Fossils of cone snails have been found from the
Eocene to the
Holocene epochs.[2] Cone snail species have shells that are roughly
conical in shape. Many species have colorful patterning on the shell surface.[3] Cone snails are almost exclusively tropical in distribution.
All cone snails are venomous and capable of stinging. Cone snails use a modified
radula tooth and a venom gland to attack and paralyze their prey before engulfing it. The tooth, which is 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
peptide-based, and contain many different toxins that vary in their effects. The sting of several larger species of cone snails can be serious, and even fatal to humans. Cone snail venom also shows promise for medical use.[4][5]
Distribution and habitat
There are over 900 different species of cone snails.[6] Cone snails are typically found in warm tropical seas and oceans worldwide. Cone snails reach their greatest diversity in the
Western Indo-Pacific region. While the majority of cone snails are found in warm tropical waters, some species have adapted to temperate/semi-tropical environments and are endemic to areas such as the
Cape coast of South Africa,[7][8] the
Mediterranean,[9] or the cool subtropical waters of southern
California (Californiconus californicus).[10]
Cone snails are found in all tropical and subtropical seas. They live on a variety of substrates, from the intertidal zone and deeper areas, to sand, rocks or
coral reefs.
Shell
Cone snails have a large variety of shell colors and patterns, with local varieties and color forms of the same species often occurring. This variety in color and pattern 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 have been assigned, with an average of 16 new species names introduced each year.[11]
The
shells of cone snails vary in size and are conical in shape. The shell is whorled in the form of an inverted cone, with the anterior end being narrower. The protruding parts of the top of the
whorls, that form the
spire, are in the shape of another more flattened cone. The
aperture is elongated and narrow with the sharp
operculum being very small. The outer lip is simple, thin, and sharp, 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 with a variety of patterns. Some species color patterns may be partially or completely hidden under an opaque layer of
periostracum. In other species, the topmost shell layer is a thin
periostracum, a transparent yellowish or brownish membrane.
Physiology and behavior
Cone snails are
carnivorous. Their prey consists of marine
worms, small
fish,
molluscs, and other cone snails. Cone snails are slow-moving, and use a venomous
harpoon to disable faster-moving prey.
The
osphradium in cone snails is more specialized than in other groups of gastropods. It is through this sensory modality that cone snails are able to sense their prey. The cone snails immobilize their prey using a modified, dartlike, barbed radular tooth, made of
chitin, along with a venom gland containing
neurotoxins.
Molecular phylogeny research has shown that preying on fish has evolved at least twice independently in cone snails.
Harpoon
Cone snails use a harpoon-like structure called a radula tooth for predation. Radula teeth are modified teeth, primarily made of
chitin and formed inside the mouth of the snail, in a structure known as the toxoglossan
radula. Each specialized cone snail tooth is stored in the radula sac, except for the tooth that is in current use.[12]
The radula 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 can paralyze smaller 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 harpoon. There is always a radular tooth in the radular sac. A tooth may also be used in self-defense when the snail feels threatened.[13][14]
The venom of cone snails contains hundreds of different compounds, and its exact composition varies widely from one species to another. The toxins in cone snail venom are referred to as
conotoxins, and are composed of various
peptides, each targeting a specific nerve channel or receptor. Some cone snail venoms also contain a pain-reducing toxin.
Relevance to humans
Dangers
Cone snails are prized for their brightly colored and patterned shells,[15] which may tempt people to pick them up. This is risky, as the snail often fires its harpoon in self defense when disturbed. 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,[16] but the sting of a few of the larger tropical fish-eating species, such as 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.[17] 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.[18]
Most of the cone snails that hunt worms are not a risk to humans, with the 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.[13][19]
Symptoms of a more serious cone snail sting include severe, 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. If stung, one should seek medical attention as soon as possible.[20]
Medical use
The appeal of
conotoxins for creating pharmaceutical drugs is the precision and speed with which the chemicals act; many of the compounds target only a particular class of
receptor. This means that 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.
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, and have been highly effective in treating postsurgical and neuropathic pain, even accelerating recovery from
nerve injury.
