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SEM image of lateral view of a love dart of the snail Monachoides vicinus. The scale bar is 500 μm (0.5 mm).
Drawing showing a side view of the love dart of the edible snail Helix pomatia.
1 = flared base of the dart
2 = position of the inner cavity
3 = longitudinal flanges or vanes
4 = sharp tip or blade of the dart
A love dart from Helix aspersa (garden snail) compared to a 50 cent euro coin about 24 mm across.

A love dart is a hard, sharp, calcareous or chitinous dart which some hermaphroditic land snails and slugs create. Love darts are made in sexually mature animals only, and are used as part of the sequence of events during courtship before actual mating takes place. Darts are quite large compared to the size of the animal: in the case of the semi slug genus Parmarion, the length of a dart may be up to one fifth that of the semi slug's foot.[1]

Prior to copulation, each of the two snails (or slugs) attempt to "shoot" one or more darts into the other snail (or slug). There is no organ to receive the dart; this action is more analogous to a stabbing, or to being shot with an arrow. The dart does not fly through the air to reach its target however; instead it is fired as a contact shot.

The love dart is emphatically not a penial stylet (in other words this is not an accessory organ for sperm transfer). The exchange of sperm between both of the two land snails is a completely separate part of the mating progression. Nevertheless, recent research shows that use of the dart can strongly favor the reproductive outcome for the snail who is able to lodge a dart first in its partner. This is because mucus on the dart introduces a hormone-like substance that allows far more of its sperm to survive.

Love darts, also known as shooting darts, or just as darts, are shaped in many distinctive ways which vary considerably between species. What all the shapes of love darts have in common is their harpoon-like or needle-like ability to pierce.

Courting Helix aspersa snails in Ireland, the one on the right has a fired love dart embedded in its body.

Contents

The mating dance

Courtship in the edible snail, Helix pomatia
Drawing of the head of a Helix pomatia prior to mating, showing the everted penis, and the dart sac in the process of shooting a love dart.
S - dart sac (bursa telae)
D - love dart
P - penis

Mating begins with a courting ritual. For example, in land snails of the genus Helix, including the escargot Helix pomatia, and the common garden snail Helix aspersa (also known as Cornu aspersa and Cantareus aspersus), copulation is preceded by an elaborate tactile courtship.

The two snails circle around each other for up to six hours, touching with their tentacles, and biting lips and the area of the genital pore, which shows some preliminary signs of the eversion of the penis. As the snails approach mating, hydraulic pressure builds up in the blood sinus surrounding the organ housing the dart. Each snail manoeuvres to get its genital pore in the best position, close to the other snail's body. Then, when the body of one snail touches the other snail's genital pore, it triggers the firing of the dart.[2]

The darting can sometimes be so forceful that the dart ends up buried in the internal organs.[3] It can also happen that a dart will pierce the body or head entirely, and protrude on the other side.[4]

After both snails have fired their darts, the snails copulate and exchange sperm.

A snail does not have a dart to fire the very first time it mates, because the first mating is necessary to trigger the process of dart formation.[1] Once a snail has mated, it fires a dart before some, but not all, subsequent matings. A snail often mates without having a dart to use [5], because it takes time to create a replacement dart. In the case of the garden snail Helix aspersa, it takes a week for a new dart to form.[6]

The dart is shot with some variation in force, and with considerable inaccuracy, such that one-third of the darts that are fired in Helix aspersa either fail to penetrate the skin, or miss the target altogether. [7] Snails have only very simple visual systems and cannot see well enough to use vision to help aim the darts.

Function

External images
Love dart which has pierced through the head of a snailNew Scientist[4]
Love darts of Cepaea nemoralis in action
Love dart of Helix aspersa in action
Love dart of Helix aspersa in action

Although the existence and use of love darts in snails has been known for at least several centuries, until recently the actual function of love darts was not properly understood.

It was long assumed that the darts had some sort of "stimulating" function, and served to make copulation more likely. It was also suggested that dart might be a "gift" of calcium. These theories have proved not to be accurate; recent research has led to a better understanding of the strange phenomenon of love darts.

The two species that have been studied the most are Helix aspersa the garden snail, and Helix pomatia the edible escargot. In Helix aspersa (aka Cantareus aspersus), the dart is coated with a special mucus which contains a hormone-like substance. This substance contracts one part of the female half of the reproductive system of the snail that is struck with the dart, and this allows many more sperm to survive, significantly increasing the likelihood of a successful fertilization.[8][9]

Morphology of darts

The love dart, also known as a "gypsobelum", is often made of calcium carbonate[10] which is secreted by a specialized organ within the reproductive system of several families of air-breathing snails and slugs, mainly in terrestrial pulmonate gastropod mollusks within the infraorder (or order) Stylommatophora.

