Cone snail

A group of shells of various species of cone snails

Cone snails, cone shells, or cones are a large group of small- to large-sized extremely venomous predatory sea snails, marine gastropod molluscs.^[1]

Until fairly recently, over 600 species of cone snails were all classified under one genus, Conus, in one family, the Conidae. However, in recent years, it was suggested that cone snails should occupy only a subfamily that should be split into a very large number of genera. A 2014 paper attempted to stabilize a newer classification of the group, significantly reducing the number of new genera but keeping a fairly large number of subgenera. Although the taxonomy has changed significantly several times during recent years, in the 2015 version of the taxonomy of these snails and their close relatives, cone snails once again compose the entire family Conidae.

Fossils of cone snails are known from the Eocene to the Holocene epochs.^[2] Cone snail species have shells that are more or less conical in shape (hence their common name). Many species have colorful patterning on the shell surface.^[3] Cone snails are almost all tropical in distribution.

All cone snails are venomous and capable of "stinging" humans; if live ones are handled their venomous sting will occur without warning and can be fatal. The species most dangerous to humans are the larger cones, which prey on small bottom-dwelling fish; the smaller species mostly hunt and eat marine worms. Cone snails use a hypodermic needle-like modified radula tooth and a venom gland to attack and paralyze their prey before engulfing it. The tooth, which is sometimes likened to a dart or a harpoon, is barbed and can be extended some distance out from the head of the snail, at the end of the proboscis.

Cone snail venoms are mainly peptides. The venoms contain many different toxins that vary in their effects; some are extremely toxic. The sting of small cones is no worse than a bee sting, but the sting of a few of the larger species of tropical cone snails can be serious, occasionally even fatal to humans. Cone snail venom is showing great promise as a source of new, medically important substances.^[4][5]

Distribution and habitat[]

There are over 900 different species of cone snails.^[6] Cone snails are typically found in warm and tropical seas and oceans worldwide, and they reach their greatest diversity in the Western Indo-Pacific region. However, some species are adapted to temperate/semi tropical environments, such as the Cape coast of South Africa,^[7][8] the Mediterranean,^[9] or the cool subtropical waters of southern California (Californiconus californicus),^[10] and are endemic to these areas.

Cone snails are found in all tropical and subtropical seas, from the intertidal zone to deeper areas, living on sand or among rocks or coral reefs. When living on sand, these snails bury themselves with only the siphon protruding from the surface. Many tropical cone snails live in or near coral reefs. Some species are found under rocks in the lower intertidal and shallow subtidal zones.

Shell description[]

This group of sea snails shows a large variety of colors and patterns, and local varieties and color forms of the same species often occur. This has led to the creation of a large number of known synonyms and probable synonyms, making it difficult to give an exact taxonomic assignment for many snails in this genus. As of 2009, more than 3,200 different species names had been assigned, with an average of 16 new species' names introduced each year.^[11]

The shells of cone snails vary in size. The shells are shaped more or less like a cone, as one might expect from the popular and scientific name. The shell is many-whorled and in the form of an inverted cone, the anterior end being the narrow end. The protruding parts of the top of the whorls that form the spire are more or less in the shape of another, much more flattened, cone. The aperture is elongated and narrow. The horny operculum is very small. The outer lip is simple, thin, and sharp, is without a callus, and has a notched tip at the upper part. The columella is straight.

The larger species of cone snails can grow up to 23 cm (9.1 in) in length. The shells of cone snails are often brightly colored and have interesting patterns, although in some species the color patterns may be partially or completely hidden under an opaque layer of periostracum. In other species, the topmost shell layer is thin periostracum, a transparent yellowish or brownish membrane.

Life habits[]

Cone snails are carnivorous and predatory. They hunt and eat prey such as marine worms, small fish, molluscs, and even other cone snails. Cone snails are slow-moving, and use a venomous harpoon (called a toxoglossan radula) to capture faster-moving prey, such as fish. The venom of a few larger species, especially the piscivorous ones, is powerful enough to kill a human.

The osphradium (a chemoreceptory organ) is more highly specialized than the same organ in any other group of gastropods. It is through this sensory modality that cone snails become aware of the presence of a prey animal, not through vision. The cone snails immobilize their prey using a modified, dartlike, barbed radular tooth, made of chitin, along with a venom gland containing neurotoxins. Small species of these cone snails hunt small prey, such as marine worms, whereas larger cone snails hunt fish.

Molecular phylogeny research by Kraus et al. (2010)^[12] based on a part of "intron 9" of the gamma-glutamyl carboxylase gene has shown that feeding on fish has evolved at least twice independently in the group.

