Love dart

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 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 during the sequence of events between courtship and mating. Darts are quite large compared to the size of the animal: in the case of the genus Parmarion, the length of a dart may be up to one fifth that of the snail's foot.

Both of the two snails (or slugs) attempt to "shoot" one or more darts into the other snail (or slug) before the mating process is completed. There is no organ to receive the darts; 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, and the exchange of sperm between both individuals 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: 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 (see the scanning electron microscope images below). What all the shapes have in common is their harpoon-like or needle-like ability to pierce.

The mating dance

Courtship in the edible snail, Helix pomatia

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 preceeded by an elaborate tactile courtship.

Drawing of the head of a mating Helix pomatia 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

The two snails circling 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.

The darting can sometimes be so forceful that the dart ends up buried in the internal organs. Sometimes the darts pierce the body or head entirely, and protrude on the other side.

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. But once a snail has mated, it fires a dart before some, but not all, subsequent matings. A snail may fairly often mate without having a dart to use because it takes time to create a replacement dart.

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. Snails have only very simple visual systems and cannot see well enough to use vision to help aim the darts.

Function

External images
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.

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.

Anatomical context

Diagram of the reproductive morphology 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

The love dart (also known as a "gypsobelum") is usually made of calcium carbonate 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.

The hermaphroditic reproductive system of pulmonate snails is complex. It is all completely internal, except for the active protrusion (eversion) of the penis during copulation. The outer opening of the reproductive system is called the "genital pore"; it is positioned on the right hand side, near 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 (bursa telae) of Helix pomatia during the process of creating a new love dart.

The love dart is created and stored before use in an internal anatomical structure known as the stylophore or dart sac (bursa telae). The neck of the stylophore is positioned next to the eversible penis, and the vagina, where all three structures open into the "atrium", a common area right inside the genital pore.

There is considerable variety in both the overall shape and the cross section of the love dart. The morphology of the dart is species-specific. For example, individuals of the two rather similar 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 are marked "MG" and are positioned off of the structure marked "V" in the anatomical diagram.)

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

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 anatomical diagram above), but not all pulmonate land snails have love darts or a dart sac.

The calcium carbonate version of the love dart is found in a limited number of pulmonate families. Most of these families are within the land snail superfamily Helicoidea: Helicidae, Bradybaenidae, Helminthoglyptidae, Hygromiidae, Humboldtianidae (previously considered to be a part of the Hygromiidae).

Calcium carbonate darts are also found in the Zonitidae, and in one family of slugs, the Philomycidae.

Darts in the family Urocyclidae are lightly calcified.

Love darts made of chitin occur in in the pulmonate land snail families Ariophantidae, Parmarionidae, Vitrinidae, and the slug family Parmacellidae.

Chitin darts are also found in the pulmonate sea slugs of the family Onchidiidae, which are within the order Systellommatophora.

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 (see the scanning electron micrographs immediately below).

Some darts have a round cross section, others are bladed or vaned. In some cases the blades are bifurcated. 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.

Scanning electron micrograph images of love darts from eight different species of pulmonate land snails
The upper images show the lateral view of the dart of each species; the scale bar is 500 μm (0.5 mm) The lower images show the cross-sections of the same darts; the scale bar on these is 50 μm
Upper row: Trichia hispida, Xerarionta kellettii, Bradybaena similaris, Chilostoma cingulatum Lower row: Humboldtiana nuevoleonis, Leptaxis erubescens, Cepaea hortensis, Monachoides vicinus

The following charts show love dart morphology on a family by family and species by species basis. The drawings show first the cross section, and then the lateral view, of the dart in that particular species.

Helicidae

Shape of love dart	Species	References
	Arianta arbustorum
	Cepaea hortensis
	Cepaea nemoralis
	Chilostoma cingulatum
	Chilostoma glaciale
	Chilostoma planospira
	Eobania vermiculata
	Helicigona lapicida
	Helix aperta (Cantareus aperta)
	Helix aspersa (Cantareus aspersus)	- image
	Helix lucorum
	Helix lutescens
	Helix pomatia	and many other works - image
	Leptaxis nivosa and Leptaxis undula
	Leptaxis erubescens
	Marmorana scabriuscula
	Marmorana serpentina
	Otala lactea
	Theba pisana

Bradybaenidae

Shape of love dart	Species	References
	Aegista vulgivaga
	Bradybaena similaris
	Euhadra amaliae
	Euhadra quaesita
	Euhadra sandai
	Fruticicola fruticum

Helminthoglyptidae

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

Hygromiidae

Shape of love dart	Species	References
	Cernuella cisalpina
	Cernuella hydruntina
	Cernuella virgata
	Helicella itala
	Hygromia cinctella
	Monachoides incarnatus (Perforatella incarnata)
	Monachoides vicinus
	Perforatella bidentata
	Pseudotrichia rubiginosa
	Trochulus hispidus (Trichia hispida)
	Trochulus striolatus (Trichia striolata)
	Xeromunda durieui
	Xerosecta cespitum
	Xerotricha conspurcata

Humboldtianidae

Shape of love dart	Species	References
	Humboldtiana nuevoleonis

Ariophantidae

Shape of love dart	Species	References

Parmarionidae

Shape of love dart	Species	References
	Parmarion sp.

