Overcoming the protective cuticle of armored opponents is a challenge faced by many organisms for both defensive and predatory reasons. Thick shells like those of crustaceans are especially difficult to penetrate without the aid of sharp and strong body parts and powerful muscles to back them up. Some organisms solve this challenge with only microscopic cellular components. Hydras, tiny animals from the cnidarian genus Hydra, feed on planktonic crustaceans and have evolved remarkable nano-structures that can penetrate the armor of their prey to inject venom.
The cells (cnidocysts) produce one large organelle called a nematocyst. The cell forms a layered polymer matrix around the nematocyst that keeps it strong and promotes the generation of 150 bar of pressure within the organelle at maturation. The “lid” of the capsule (operculum) associates with the cell membrane facing out. When the sensory portion of the cell (cnidocil) is mechanically disturbed (e.g., by contact with prey) it causes a rapid increase in the calcium ion concentration in the cell. This causes molecular rearrangement of the opeculum allowing the release of the nematocyst’s stored pressure towards the outside of the organism. The stylet, composed of strong and flexible protein tubules with spiked barbs at the end, ejects from the cell with an acceleration of ~5.4 million times gravity. Because of the tiny cross-sectional area at the tip of the stylet, 7.7 billion Pascals of pressure are exerted upon the cuticle of the prey and drives it deep into the underlying tissue. In fact, this is not only the fastest animal system observed to date, the pressure on impact is also comparable to that produced by a bullet.
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“The cnidocyst is the defining organelle of the cnidarians, used for capture of prey and defense…nematocysts comprise a powerful molecular spring mechanism releasing energy stored in the wall polymer in the nanosecond time range…The nematocyst is product of a giant post-Golgi vesicle…It consists of several different protein species that assemble into a large capsular structure with a long spiny tubule inside. The matrix is built up by poly-γ-glutamate, which binds a 2 M concentration of cations, thereby creating a high osmotic intra-capsular pressure of more than 150 bar…the nematocyst vesicle is docked at the apical side of the cell. Mechanical stimulation of the sensory receptor of the nematocyte, the cnidocil, elicits an action potential, which in turn triggers Ca2+-dependent discharge.” (Özbek et al. 2009:1038)
“Within less than 3 ms the tubule evaginates and injects toxins into the prey…On the molecular level, the nematocyst is designed to withstand this extreme mechanical stress by combining high resistance and flexibility. The major constituents of the capsule wall are mini-collagens, a family of unusually short collagens…in a three-dimensional polymer stabilized by disulfide bonds…and [cross-linked] NOWA protein. They form a continuous supra-structure of the nematocyst wall…All nematocysts have a solid, elastic wall and a lid structure, which closes the capsule before discharge…the capsules of the highest structural complexity are found in hydrozoans. Here, the stenoteles represent one of the most sophisticated capsule types. The stenotele tubule is enlarged at its basal region, which is called the shaft. The distal shaft carries the stylet apparatus, which contains three huge stylets and three arrays of thinner spines. This stylet apparatus forms a dart when the tubule evaginates, which punches a whole into the prey’s cuticle through which the long tubule enters, releasing hemolytic and neurotoxic substances…The dart-like stylet apparatus can penetrate even thick crustacean cuticles…It is generally accepted today that the high osmotic pressure that is built up at the end of capsule morphogenesis in conjunction with the elastically stretched capsular wall is the main driving force of discharge. At the end of morphogenesis the capsule is maximally expanded and during discharge the volume gets suddenly reduced again by about 50%…The high osmotic pressure is generated by poly-γ-glutamate and…anions…They are responsible for the generation and regulation of an internal osmotic pressure that amounts up to 150 bar…a high osmotic pressure inside the capsule is the basic principle of all nematocysts.” (Özbek et al. 2009:1039)
“[T]he molecular assembly of the operculum enables it to react to the ionic changes preceding discharge by a molecular rearrangement facilitating capsule wall disintegration at this position. Thus, the release of the increased osmotic pressure would be directed to the preformed opening preventing a random rupture of the wall structure.” (Özbek et al. 2009:1040)
“[A]n action potential that is normally elicited when a prey mechanically deflects the sensory receptor of the cell, the cnidocil…extracellular calcium entering the nematocyte leads to fusion pore formation during nematocyst exocytosis…In phase B the nematocyst vesicle fuses with the nematocyte membrane, and the capsule’s discharge process starts. The most obvious event is that the lid is opened and the stylet-bearing portion of the tubule is ejected…an average velocity (y) of the stylet tip of 18.6 m/s is generated…5,413,000 x g is required to produce this average velocity (y) from a standing start. This high acceleration can explain how a small mass can generate sufficient force at the site of impact. The stylets are composed of a keratin-like protein and have an extremely narrow tip. Based on the accelerated mass and the stylets tip (80 nm2), a pressure (p) of 7.7 GPa was estimated when the stylet tip hits the prey. This force is in the range of technical bullets and explains how a 5 mm thick solid cuticle of a crustacean prey can be effectively perforated by a cnidarian nematocyst with a minimum of mass. This seems to be the fastest known process generated in animal systems…[prior to discharge] the cyst is in its maximally expanded state and only a minimal increase in pressure is sufficient to elicit discharge…Spinalin, a keratin-like glycine- and histidine-rich protein, is involved in the formation of spine and operculum structures.” (Özbek et al. 2009:1041)