Harpoon and venoms[]

An individual Conus pennaceus attacking one of a cluster of three snails of the species Cymatium nicobaricum, in Hawaii

Cone snails use a radula tooth as a harpoon-like structure for predation. Each of these harpoons is a modified tooth, primarily made of chitin and formed inside the mouth of the snail, in a structure known as the toxoglossan radula. (The radula in most gastropods has rows of many small teeth, and is used for grasping at food and scraping it into the mouth.) Each specialized cone snail tooth is stored in the radula sac (an everted pocket in the posterior wall of the buccal cavity), except the tooth that is currently ready to be used. The radular-tooth structures differ slightly according to the feeding mode of vermivorous, molluscivorous and piscivorous species.^[13]

The tooth is hollow and barbed, and is attached to the tip of the radula in the radular sac, inside the snail's throat. When the snail detects a prey animal nearby, it extends a long flexible tube called a proboscis towards the prey. The radula tooth is loaded with venom from the venom bulb and, still attached to the radula, is fired from the proboscis into the prey by a powerful muscular contraction. The venom paralyzes small fish almost instantly. The snail then retracts the radula, drawing the subdued prey into the mouth. After the prey has been digested, the cone snail will regurgitate any indigestible material, such as spines and scales, along with the then-disposable harpoon. There is always a dart stored in the radular sac. A dart may be used in self-defense when the snail feels threatened.^[14][15]

The venom of cone snails contains hundreds of different compounds, and its exact composition varies widely from one species to another. The toxins in these various venoms are called conotoxins. These are various peptides, each targeting a specific nerve channel or receptor. Some cone snail venoms also contain a pain-reducing toxin, which the snail uses to pacify the victim before immobilising and then killing it.

Relevance to humans[]

Risks[]

A live textile cone, Conus textile, one of the dangerous cones to handle.

The bright colors and patterns of cone snails are attractive,^[16] hence people sometimes pick up the live animals. This is risky, because the snail often fires its harpoon in these situations. The harpoons of some of the larger species of cone snail can penetrate gloves or wetsuits.

The sting of many of the smallest cone species may be no worse than a bee or hornet sting,^[17] but the sting of a few of the larger tropical fish-eating species, especially Conus geographus, Conus tulipa and Conus striatus, can be fatal. Other dangerous species are Conus pennaceus, Conus textile, Conus aulicus, Conus magus and Conus marmoreus.^[18] According to Goldfrank's Toxicologic Emergencies, about 27 human deaths can be confidently attributed to cone snail envenomation, though the actual number is almost certainly much higher; some three dozen people are estimated to have died from geography cone envenomation alone.^[19]

Most of the cone snails that hunt worms rather than fish are probably not a risk to humans, with the possible exception of larger species. One of the fish-eating species, the geography cone, Conus geographus, is also known colloquially as the "cigarette snail", a gallows humor exaggeration implying that, when stung by this creature, the victim will have only enough time to smoke a cigarette before dying.^[20][14]

Symptoms of a more serious cone snail sting include intense, localized pain, swelling, numbness and tingling and vomiting. Symptoms can start immediately or can be delayed for days. Severe cases involve muscle paralysis, changes in vision, and respiratory failure that can lead to death.

Medical use of the venom[]

The appeal of the cone snail's venom for creating pharmaceutical drugs is the precision and speed with which the various components act; many of the compounds target a particular class of receptor, to the exclusion of any other. This means that, in isolation, they can reliably and quickly produce a particular effect on the body's systems without side effects; for example, almost instantly reducing heart rate or turning off the signaling of a single class of nerve, such as pain receptors.

Ziconotide, a pain reliever 1,000 times as powerful as morphine, was initially isolated from the venom of the magician cone snail, Conus magus.^[21] It was approved by the U.S. Food and Drug Administration in December 2004 under the name "Prialt". Other drugs are in clinical and preclinical trials, such as compounds of the toxin that may be used in the treatment of Alzheimer's disease, Parkinson's disease, depression, and epilepsy.^[22][23]

Many peptides produced by the cone snails show prospects for being potent pharmaceuticals, such as AVC1, isolated from the Australian species, the Queen Victoria cone, Conus victoriae. This has proven very effective in treating postsurgical and neuropathic pain, even accelerating recovery from nerve injury.

Geography cone and tulip cone are known to secrete a type of insulin to cause hypoglycaemic shock in nearby fish, paralyzing them. They are the only two species known to use insulin as a natural weapon. This is interesting for biochemists for determining structure-function relationships in this protein.^[24]

Cone snail venom is currently being examined and studied for therapeutic usage^[25] to treat and inhibit various diseases and the side effects produced by drugs treating those diseases. While cone snail venom is primarily utilized to paralyze the host of the snail, the venom has shown to provide relief to human diseases. By deconstructing the highly potent peptides and proteins present in cone snail venom, scientists have been able to produce drugs to treat chronic pain, such as Ziconotide.^[26] The peptides and proteins are released through the venom gland of the cone snail and target the membrane proteins of the host. The conotoxins contain target specificity and short sequences. An example of this phenomenon is ω-MVIIA which targets Ca2+ channels and has a specificity for N-type calcium channels. There are multiple known conotoxin peptides which code for specific receptors such as: Contulakin-G, χ-MrIA, α-Vc1.1, Conantokin-G, κ-PVIIA, and μO-MrVIB.^[27] Most of these peptides have been shown to inhibit pain receptors in patients undergoing treatment for cancer and HIV.

