There are about 22,000 species of snails in the world. Most of them belong to the gastropod order. Pulmonata, a minority of which belongs to the suborder Pulmonata. Snails feed on rotten plant matter and lay eggs in the soil. Snails are widely distributed, most commonly found on tropical islands (such as Cuba), but also found in cold regions (dormant in winter). Arboreal species are brightly colored, while ground-dwelling ones are usually monochromatic. Achatina is the largest in Africa, more than 25 cm. One type of snail is particularly special, its shell contains iron sulfide nanoparticles, showing a characteristic black metallic luster. And it is extremely strong, even bullets can not penetrate, it is a scaly gastropod snail.
The scaly-horned gastropod snail was first spotted in 2001 at the bottom of the indian ocean more than 2,000 meters below the surface. Two species of scaly-horned gastropod snails have been found. Living in the Kairei district (located on the ridge of the southwest Indian Ocean) is black with an abundance of iron, where the snail shell is magnetic; In the Solitaire district (on the central Indian Ocean ridge), the snails here are iron-deficient, white, and non-magnetic.
Although scientists discovered the scaly-horned gastropod snail more than a decade ago, it wasn't until 2015 that researchers officially published a biological description of the species and determined its scientific name was Chrysomallon squamiferum. The genus name "Chrysomallon" is derived from the ancient Greek language meaning "golden hair" because the ferrous disulfide in their snail shells appear golden; The species name "squamiferum" comes from Latin and means "scaly".
The head of the scaly-horned gastropod snail has two smooth, tapering antennae. They have no eyes and no specialized adapters. Their gastropods are red and large in size, unable to fully retract the shell. In addition, they do not have Suprapedal glands like other snails and slugs, nor do they have upper foot tentacles.
The shell of the scaly gastropod snail is covered mainly with iron disulfide and magnetic pyrite. These metals come from mineral-rich nozzles. The shell gives the snail a natural armor to evade predators, not only defending itself but also causing damage to the claws of attacking enemies.
The study found that the gastropods of scaly-horned gastropod snails are covered with iron-containing scales and have an unusual three-layer iron-rich shell, the first layer of 250 micron-thick aragonite layer (CaCo3) (aragonite is a common shell material), this layer of aragonite structure is wrapped in another layer of 150 microns thick paste-like organic layer, the organic layer also has the function of heat dissipation, and the outside of the paste-like organic layer is the snail's hard and thin outer layer (about 30 microns thick).
How do these shells of extraordinary strength grow? The study found that the shell of the scaly gastropod snail is not the result of its own growth with such a high intensity, but the result of continuous mineralization in the living environment. Of course, this mineralization process is related to the unique physiological structure of the scaly-horned gastropod snail.
The process of biomineralization is ubiquitous in nature, and mollusks produce hard tissue through biominalization for bone support and other roles. These hard tissues are usually found in teeth and bones.
Scientists placed iron-deficient white scaly-horned gastropod snail scales in the deep-sea black smoke hydrothermal vents black-scaled gastropod snail habitat. However, the scales of the white scaly gastropod snail were not biominerized by iron sulfide, and their iron atom content did not exceed the crystallization limit of EDS analysis. It shows that the mineralization process is related to the physiological activity of scaly gastropod snails.
The iron atoms that make up the sulfide minerals in the scales of the scaly gastropod snails may have two sources: the snail's body tissues or the surrounding seawater. To further understand the sources of snail utilization, the scientists used STEM/EDS to study the interface between the secreted epithelium of the black sheath foot and the newly formed scales. The electron spectrogram clearly shows that the newly secreted shell contains sulfur, but does not contain iron. The sulfur content in the snail tissue of the scaly gastropod is low and contains detectable sulfur. If the iron atoms in the iron sulfide nanoparticles come from the snail's body, then the accumulation of iron around the surface and newly formed scales should be observable. Thus, the iron atoms do not come from the snail's body, but from the surrounding seawater. The depletion of sulfur in the scale-secreting tissue and the enrichment of sulfur in the newly formed part of the scales indicate that the sulfur in the iron sulfide nanoparticles is provided by animals and deposited in the scales when the scales are formed.
In the process of subcutaneous epithelial secretion of scales, the pore size of the sulfur diffusion channel is constructed, and then the organic matrix is filled, thereby promoting the diffusion of sulfur ions. The scales grow by increasing layers consisting mainly of an organic matrix, and sulfur ions, as well as unknown oxidants from the fluid circulation of the sheathed foot, diffuse into the columnar. Iron ions diffuse inward from the surface of scale and react with sulfur ions in the cylinder to form iron sulfide. The growth of iron sulfide crystals is regulated by a columnar organic matrix to produce nanoparticles, and the size of the nanoparticle aggregation is mainly limited by the diameter of the columnar structure. Co-existing sulfides can lead to the chemical conversion of iron sulfide and sulfide ions, which are then trapped on the walls of the channel to form sulfur-rich areas. Since the concentration of iron ions around the surface increases while other conditions remain unchanged, an iron-rich mineral [i.e., iron ions]. Gray crystals (Fe3S4) tend to form near the surface of the scales, resulting in the formation of 3 different chemical species of ferric sulfide observed before.
Although much research has been done on the formation of scaly gastropod snail shells and some evidence of the shell formation process has been obtained, how scaly gastropod snails control this process remains a mystery to this day. Speculation about how scaly-horned gastropod snails are injected into iron sulfide nanoparticles is currently divided into three processes, including one abiotic process and two biomineralization processes. First, abiotically formed nanoparticles may be incorporated into these particles, as mineral nanoparticles are produced by the hydrothermal vents themselves as high-temperature flow reactors with a fluid temperature of 315 °C; These nanoparticles may then find a way to enter this scale — for example, reactions bind together during secretion. Secondly, sulfur oxide bacteria are abundant around hydrothermal vents and often appear on the surface of animals; This bacterium may biominerize iron sulfide, which is subsequently incorporated into scaly gastropod snail scales. Third, whether in the epidermis secreted under the scales or under the scales, it is indeed possible for the scaly foot snail to biotransform or biologically control the mineralization of ferric pyrite through its own biotransformation.
The mechanism of mineralization of the shell of the scaly-horned gastropod snail is suitable for low temperature and dynamic environments. Although this process is mediated by the scaly-horned gastropod snail itself, some experiments have demonstrated that the formation of iron sulfide nanomaterials through this mechanism does not require the involvement of living animals, as long as the chemical and physical framework (i.e., the scales are saturated by sulfur-rich regions, for example). This greatly improves the utility of its applications in future industrial processes, not just iron nanoparticles.
The biominalization of unusual materials in deep-sea organics provides inspiration for the low-energy fabrication of functional materials. Now the U.S. military is currently developing a new military armor for the peculiar shell construction of this snail. Perhaps in the near future, military equipment or other materials for bionic scaly-horned gastropod snails will appear in reality.