Darts can range in size from about 30 mm long in the larger snail species, down to about 1 mm in the smallest snails that have darts. Typically most darts are less than 5 mm long, but still they are substantial compared with the size of the animal.[1]

Simplified diagram of the reproductive system of a pulmonate land snail
D = love dart
S = stylophore or dart sac (bursa telae)
P = penis
V = vagina
G = genital pore
MG = mucus glands

There is considerable variety in both the overall shape and the cross section of the love dart. The morphology (shape and form) of the dart is species-specific. For example, individual snails of the two rather similar helicid species Cepaea hortensis and Cepaea nemoralis can sometimes only be distinguished by examining the shape of the love dart and the vaginal mucus glands (which in the anatomical diagram are marked "MG" and are positioned off of the structure marked "V".)

Anatomical context

Note: The taxonomic placement of all the families mentioned in this article follows the taxonomy of the Gastropoda by Bouchet & Rocroi (2005).

There is a complex hermaphroditic reproductive system in pulmonate snails (those snails that have a lung rather than a gill or gills.) Their reproductive system is completely internal, except for the active protrusion (eversion) of the penis for copulation. The outer opening of the reproductive system is called the "genital pore"; it is positioned on the right hand side, very close to the head of the animal. This opening is virtually invisible however, unless it is actively in use.

Drawing of a transverse section of the dart sac (also known as the bursa telae) of Helix pomatia during the process of creating a new love dart.

The love dart is created and stored before use in a highly muscular internal anatomical structure known as the stylophore or dart sac (also known as the bursa telae). The exact positioning of the stylophore varies, but it is in the vicinity of the eversible penis and the vagina, where these two structures open into the "atrium", a common area right inside the genital pore.

The opening of the stylophore leads directly into the atrium in certain species in the families Vitrinidae, Parmacellidae, Helminthoglyptidae, Bradybaenidae, Urocyclidae, Ariophantidae, and Dyakiidae. The opening of the stylophore can instead lead to the penis, as is the case in some species of Aneitinae (a subfamily of Athoracophoridae), Sagdidae, Euconulidae, Gastrodontidae and Onchidiidae. Alternatively, it can lead to the vagina, as in the case in some species of Ariopeltinae (a subfamily of Oopeltidae), Ariolimacinae (a subfamily of Ariolimacidae), Philomycidae, other species within the Bradybaenidae, and also in the Hygromiidae, Helicidae and Dyakiidae.[11]

Only two families have darts present in every species: the Bradybaenidae and in the Dyakiidae. In all the other families there is reduction or loss of dart-making ability in some of the species (cf.[11]).

Many species have only one dart sac, however other species have several. Snails in the family Bradybaenidae have more than one dart sac, and some species of Hygromiidae and Helmintoglyptidae have four dart sacs.[1] Some Urocyclidae have up to 70 darts.[1]

Occurrence within the pulmonate snails and slugs

All pulmonate land snails are hermaphrodites, and have a complete and rather elaborate set of both male and female reproductive organs (see the simplified anatomical diagram above), but by no means do all pulmonate land snails have love darts or a dart sac.

Calcareous darts

Calcareous (composed of calcium carbonate) darts are found in a limited number of pulmonate families[1] within the Stylommatophora.

Most of these families are within the land snail superfamily Helicoidea[12]: Helicidae, Bradybaenidae, Helminthoglyptidae, Hygromiidae, Humboldtianidae (previously considered to be a part of the Hygromiidae).

Calcium carbonate darts are also found in the family Zonitidae within the superfamily Zonitoidea, and in one family of slugs, the Philomycidae, which are within the superfamly Arionoidea.[13]

Lightly calcified darts occur in the snail and semi-slug family Urocyclidae, within the superfamily Helicarionoidea.[1]

Chitinous darts

Chitinous (composed of chitin) love darts occur in the pulmonate land snail families Ariophantidae (superfamily Helicarionoidea), in the family Helicarionidae (superfamily Helicarionoidea), in the Vitrinidae (superfamily Limacoidea), and in the slug family Parmacellidae (superfamily Parmacelloidea).