Shell collecting[]

The intricate color patterns of cones have made them one of the most popular collectible shells.^[28][29]

Conus gloriamaris, the "Glory of the Seas" cone, was, in earlier centuries, one of the most famous and sought-after seashells, with only a few specimens in private collections. This apparent rarity meant that shells of this species fetched very high prices, until finally the habitat for this cone was discovered. Sizable populations were then located, and this brought the price down dramatically.^[30]

As jewelry[]

Naturally occurring, beach-worn cone shell "tops" (the broken-off spire of the shell, which usually end up with a hole worn at the tip) can function as beads without any further modification. In Hawaii, these natural beads were traditionally collected from the beach drift to make puka shell jewelry. Since it is difficult to obtain enough naturally occurring cone tops, almost all modern puka shell jewelry uses cheaper imitations, cut from thin shells of other species of mollusk, or made of plastic.

Species[]

Until 2009 all species within the family Conidae were placed in one genus Conus. Testing in order to try to understand the molecular phylogeny of the Conidae was begun by Christopher Meyer and Alan Kohn,^[31] and continued, particularly with the advent of nuclear DNA testing in addition to mDNA testing.

In 2009, J.K. Tucker and M.J. Tenorio proposed a classification system consisting of three distinct families and 82 genera for the living species of cone snails. This classification was based on shell morphology, radular differences, anatomy, physiology, and cladistics, with comparisons to molecular (DNA) studies.^[32] Published accounts of genera within the Conidae that use these new genera include J.K. Tucker & M.J. Tenorio (2009), and Bouchet et al. (2011).^[33] Tucker and Tenorio's proposed classification system for the cone shells and their allies (and the other clades of Conoidean gastropods) is shown in Tucker & Tenorio cone snail taxonomy 2009.

Some experts, however, preferred to use the traditional classification, where all species are placed in Conus within the single family Conidae: for example, according to the November 2011 version of the World Register of Marine Species, all species within the family Conidae were placed in the genus Conus. The binomial names of species in the 82 genera of living cone snails listed in Tucker & Tenorio 2009 were recognized by the World Register of Marine Species as "alternative representations".^[34] Debate within the scientific community regarding this issue continued, and additional molecular phylogeny studies were being carried out in an attempt to clarify the issue.^{[32][35][36][37][38][39][40][41][42][43]}

In 2015, in the Journal of Molluscan Studies, Puillandre, Duda, Meyer, Olivera & Bouchet presented a new classification for the old genus Conus. Using 329 species, the authors carried out molecular phylogenetic analyses. The results suggested that the authors should place all cone snails in a single family, Conidae, containing four genera: Conus, Conasprella, Profundiconus and Californiconus. The authors group 85% of all known cone snail species under Conus. They recognize 57 subgenera within Conus, and 11 subgenera within the genus Conasprella.^[1]

List of synonyms[]

See also[]

• ConoServer, a database of cone snail toxins, known as conopeptides.^[44] These toxins are of importance to medical research.
• Conotoxin

References[]

Further reading[]

External links[]

• SEM images of the radula can be found at Thompson, T.E.; Bebbington, A. (1973). "Scanning electron microscope studies of gastropod radulae". Malacologia. 14: 147–165.
• Tucker J.K. (2009). Recent cone species database. September 4th 2009 Edition
• Filmer R.M. (2001). A Catalogue of Nomenclature and Taxonomy in the Living Conidae 1758–1998. Backhuys Publishers, Leiden. 388pp
• Bouchet, P.; Fontaine, B. (2009). "List of new marine species described between 2002–2006". Census of Marine Life.
• Natural History Museum Rotterdam - photos of Conus shells
• Cone snail and conotoxins page
• The Conus Biodiversity website
• ConoServer
• Conidae from worldwide.conchology.com. Scroll down for many photographs.
• Pain-killer comes out of its shell (The Age news article)
• Venomous snails aid medical science (BBC News Article).
• ConeShell Collection Giancarlo Paganelli
• Cone Shells - Knights of the Sea. Alexander Medvedev's collection
• Cone Snail Video - Hunting Footage and Physiology
• Deadly Critters That Might Save Your Life (CNN)
• Baldomero "Toto" Olivera's short talk: Conus Peptides
• Zonatus Gallery
• Miller, John A. (1989). "The toxoglossan proboscis: structure and function". Journal of Molluscan Studies. 55 (2): 167–181. doi:10.1093/mollus/55.2.167.
• BBC Nature Video Cone snails are silent assassins of the sea, drugging sleeping fish before poisoning them