The love-darts of land snails: integrating physiology, morphology and behaviour

Several land snail species of the helicoid and limacoid superfamilies possess one or more love-darts, which seem to have evolved as a result of conflict over the fate of donated sperm and/or as a way to select the most fit sperm donor. A love-dart is a calcareous stylet used during mating encounters to pierce the partner's body wall. When used, it carries accessory gland mucous products that influence the partner's physiology. Most of the knowledge on the effects of the glands' mucus derives from a single well-studied species, Cornu aspersum, in which the mucus increases the male reproductive success of the dart user. However, detailed descriptions on the use of the dart are limited to just a few other species. Hence, here we compare physiological, morphological and behavioural aspects concerning love-darts in several dart-bearing species. Patterns in the use of the dart are identified according to family and we discuss the coevolution of the morphology of the dart and anatomical traits of the reproductive system. The reported physiological effects caused by the dart mucus suggest a common function of the dart in increasing male reproductive success. Nevertheless, caution is needed when generalizing the use and effects of the love-dart, which are predominantly based on one model organism.