ADAPTATION

The dart-shooting behavior displayed during the simultaneous hermaphroditic reproduction of land snails is a phenomenon that has only recently received sufficiently credible explanation with respect to its adaptive value.  The adaptive value of a behavior, defined as its ultimate causation, is an explanation of its function in contribution to the organism’s reproductive fitness (Tinbergen 1963).  Several theories have been posited in attempt to account for this exceedingly unusual reproductive behavior.  While the theory of sexual conflict is currently considered to be the model for the most plausible function of the evolution of dart-shooting behavior across phylogenetic boundaries (See PHYLOGENY), it is entirely possible that the function of dart-shooting can be attributed to several different modes of enhancing fitness on the level of particular species.


Snails in courship. Courtesy of Ronald Chase, McGill University.


Nuptial Gift Theory

This theory hypothesized that the calcium carbonate in the love dart serves as a nuptial gift by contributing a material essential for egg production to the receiver of the dart.  According to this model, the dart provides the receiver and the pair’s offspring with a direct benefit by aiding in the production of eggs and thus increasing the probability of the offspring’s survival.  In a study in 1998, Koene and Chase determined that some species do not produce enough calcium carbonate to impact the production of eggs, and the recipients in several species do not entirely incorporate the dart, disallowing the absorption of the available calcium carbonate (Koene and Chase 1998).  Thus the nuptial gift theory may not be applied universally as an explanation of the function of love-darts in all snails that display the behavior.


Sexual Signal / Female Choice Theory

In 2000, Baminger et al. explored the possibility that the dart may serve as a sexual signal that conveys information to the receiver concerning the shooter’s reproductive fitness.  This theory suggests that receivers select for effective shooters, and implies that the production of the costly dart might be an indicator of the shooter’s overall reproductive fitness.  This theory also assumes that shooting ability is a heritable trait that is displayed consistently by individuals, since it is an indicator of overall fitness.  However, Baminger et al. found that calcium deficiency doesn’t prevent the production of darts (implying that dart production does not reflect the health of the individual), and in some species there are large differences in an individual’s performance of the behavior (Baminger et al. 2000).  Thus we can conclude that dart production is not an accurate reflection of an individual’s overall reproductive fitness in all species, and therefore we are encouraged to look for a more illuminating answer to the problem of this behavior’s function.


Courtesy of www.kingsnake.com/westindian/metazoa1.html. Mating Pleurodontid snails in the US Virgin Islands.


Sexual Conflict / Control Theory

The existence of conflicting interests between the sexes has been observed and identified in many species with two sexes, but the possibility for its importance in hermaphroditic species has until relatively recently been left unexplored.  In fact, Darwin denied the possibility of sexual selection acting on hermaphrodites (Darwin 1871).  This conception might be attributed to the fact that the idea of sexual selection is based on the condition that some traits that are advantageous to the reproductive fitness of one sex are maladaptive to the other sex.  Thus a conflict of interest between the individuals acting as the sperm recipient and the individual acting as the sperm donor might be considered impossible in hermaphrodites because all individuals play both recipient and donor roles at the same time.  In species with two sexes, the conflicting interests between genders results in a kind of “arms race”, manifested in the co-evolution between male and female reproductive traits.  Interestingly, recent findings have shown that the dart shot by snails induces a physiological change in the receiver that inhibits the digestion of sperm and thus the execution of cryptic female choice that a receiver might practice without the dart’s interruption (Koene and Chase 1998).  The dart can thus be seen to increase the shooter’s reproductive success by enhancing the ability of the shooter’s sperm to avoid digestion and to fertilize a larger number of the receiver’s eggs, overriding the receiver’s mechanism for cryptic female choice.  In their 2001 study, Landolfa et al. found that the effective shooting of darts positively affected the paternity percentage of offspring of the receiver’s clutch when the receiver has mated with individuals of differing dart shooting efficiencies (Landolfa et al. 2001).


Figure from study by Landolfa et. al. When two snails were permitted to inseminate a single snail acting as a female, they found that the "male" that shot the dart deeper into the "female" (thereby achieving a better "dart shoot score") had a higher percentage of paternity of the resulting offspring.

The control hypothesis that dart shooting increases reproductive success by inhibiting cryptic female choice is supported by the fact that in all species that exhibit this behavior, mating involves the intromission of the penises of both individuals in a single mating event.  Thus, both snails benefit from the behavior of dart-shooting, since it has the potential to increase the reproductive success of both individuals; the snails do not act only as a receiver during one mating and then only as a donor during mating with another partner, and therefore the plausibility of the dart-shooting behavior as evidence of sexual conflict is supported (Schilthuizen 2005).  To put it simply, the efficient shooting of the dart allows for the shooter to have more reproductive success and conflicts with the interests of the receiver, but since both snails in a pair play both roles at the same time this conflict between the shooter and the receiver is embodied in each individual, and the possibility of an “arms race” of coevolutionary reproductive traits is possible.  Despite strong phylogenetic evidence for this hypothesis (see PHYLOGENY), no real evidence has yet been found that suggests that the shooting of darts directly negatively affects the fitness of the receiver (Koene and Schulenburg 2005), but it does have a very real consequence in preventing sperm selection by the receiver and thus potentially the reproductive fitness of its offspring