By Rebecca Campbell Gibbel, MS, DVM
Pulsing Xenia elongata for sale online for $18.41.
WHAT ARE PULSE CORALS?
Pulsing octocorals or xeniids, are Indo-Pacific soft coral species that are favored in the aquarium trade for their beauty and charismatic behavior. Their eight large tentacles have stalks with feathery ends that open and close continuously. Their closely packed array of polyps resemble a field of flowers pulsing independently day and night, in a mesmerizing display that only stops if they are disturbed. They are easy to maintain in aquaria because they tolerate a range of water conditions and are described as peaceful specimens because they do not sting neighboring corals, but they are notorious for growing so vigorously that they can overgrow a tank.. Recently, pulsing corals were found in the Caribbean and are now causing havoc in the new locations where they have no natural controls.
The Unomia stolonifera species appeared in Venezuelan waters in the early 2000’s and have rapidly overtaken 50-100% of the coral cover on the country’s native reefs (Zangroniz, 2023). The same species is spreading in Hawaii, and intensive efforts are underway to combat them there, although it may seem odd to be in a position of eliminating corals rather than safeguarding them! A different xeniid, the Xenia umbellata pom-pom species, has recently been seen in Puerto Rico and Cuba, and biologists are working hard to find ways to stop them before they smother the remains of the dwindling Caribbean reefs. Pulsing corals grow readily on sand, sea grass, hard coral reefs and in poor water quality conditions. Like other invasive species they have rapid growth, hardiness, fast reproduction, and very few known predators – the perfect invader!
Unomia stolinfera overtaking a reef in Venezuela, smothering stony corals and all other reef organisms
WHY DO THEY PULSE?
This question has long piqued the interest of biologists; does pulsing clear debris, defend against competitors, or capture planktonic prey? But investigations revealed that the gut cavities of pulse corals were not found to contain food (Fabricius and Alderslade, 2001) and these corals will actually eject any particle that finds its way into their mouths (Lewis, 1982). Most other corals depend on their symbiotic Symbiodiniaceae algae for the majority of their nutrition and capture the rest from floating plankton. The Symbiodiniaceae reside within the corals and create a food source for the host corals through photosynthesis. In exchange, the Symbiodiniaceae gain a protected dwelling and vital nutrients from the coral. Pulse corals are now understood to have the unusual approach of relying completely on their algae to feed them, so they must pursue strategies to optimize and retain their Symbiodiniaceae.
Kremien et al. (2013) examined the miniscule currents produced by pulsing coral tentacles. They found that this water flow creates mixing of the water surrounding the tentacle ends, and an upward jet of water away from the polyp. The stirring of the water around the tentacles allows 5-fold greater algal photosynthesis by flushing away the oxygen that accumulates as a product of daytime photosynthesis. Too much oxygen is harmful to the coral host and suppresses photosynthesis, and its accumulation is a main reason why corals must limit the number of algae that they harbor. At night, when no photosynthesis is occurring, the very low level of oxygen in the surrounding water is also stressful to the coral polyps and the nocturnal mixing process creates turbulence to bring small amounts of dissolved oxygen and other nutrients to the polyp.
From Samson et al., (2019): Images of tracer particles introduced around a purple pulsing coral polyp, revealing microcurrents. Mixing is pronounced near the tentacles where dissolved nutrients are first absorbed and then an upward pulse of water drives oxygen-enriched water away from the polyp.
The pulsation that greatly increases Symbiodiniaceae photosynthesis allows the coral to grow quickly, and this benefit outweighs the calculated energy cost of the pulsing behavior (Kremien et al., 2013). This significant advantage contributes to the xeniids’ extremely vigorous growth.
HOW DO THEY PULSE?
Most coral polyps can withdraw into the skeletal structure, and then inflate with water to extend or move. The center of the arm-like tentacles are hollow and communicate with the polyps’ central chamber, while the polyp’s mouth expels or brings water in. The expansion of tentacles is similar to the inflation of the goofy puppet tubes that are used for advertising purposes.
“Air dancer” tubes can be inflated and deflated similar to coral polyps.
Although corals do not have brains, they do have a well-developed system of nerves and muscle tissue, with connections to specialized cells that sense chemical and mechanical stimuli as well as light (Corals of the World.org). Polyps extend and contract in all corals in response to signals from the nerve net. In xeniid corals this behavior is repurposed into the pumping maneuver which maximizes Symbiodiniaceae function. The nerve net allows signals to be transferred from polyp to polyp to communicate danger, which is how numerous polyps will contract or stop pulsing when only one is disturbed.
CAN THEY BE STOPPED?
Some facts about the challenge ahead:
- Pulse corals spread extremely easily through fragmentation and can be relocated through fishing, shipping, and other human activities
- Xeniids can also spread by sexual reproduction.
- When ocean temperatures increase, soft corals are usually more resilient to bleaching and respond by acquiring even more symbiotic algae, rather than ejecting them (Steinberg et al., 2022). This helps the pulsing corals withstand the heatwave and outcompete native species.
So now that pulse corals have been introduced into the Caribbean Sea, can they be stopped? Various methods are effective for killing these corals, including manual removal through ultrasound machines and vacuums, and chemical injections to the colony. But the simplest method is to turn their essential supercharged algae into their Achilles heel. If a piece of light-occluding fabric is weighted on top of an area of pulse coral invasion and left in place for 8 days, the underlying colony will die from lack of light. Since pulse corals do not capture food from the water column, when their algae do not photosynthesize, they will quickly starve. What is key in trying to remove them is early identification of invading colonies while they are still small. Sending divers out to suppress each patch of Unomia or Xenia is clearly a labor-intensive effort, but it will be worth it if this beautiful but lethal threat to our struggling reefs can be controlled.
Clownfish and pulse coral in the Pacific, where they belong.
REFERENCES
- Corals of the World.org. Coral Structure and Growth. https://www.coralsoftheworld.org/page/structure-and-growth/
- Fabricius, K. E. and Alderslade, P. (2001). Soft Corals and Sea Fans: A Comprehensive Guide to the Tropical Shallow Water Genera of the Central-West Pacific, the Indian Ocean and the Red Sea. Townsville, QLD: Australian Institute of Marine Science.
- Kremien, M., Shavit, U., Mass, T., & Genin, A. (2013). Benefit of pulsation in soft corals. Proceedings of the National Academy of Sciences, 110(22), 8978-8983.
- Lewis, J. B. (1982). Feeding behavior and feeding ecology of the Octocorallia (Coelenterata: Anthozoa). J. Zool. 196, 371-384. doi:10.1111/j.1469-7998.1982. tb03509.x
- Samson, J. E., Miller, L. A., Ray, D., Holzman, R., Shavit, U., & Khatri, S. (2019). A novel mechanism of mixing by pulsing corals. Journal of Experimental Biology, 222(15), jeb192518.
- Steinberg, R. K., Ainsworth, T. D., Moriarty, T., Bednarek, T., Dafforn, K. A., & Johnston, E. L. (2022). Bleaching susceptibility and resistance of octocorals and anemones at the world’s southernmost coral reef. Frontiers in Physiology, 13, 804193
- Zangroniz, A. (2023, February 28). Be on the Lookout: a potential new invasive species-Xeniid soft corals . UF IFAS Blogs. https://blogs.ifas.ufl.edu/miamidadeco/2023/02/28/be-on-the-lookout-a-potential-new-invasive-species-xenia-soft-corals/
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