Philomycidae

A slug family

Shape of love dart	Species	References
	Philomycus carolinianus	The dart is thick and curved.
	Philomycus togatus

Urocyclidae

Some species in this family have spiral darts. There are also some darts (citation:) "with minute barbs pointing toward the tip".

Shape of love dart	Species	References

Vitrinidae

Shape of love dart	Species	References

Parmacellidae

Species of slugs within this family have spiral darts.

Zonitidae

The "true glass snails"

Shape of love dart	Species	References
	Zonitoides arboreus
	Zonitoides sp. (probably Zonitoides nitidus and Zonitoides excavatus)	(image - with darts of other species)

The Cupid connection

Some writers have commented on the parallel between the love darts of snails, and the love darts fired by the mythological being Cupid, also known as Eros in Greek mythology. It is possible there is a connection between the behaviour of the snails and the myth. Professor Ronald Chase of McGill University in Montreal commented, "I believe the myth of Cupid and his arrows has its basis in this snail species, which is native to Greece". Chase added, "The Greeks probably knew about this behavior because they were pretty good naturalists and observers."

In some languages other than English, the love dart 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 "an arrow of love").

The use of darts and stabbing in other gastropod groups

Cone snails

A live textile cone, Conus textile, one of the three most dangerous cone snails to handle.

Marine cone snails (not at all closely related to pulmonate land snails) use a different type of dart for a very different purpose: not for reproduction, but for predation.

The darts of cone snails physically resemble small harpoons, as do love darts, but cone snail darts have a totally different anatomical origin: each cone snail dart is a modified tooth, primarily made of chitin. Cone snail darts are formed inside the mouth of the snail, in a structure known as the radula. (Pulmonate land snails also have a radula, as do almost all snails and indeed, the majority of mollusks. The radula in most gastropods has rows of many small teeth, and is used for rasping at food and scraping it into the mouth.)

All cone snails are predatory; small species of cone snails hunt small prey such as marine worms. Larger cone snails hunt fish. The tropical cone snail Conus purpurascens uses its special modified radular teeth to fire a retrievable hollow dart at small fish and injects a toxin. The toxin rapidly paralyses the fish, which the cone snail then swallows.

All cone snail species are equipped with a battery of toxic darts which can fire in any direction, even backwards. Some of these toxins can be fatal to humans.

Opisthobranchs

Like pulmonate gastropods, Opisthobranch gastropods are hermaphrodites, but they 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, (Anthes, N and Michiels, NK (2007).

Further reading

• 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
• Chase, R and Vaga, K (2005) Independence, not conflict, characterizes dart-shooting and sperm exchange in a hermaphroditic snail Journal Behavioral Ecology and Sociobiology, 59(6):732-739. Preview
• Chase, R (2007) Gastropod reproductive behavior Scholarpedia, 2(9):4125
• Chung DJD (1986) Initiation of growth of the first dart in Helix aspersa Journal of Molluscan Studies, 52:253–255.
• Davison, A; Wade, CM; Mordan, PB and Chiba, S (2005) Sex and darts in slugs and snails (Mollusca: Gastropoda: Stylommatophora) Journal of Zoology, 267 : 329-338. Cambridge University Press.
• Koene, JM and Schulenburg, H (2005) Shooting darts: co-evolution and counter-adaptation in hermaphroditic snails BMC Evolutionary Biology, 5:25
• Koene, JM and Chiba, S (2006) The way of the samurai snail American Naturalist, 168(4):553-5. Download
• Koene, JM (2006) Tales of two snails: sexual selection and sexual conflict in Lymnaea stagnalis and Helix aspersa Integrative and Comparative Biology, 46(4):419-429,
• Landolfaa, MA; Greena, DM and Chase, R (2001) Dart shooting influences paternal reproductive success in the snail Helix aspersa (Pulmonata, Stylommatophora) Behavioral Ecology, 12(6):773-777. Download Lay summary
• Leonard, JL (2006) Sexual selection: lessons from hermaphrodite mating systems Integrative and Comparative Biology, 46(4):349-367. Download
• Michiels, N (2006) Battle of the hermaphrodites: sexes clash even when sharing the same body Science News, 16 Sep 2006.
• Pomiankowskia, A and Reguera, P (2001) The point of love Ecology & Evolution. 16(10)533-534.

External links

• Lovebirds and Love Darts: The Wild World of Mating National Geographic News: Valentine's Day Special Report, 13 Feb 2004.
• A dart in the dark
• Mating and Reproduction of the Roman snail
• Snail darts A moment of science.
• Love that love dart
• Cupid myth influenced by snails?
• Snail-Sex: How do they do it?
• Reproduction of Helix pomatia
• The evolution of love darts and shooting behaviour in land snails
• Love Darts in the Backyard
• The anatomy of a snail
• Shot Through the Head, for a Reason. New York Times, 6 Jun 2006.
• Snail sex
• Reproduction of the Roman snail
• Strawberry snail (Trichia striolata). Life on an Oxfordshire lawn.
• Hunt S.: The structure and compositon of the love dart (gypsobelum) in Helix pomatia.. Tissue Cell. 1979; 11(1): 51-61.