Geography and
tulip cone snails are known to secrete a type of
insulin that paralyzes nearby fish by causing
hypoglycaemic shock. They are the only two non-human animal species known to use insulin as a weapon.[24] Cone snail insulin is capable of binding to human insulin receptors and researchers are studying its use as a potent fast-acting therapeutic insulin.[25]
Shell collecting
The intricate color patterns of cone snails have made them one of the most popular species for
shell collectors.[26][27]
Conus gloriamaris, also known as "Glory of the Seas", one of the most famous and sought-after seashells in past centuries, with only a few specimens in private collections. The rarity of this species' shells led to high market prices for the objects, until the habitat of this cone snail was discovered, which decreased prices dramatically.[28]
As jewelry
Naturally occurring, beach-worn cone shell tops can function as beads without any further modification. In
Hawaii, these natural beads were traditionally collected from the beach drift to make
puka shelljewelry. Since it is difficult to obtain enough naturally occurring cone snail tops, almost all modern puka shell jewelry uses cheaper imitations, cut from thin shells of other species of mollusk, or made of plastic.
Until 2009 all species within the family
Conidae were placed in one genus, Conus. Testing of the
molecular phylogeny of the Conidae was first conducted by Christopher Meyer and Alan Kohn,[29] and has continued, particularly with the advent of nuclear DNA testing.
In 2009, J.K. Tucker and M.J. Tenorio proposed a classification system consisting of three distinct families and 82 genera for living species of cone snails. This classification is based on
shellmorphology,
radular differences,
anatomy,
physiology, and
cladistics, with comparisons to molecular (DNA) studies.[30] Published accounts of Conidae that use these new genera include J.K. Tucker & M.J. Tenorio (2009), and Bouchet et al. (2011).[31] Tucker and Tenorio's proposed classification system for the cone shells and other clades of
Conoidean gastropods is shown in
Tucker & Tenorio cone snail taxonomy 2009.
Some experts, however, still prefer to use the traditional classification. For example, in 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".[32] Debate within the scientific community regarding this issue has continued, and additional
molecular phylogeny studies are being carried out in an attempt to clarify the issue.[30][33][34][35][36][37][38][39][40][41]
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]
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^Tenorio, M. J. & Monteiro, A. J. (2008). The Family Conidae. The South African species of Conus. In: Poppe, G. T. & Groh, K. (eds): A Conchological Iconography. Hackenheim: ConchBooks. 47 pp., 60 pls.
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^Tenorio MJ, Tucker JK, Chaney HW (2012). "The Families Conilithidae and Conidae. The Cones of the Eastern Pacific". In Poppe GT, Groh K (eds.). A Conchological Iconography. Hackenheim: ConchBooks. p. 112.
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"Conidae J. Fleming, 1822". World Register of Marine Species (WoRMS). ' Traditionally, all cone shells were included in the Linnaean genus Conus. Tucker & Tenorio (2009) proposed an alternative shell- and radula-based classification that recognized 4 families and 80 genera of cones. In 2011, WoRMS, still recognized a single family Conidae (following Puillandre et al. 2011), but Tucker & Tenorio's 80 genera classification was presented as "alternative representation"
^C.M.L. Afonso & M.J. Tenorio (August 2011), A new, distinct endemic Africonus species (Gastropoda, Conidae) from Sao Vicente Island, Cape Verde Archipelago, West Africa, Gloria Maris 50(5): 124–135
^P. Bouchet, Yu I. Kantor, A. Sysoev, and N. Puillandre (March 2011), A New Operational Classification of the Conoidea, Journal of Molluscan Studies 77:273–308, at p. 275.
^N. Puillandre, E. Strong, P. Bouchet, M. Boisselier, V. Couloux, & S. Samadi (2009), Identifying gastropod spawn from DNA barcodes: possible but not yet practicable, Molecular Ecology Resources 9:1311–1321.
^R.L. Cunha, R. Castilho, L. Ruber, & R. Zardoya (2005), Patterns of cladogenesis in the venomous marine gastropod genus Conus from the Cape Verde Islands Systematic Biology 54(4):634-650.
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BBC Nature Video Cone snails are silent assassins of the sea, drugging sleeping fish before poisoning them