They can hide, but not forever! So how does the sea anemones defend itself from potential predators? Let's find out in this post!
Cnidae - The Microscopic Weapons of Death
Instead of using knives, swords or throwing stars, the sea ninja (aka the sea anemones) uses microscopic weapons known as cnidae for defending against predators and capturing prey. Cnidae are microscopic capsules, which forcefully shoot out threads when there is mechanical and/or chemical stimulations. For example, the freshwater polyp Hydra magnipapillata can discharge its cnidae due to changes in light levels. In general, there are three types of cnidae: nematocysts, ptychocysts, and spirocysts.
|Cnidae of Cnidaria. They come in all shapes and sizes! Source: Snail's Odyssey.|
The sea ninja uses nematocysts to store and inject venom into its victims. The exact mechanism of how nematocysts discharge is still a mystery because this discharge occurs in a matter of nanoseconds. So far, we know that after stimulation, positively charged calcium ions leave the capsule behind, and the negatively charged polyglutamic acid is still bound to the nematocyst. This results in water entering the nematocysts, causing a rise in osmotic pressure and leading to the nematocysts to burst. The hydrostatic pressure and spring mechanism of the nematocyst evert the tubule and it injects into the skin of the target. The force of the tubule is enough to puncture the rock-hard scales of fish! The discharged tubule can have an acceleration of 5,400,000g and a pressure 7.7 GPa, which makes being injected by nematocysts the microscopic version of being shot by a speeding bullet.
|Fire anemone (Actinodendron arboreum) may look deceptively like a soft coral, but it packs a powerful and painful sting!|
Aside from using cnidae for offense, sea anemones also use cnidae to stick to things. The tubules within the spirocysts are interconnected in a hexagonal array before discharge. Once the spirocysts discharge, the tubules dissolve in water and adhere to substrata (e.g., sand) or prey. The traction between the tubule and surface leads to the tubule spinning out and forms a microscopic web. Sea anemones not only use nematocysts to envenomate prey, but they also use their sticky spirocysts to prevent prey from escaping their tentacles. The banded bead anemone, Anthopleura dixoniana use spirocysts to stick to rocks stubbornly. Ptychocysts are only produced by tube-dwelling peacock anemones (cerianthids). Once ptychocysts discharge the everted threads act as an adhesive for sand and other sediments which helps cerianthids form tubes.
|Glass sea anemone (Dofleinia sp.) - another beautiful anemone that is considered dangerous as it can inflict painful stings that take months to heal!|
Nematocysts have provided a new way to deliver drugs since they are like microscopic injections. In theory, the nematocysts would be loaded with the drug, put in a patch like a device, and then once cnidae discharge, the drug will be injected into the patient’s skin. The best part is that this form of injection hurts a lot less. Spirocysts and ptychocysts could provide inspiration for the development of novel biomaterials. Cnidae is one of the complex secretion products in nature and it a unique feature which not only distinguished sea anemones, but all Cnidarians.
About the writer:
Shabdita Vatsa recently graduated from the National University of Singapore with a Bachelors in Life Sciences (Molecular and Cell Biology). She spent most of her undergraduate days obsessing over sea anemones and their toxins. Her fascination became research projects, working with scientists from the Reef Ecology Lab and Protein Chemistry Lab to study anemone toxins.
A passionate writer, Shabdita loves creative writing, popular science writing, and scientific writing. You can find more of her writings on her personal page: https://lifesciencepotluck.tumblr.com/
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