GASTROPOD CHEMORECEPTION

Preference of Biomphalaria tenagophila among macrophytes and their periphytons determined through the degree of attractiveness

In presence of extracts of six flowering plants the Biomphalaria tenagophila was more attracted to four them in the following sequence: Nasturtium pumilum > Polygonum acre > Commelina sp. = Echinochloa crusgalli. The periphyton of these flowering plants attracted in the same way the B. tenagophila but without no preference for either of them. Reporting the results that behavior may be evaluated as a co-evolution between snail and plants.

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

The 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.

Autoradiographic evidence for receptor cell renewal in the olfactory epithelium of a snail

The tentacles of the terrestrial snail Achatina fulica contain an epithelium at their tips which is specialized for olfaction. The histology of the snail's olfactory organ bears a striking resemblance to that of the olfactory mucosa in the nose of vertebrates, where the receptor cell population is known to undergo a continuous process of renewal. In the present experiments, [3H]thymidine was delivered as a single pulse that was determined to have a maximum duration of about 1 h. Thirty minutes after an injection of [3H]thymidine, presumptive precursor cells were found labeled within, or at the edges of, receptor cell lobules. At later survival times, label was seen over cells that were identified as receptors. The mean position of the labeled cells within the layer of receptor cells became progressively more superficial with increasing survival times, indicating an upward migration of newly differentiated cells. The labeling index in the snail is ca. 0.7%, compared to 0.9% in the mouse. The turnover time is about 45 days, compared to 30-45 days in the mouse.

Interactions between the leech Glossiphonia complanata and its gastropod prey

Predator-prey interactions between the predatory leech, Glossiphonia complanata, and its gastropod prey were investigated in laboratory experiments, including behavioural observations with the aid of time-lapse video technique. Six gastropod species were investigated, viz. Lymnaea peregra, Planorbis planorbis, Physa fontinalis, Ancylus fluviatilis, Bithynia tentaculata, and Theodoxus fluviatilis. The species studied exhibited anti-predator defences, which had their maximum efficiency at different stages of encounter with G. complanata. The activity of B. tentaculata decreased with increasing leech activity, but was relatively higher when food was present than when not. Handling times were dependent on the time elapsed since the previous meal was captured (intercatch interval), which in turn was related to the size of the previous prey. Handling time was also related to the size of both predator and prey. The capture efficiency was high for small prey and the leeches spent more time in patches with higher yield. They were, however, unable to discriminate between patches of different prey density.

The carboxylic and amino acid chemoreception niche of Bulinus rohlfsi (Clessin), the snail host of Schistosoma haematobium

The response of the freshwater snail, Bulinus rohlfsi, to gradients of carboxylic acids, amino acids and some related compounds was investigated by means of diffusion olfactometers. The snails proved to be very discriminating in their responses to the chemical species tested and there were also very marked ontogenetic differences. Although 15 distinctive types of amino and carboxylic acids proved to be attractants and arrestants, only octanoic acid proved effective for snails of all ages. Possible mechanisms involved in the chemoreception of active compounds, the ecological, physiological and biochemical relevance of the responses and possible application to controlled release technology are discussed.