Within the more ancient suborder Systellommatophora, chitin darts are found in the pulmonate sea slugs of the family Onchidiidae, in the superfamily Onchidioidea.[1]

Cartilaginous darts

Love darts made of cartilage occur in the family Gastrodontidae.[11]

Evolution of love darts

Because of the presence of darts in many superfamilies of the Stylommatophora, it seems likely that love darts appeared during the early evolution of the Pulmonata, and that the ancestors of the Stylommatophora possessed darts already.

During evolution, darts appear to have been lost secondarily, i.e., after they had evolved and been functional. Vestigial darts (ones that exist only in a rudimentary condition) occur in the family Sagdidae.[11], and in many Helicoidea, the surrounding organs have also degenerated (become non-functional). The sarcobelum is a fleshy or cuticle-coated papilla which is considered to be a degenerated, previously dart-bearing, organ.[11]

Species variability

Love darts are shaped in many distinctive ways, and vary considerably between species. The morphology of the dart is almost always species specific.

Some darts have a round cross section, others are bladed or vaned. In some cases the blades on the sides of the dart are bifurcated or divided into two parts. Some darts are shaped like a needle or a thorn, others have a tip like an arrowhead, or look like a dagger. What all the shapes have in common is their ability to pierce.

Images

Note: both the Scanning electron micrographs (SEMs) and the drawings below are taken from, or modified from, Koene & Schulenburg, 2005.

SEM images of love darts from eight different species of pulmonate land snails. The upper images show a lateral view, the lower images show a cross-section.

Trichia hispida. The scale bar is 500 μm (0.5 mm).
Xerarionta kellettii. The scale bar is 500 μm.
Bradybaena similaris. The scale bar is 500 μm (0.5 mm).
Chilostoma cingulatum. The scale bar is 500 μm.
Humboldtiana nuevoleonis. The scale bar is 50 μm.
Leptaxis erubescens. The scale bar is 50 μm.
Cepaea hortensis. The scale bar is 50 μm (0.5 mm).
Monachoides vicinus. The scale bar is 50 μm.

The following tables or charts show numerous examples of love dart morphology, on a family by family and species by species basis. Not all families and species are included. The drawings show first the cross section, and then the lateral view, of the dart in that particular species. Darts vary in size according to the size of the snail or slug species, but here they are all shown at the same size, for purposes of comparison.

Helicidae

Shape of love dart Species References
Arianta arbustorum dart.svg Arianta arbustorum
Cepaea hortensis dart.jpg Cepaea hortensis
(also shown in the SEM images)
Cepaea nemoralis dart.jpg Cepaea nemoralis
Chilostoma cingulatum dart.jpg Chilostoma cingulatum
(also shown in the SEM images)
Chilostoma glaciale dart.jpg Chilostoma glaciale
Chilostoma planospira dart.jpg Chilostoma planospira
Eobania vermiculata dart.jpg Eobania vermiculata
Helicigona lapicida dart.jpg Helicigona lapicida
Cantareus aperta dart.jpg Helix aperta = Cantareus aperta
Cantareus aspersus dart.jpg
(more images)
Helix aspersa = Cantareus aspersus
Helix lucorum dart.jpg Helix lucorum
Helix lutescens [14]
Helix pomatia dart.jpg
(another image)
Helix pomatia [10][14][15][16][17] and many other works
Leptaxis nivosa and Leptaxis undata [18]
Leptaxis erubescens dart.jpg Leptaxis erubescens
(also shown in the SEM images)
[18]
Marmorana scabriuscula dart.jpg Marmorana scabriuscula
Marmorana serpentina dart.jpg Marmorana serpentina
Otala lactea dart.jpg Otala lactea
Theba pisana dart.jpg Theba pisana

Elonidae

Shape of love dart Species References
Elona quimperiana [19]
Norelona pyrenaica [19]

Bradybaenidae

Shape of love dart Species References
Aegista vulgivaga dart.jpg Aegista vulgivaga
Bradybaena similaris dart.jpg Bradybaena similaris
(also shown in the SEM images)
Euhadra amaliae dart.jpg Euhadra amaliae
Euhadra quaesita dart.jpg Euhadra quaesita
Euhadra sandai dart.jpg Euhadra sandai
Fruticicola fruticum dart.svg Fruticicola fruticum

