The function of oral nuptial gifts in Pomacea canaliculata

BURELA S.; MARTIN P. R.

San Miguel de Tucumán

Workshop; 4th International Workshop on the Biology of Ampullariidae; 2010

Pomacea canaliculata is a dioecious snail that shows a multiple mating behavior and males face up to the consequent risk of sperm competition. Males have a copulatory apparatus with a delicate penis that runs through a muscular channeled sheath that present three different glands. The most conspicuous one is the outer gland which during copulation often secrets a drop of mucus whose function is still puzzling. It has been previously suggested that the function of the secretion would be to repel prowler males. However, during copulation, females eat this secretion, a behavior can be interpreted as an oral nuptial gift. This behavior could constitute an additional mating effort of the male to entice the female to remain in copulation for longer periods than her optimum or to increase her reproductive output in the short-term in order to raise his paternity rate on her progeny. The objective of this study was to test the three aforementioned functions of this secretion in order to elucidate the evolutionary significance of this behavior. To test for the aversive effect of the mucus secretion, the penis sheath of 15 freshly killed field males was cut in two, one part with the outer gland (+G) and the other without it (-G), which were macerated separately. Each macerate was suspended in distillated water as to achieve a concentration of 0.05 g/ml. Aliquots of 1 ml of each suspension were placed in Eppendorf tubes and frozen. For the trials, experimental snails were positioned in the base of a T-maze and Eppendorf tubes with +G and –G were randomly placed at opposite arms of the maze. The T-mazes were covered with plastic film and filled with tap water at 25 °C in each observation. We observed the choice of 25 females and 25 males (from the field). The observation period lasted 30 minutes and each reaction and behavior was registered. The males showed no preference for any suspension, while the females chose the side with the solution suspension +G more frequently than expected at random, indicating that they had been attracted by the mucus secretion. To evaluate if the oral gifts prolong the copulation duration, we fed females during copulation with doses of jellified suspensions of +G and -G, which in order to facilitate the feeding, were provided through plastic cannulae. The snails were paired in aquaria filled with tap water at 25 °C. After 2 h of copulation, a group of females was fed with one dose of +G jelly and second group with one of –G; the treatments were repeated at 30 min intervals until the copulation ended. No jelly was given to the females of a third control group of couples. There was no difference between treatments, indicating that the consume of the mucus secretion did not affect the copulation duration. To test the effect of the oral gift on the female reproductive output, we conducted an experiment with four treatments. The experimental females were collected from the field one week before the start of the experiment. The females collected were sexually mature, assuming that all of them had the similar reproductive history. They were acclimatized during one week in individual aquaria filled with tape water and kept in controlled conditions of light (14 h light) and temperature (25±2 °C). Afterwards they were randomly assigned to one of the following treatments: 1) only lettuce, 2) lettuce + 1 ml of solution –G, 3) lettuce + 1ml of solution +G, 4) lettuce + 3 ml of solution +G. Females were kept isolated during three weeks and their eggs masses were separated in order to count the number of eggs per egg masses, egg masses, etc. There were no differences in the total number of eggs or egg masses deposited by females subjected to the different treatments. These results provided little support for the hitherto tested functional explanations of the oral gift behavior: the oral gifts do not modify the female reproductive output in the short-term and males cannot prolong their copulations with them; no repellent effect on prowler males seems evident either. The female attraction to the mucus secretion is, until now, the only clue to find out the significance of this behavior in Pomacea canaliculata.