Bruno Baur

Repeated mating and female fecundity in the simultaneously hermaphroditic land snail Arianta arbustorum

Summary To evaluate the influence of repeated mating on female fecundity in the simultaneously hermaphroditic, self-incompatible land snail Arianta arbus-torum, the number and size of clutches, egg size and hatching success of individuals from 3 populations were determined under conditions of isolation and grouping during one breeding season in a field cage experiment. Only adult snails which had mated and oviposited in the preceding year were used. Sperm storage enabled isolated individuals to continue with the production of fertilized eggs. Snails kept singly or in groups differed neither in number of clutches laid nor in egg size. But isolated snails laid smaller clutches than did grouped snails. As a result isolated snails produced fewer eggs per breeding season. Hatching success varied greatly between populations and rearing conditions. In general, isolated snails had fewer hatchlings than grouped snails, indicating that individuals prevented from remating suffered a reduced fitness.

Population regulation in the land snail Arianta arbustorum: density effects on adult size, clutch size and incidence of egg cannibalism

SummaryThe relationships between local population density and adult size, clutch size and spatial distribution of egg batches were investigated in 11 natural populations of the land snail Arianta arbustorum in a forest near Uppsala, Sweden. Shell size of adults decreased with increasing population density as did clutch size. Within populations, clutch size scaled allometrically with shell size indicating size-specific fecundity. It is hypothesized that food unpalatability caused by mucus deposition slows down juvenile growth rate in high density populations, resulting in small adults and thus reducing their fecundity in subsequent years. The influence of the distance between batches on the incidence of egg cannibalism by hatchlings was examined in a laboratory experiment. In this experiment the number of eggs cannibalized increased with decreasing distance to the batch of hatching snails. Thus, in the field, eggs of highly aggregated batches suffer a high risk of cannibalism. In the 3 populations with the highest snail density, 21–39% of all batches were deposited close to each other (nearest neighbour distance ≤5 cm, i.e. less than hatchlings more within 1 day). These findings indicate that egg cannibalism can act as a population regulating factor.

Proximate factors influencing egg cannibalism in the land snail Arianta arbustorum (Pulmonata, Helicidae)

SummaryFactors affecting oophagy among siblings in the land snail Arianta arbustorum were studied in 3 populations from different altitudes in Switzerland. The degree of egg cannibalism in A. arbustorum is a function of hatching asynchrony since the earliest hatched snails will devour the unhatched eggs in the same clutch. Clutch size, egg density and amount of vegetable food available to newly hatched snails did not affect the degree of cannibalism. Snails from 3 populations were similar in terms of incubation time and intrinsic hatching asynchrony of the clutches. However, they differed in degree of cannibalism when the hatching asynchrony had been experimentally increased. Snails from a lowland forest showed a higher degree of cannibalism than did those from an alpine mountain slope. The parent snails differed in terms of incubation time and hatching synchrony in their clutches. Under natural conditions, the length of the hatching spread and, as a result, the degree of cannibalism will depend additionally on the mode of oviposition (batches or single; clumped or dispersed), on the spatial heterogeneity of egg-laying places and on climatic conditions (e.g. drought).

Egg-species recognition in cannibalistic hatchlings of the land snailsArianta arbustorum andHelix pomatia

Laboratory experiments were conducted to determine whether cannibalistic hatchling of the land snailsArianta arbustorum andHelix pomatia discriminate between eggs of the two species. Hatchlings from both species showed a significant choice for conspecific eggs and consumed on average 1 egg in 4 days. Eggs from the other species were only occasionally eaten.

Andrena wilkella male bees discriminate between enantiomers of cephalic secretion components.

Diastereomers of the spiroacetal, 2,8-dimethyl-1,7-dioxaspiro [5.5]undecane, represent main components of the cephalic secretion from males of the solitary bee,Andrena wilkella. The major compound proved to be of high enantiomeric purity, showing (2S,6R,8S) configuration. Only the naturally occurring enantiomer attracted patrolling males in the field; its antipode was behaviorally inactive and in a racemic mixture did not inhibit response. The (E,Z) diastereomers were also found to be almost inactive. EAG studies gave the same result as the behavioral tests. The biological function of the spiroacetal is discussed in view of the evolution of the mating behavior inA. wilkella.

Can Cannibalistic Hatchlings of the Land Snail Arianta Arbustorum Distinguish Between Sib and Non-Sib Eggs?

In the field hatchlings of the land snail Arianta arbustorum have been observed devouring eggs from sib as well as non-sib clutches. In the laboratory newly hatched A. arbustorum were tested for their ability to distinguish between eggs of different degrees of relatedness. In these experiments hatchlings did not discriminate between sib, other kin and non-kin eggs. Snails from three populations showed the same egg preferences, but they differed in their propensity for cannibalism. Despite the apparent lack of sib and kin recognition, egg cannibalism in A. arbustorum may be adaptive, even among siblings, because of the nutritional and energetic benefits.