/10.1038/nature21707

3. Nüchter, T., Benoit, M., Engel, U., Ozbek, S., & Holstein, T. W. (2006). Nanosecond-scale kinetics of nematocyst discharge. Current Biology : CB, 16(9), R316-8. https://doi.org/10.1016/j.cub.2006.03.089