Helminthoglyptidae

Shape of love dart Species References
Helminthoglypta nickliniana dart.jpg Helminthoglypta nickliniana
Helminthoglypta tudiculata dart.jpg Helminthoglypta tudiculata
Monadenia fidelis dart.jpg Monadenia fidelis
Polymita picta dart.jpg Polymita picta
Xerarionata kellettii dart.jpg Xerarionta kellettii

Hygromiidae

Shape of love dart Species References
Cernuella cisalpina dart.jpg Cernuella cisalpina
Cernuella hydruntina dart.jpg Cernuella hydruntina
Cernuella virgata dart.jpg Cernuella virgata
Helicella itala dart.jpg Helicella itala

Each snail in this species has 2 darts

Hygromia cinctella dart.jpg Hygromia cinctella
Perforatella incarnata dart.jpg Monachoides incarnatus = Perforatella incarnata
Monachoides vicinus dart.jpg Monachoides vicinus
(also shown in the SEM images)
Perforatella bidentata dart.jpg Perforatella bidentata
Pseudotrichia rubiginosa dart.jpg Pseudotrichia rubiginosa
Trichia hispida dart.jpg Trochulus hispidus = Trichia hispida
(also shown in the SEM images)

Each snail in this species has 2 darts

Trichia striolata dart.jpg Trochulus striolatus = Trichia striolata

Each snail in this species has 2 darts

Xeromunda durieui dart.jpg Xeromunda durieui
Xerosecta cespitum dart.jpg Xerosecta cespitum
Xerotricha conspurcata dart.jpg Xerotricha conspurcata

Humboldtianidae

Shape of love dart Species References
Humboldtiana nuevoleonis dart.svg Humboldtiana nuevoleonis
(also shown in the SEM images)

Each snail in this species has 2 darts

Ariophantidae

Shape of love dart Species References
reproductive system,
d = dart
Ariophanta laevipes [20]

Helicarionidae

Shape of love dart Species References
Parmarion sp. [1]

Philomycidae

A slug family

Shape of love dart Species References
The dart is thick and curved. Philomycus carolinianus [21][22]
Philomycus togatus [23]
Philomycus virginicus [24]

Urocyclidae

Some species in this family have spiral darts[1], and some darts have (citation:) "minute barbs pointing toward the tip".[1]

Shape of love dart Species References

Vitrinidae

Shape of love dart Species References

Parmacellidae

Species of slugs within this family have spiral darts.[1]

Shape of love dart Species References

Gastrodontidae

Shape of love dart Species References
Zonitoides arboreus [1]
image
(with darts of other species)
Zonitoides sp.

(This species was probably either Z. nitidus or Z. excavatus)

[25]

The Cupid connection

AmorI.svg

Some writers have commented on the parallel between the love darts of snails, and the love darts fired by the mythological being Cupid, known as Eros in Greek mythology.[7] It is even possible that there is a connection between the behavior of the snails and the myth. Malacologist (mollusk expert) Ronald Chase of McGill University said about the garden snail Helix aspersa, "I believe the myth of Cupid and his arrows has its basis in this snail species, which is native to Greece". He added, "The Greeks probably knew about this behavior because they were good naturalists and observers."[26]

In some languages, the dart that these snails use before mating is known as an "arrow". For example, in the German language it is called a Liebespfeil or "love arrow", and in the Czech language it is šíp lásky (which means "arrow of love").

Dart-like structures in other gastropod groups

The toxoglossans

Marine gastropods in the families Terebridae, Turridae and Conidae, (collectively known as the toxoglossans, meaning "poison tongue") use a poison dart or harpoon, which is a modified radula tooth, primarily made of chitin. These snails are carnivorous hunters: the harpoon is used in predation. When the snail reaches its prey, it injects a toxin into it. In most cases the prey is marine worms, but in the case of some larger cone snails, it is small fish.

Opisthobranchs

Opisthobranch gastropods are hermaphrodites, as are the pulmonates, however opisthobranchs do not have love darts. Nonetheless, some of them do stab one another during mating, using hardened anatomical structures. For example in the Cephalaspidean genus Siphopteron, both seaslugs attempt to stab their partner with a two-part, spined penis.[27]

References

  1. ^ a b c d e f g h i j k l m Chung D. J. D.: Molluscan 'Love darts'? Hawaiian Shell News, 1986 May, Vol 34(5), pages 3-4. http://s190418054.onlinehome.us/HSN/1980/8605.pdf
  2. ^ Rogers, DW & Chase, R (2001): Dart receipt promotes sperm storage in the garden snail Helix aspersa. Behavioral Ecology and Sociobiology, 50(2):122–127. Download
  3. ^ Reproductive behaviour (2008) In Encyclopædia Britannica. Retrieved October 03, 2008, from Encyclopædia Britannica Online
  4. ^ a b 'Love darts' double snail's chance of offspring New Scientist, 20 March 2006. The photo in this article shows a dart which has pierced the body entirely, and is protruding on the other side.
  5. ^ and Reproduction written by Robert Nordsiek, Retrieved August 8th 2009
  6. ^ [1] In New Scientist, Roxanne Khamsi, published 20th March, 2006, cited August 5th 2009
  7. ^ a b Are Snails' Love Darts Source of Cupid Lore? McGill Tribune, 13 February 2002.
  8. ^ Chase, R and Blanchard, KC (2006) The snail's love-dart delivers mucus to increase paternity Proceedings of the Royal Society of London B, Biological Sciences, 273(1593):1471-1475. Download.
  9. ^ News: Further secrets of the snail love dart, published 23 March 2006, cited 6 October 2008.
  10. ^ a b Hasse, B; Marxen, JC; Becker, W; Ehrenberg, H and Epple, M (2002) A crystallographic study of the love dart (gypsobelum) of the land snail Helix pomatia Journal of Molluscan Studies, 68 : 249-254.
  11. ^ a b c d e Barker G. M.: Gastropods on Land: Phylogeny, Diversity and Adaptive Morphology. in Barker G. M. (ed.): The biology of terrestrial molluscs. CABI Publishing, Oxon, UK, 2001, ISBN 0-85199-318-4. 1-146, cited pages: 97, 141, 144.
  12. ^ Joris M. Koene and Hinrich Schulenburg: Shooting darts: co-evolution and counter-adaptation in hermaphroditic snails. - BMC Evolutionary Biology, 2005, 5:25. doi:10.1186/1471-2148-5-25.
  13. ^ B. J. Gómez: Structure and Functioning of the Reproductive System. 307-330, page 324. in: Barker G. M. (ed.): The biology of terrestrial molluscs. CABI Publishing, Oxon, UK, ISBN 0-85199-318-4.
  14. ^ a b Koralewska-Batura E.: Die Struktur der Liebespfeile Gattung Helix Linnaeus (Gastropoda: Pulmonata: Helicidae). Folia Malacologica, 1994, Volume 5, 115-118. (abstract)
  15. ^ Reproduction of the Roman snail
  16. ^ Reproduction of Helix pomatia
  17. ^ Mating and Reproduction of the Roman snail
  18. ^ a b Koene, JM and Muratov, IV (2004) Revision of the reproductive morphology of three Leptaxis species (Gastropoda, Pulmonata, Hygromiidae) and its implication on dart evolution. Malacologica, 46(1): 73-78. Download
  19. ^ a b Gittenberger E. 1979. On Elona (Pulmonata, Eloniadae fam. nov.) Malacologia Volume 18, 1-2, Sixth European Malacological Congress, Amsterdam, 139-145.
  20. ^ Godwin-Austen H. H. 1908. Mollusca. Testacellidae and Zonitidae. Taylor & Francis, London, page 25.
  21. ^ http://snailstales.blogspot.com/2005/04/dissection-selection-philomycus.html "Snail's Tales" blog of Aydin Örstan
  22. ^ Tompa AS: The ultrastructure and mineralogy of the dart from Philomycus carolinianus (Pulmonata: Gastropoda) with a brief survey of the occurrence of darts in land snails. Veliger 1980, 23:35-42.
  23. ^ Ken Hotopp: Land Snails of Pennsylvania: Philomycus togatus (Gould, 1841), published online January 2 2006.
  24. ^ Fairbanks H. L. March 24 1993. The reproductive anatomy of Philomycus sellatus Hubricht, 1972 and Philomycus virginicus Hubricht, 1953 (Gastropoda: Philomycidae). The Nautilus, vol. 107, number 1, pages 9-13. page 10.
  25. ^ http://delta-intkey.com/britmo/www/zonitida.htm cited 6 October 2008
  26. ^ Lovebirds and Love Darts: The Wild World of Mating
  27. ^ Anthes, N and Michiels, NK (2007) Precopulatory stabbing, hypodermic injections and unilateral copulations in a hermaphroditic sea slug Royal Society publishing. Biology Letters, 3(2):121-124. Download

Further reading

External links








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