Designer snails

The Algorithmic Beauty of Sea Shells (2nd edn) by Hans Meinhardt Springer-Verlag, 1998. DM89.00/$54.95/￡34.00 hbk (xi+236 pages) ISBN 3 540 63919 5.     

O-Glycosylation of snails

The glycosylation abilities of snails deserve attention, because snail species serve as intermediate hosts in the developmental cycles of some human and cattle parasites. In analogy to many other host-pathogen relations, the glycosylation of snail proteins may likewise contribute to these host-parasite interactions. Here we present an overview on the O-glycan structures of 8 different snails (land and water snails, with or without shell): Arion lusitanicus, Achatina fulica, Biomphalaria glabrata, Cepaea hortensis, Clea helena, Helix pomatia, Limax maximus and Planorbarius corneus. The O-glycans were released from the purified snail proteins by β-elimination. Further analysis was carried out by liquid chromatography coupled to electrospray ionization mass spectrometry and - for the main structures - by gas chromatography/mass spectrometry. Snail O-glycans are built from the four monosaccharide constituents: N-acetylgalactosamine, galactose, mannose and fucose. An additional modification is a methylation of the hexoses. The common trisaccharide core structure was determined in Arion lusitanicus to be N-acetylgalactosamine linked to the protein elongated by two 4-O-methylated galactose residues. Further elongations by methylated and unmethylated galactose and mannose residues and/or fucose are present. The typical snail O-glycan structures are different to those so far described. Similar to snail N-glycan structures they display methylated hexose residues.     

Lichen endozoochory by snails

Endozoochory plays a prominent role for the dispersal of seed plants. However, for most other plant taxa it is not known whether this mode of dispersal occurs at all. Among those other taxa, lichens as symbiotic associations of algae and fungi are peculiar as their successful dispersal requires movement of propagules that leaves the symbiosis functional. However, the potential for endozoochorous dispersal of lichen fragments has been completely overlooked. We fed sterile thalli of two foliose lichen species (Lobaria pulmonaria and Physcia adscendens) differing in habitat and air-quality requirements to nine snail species common in temperate Europe. We demonstrated morphologically that L. pulmonaria regenerated from 29.0% of all 379 fecal pellets, whereas P. adscendens regenerated from 40.9% of all 433 fecal pellets, showing that lichen fragments survived gut passage of all snail species. Moreover, molecular analysis of regenerated lichens confirmed the species identity for a subset of samples. Regeneration rates were higher for the generalist lichen species P. adscendens than for the specialist lichen species L. pulmonaria. Furthermore, lichen regeneration rates varied among snail species with higher rates after gut passage of heavier snail species. We suggest that gastropods generally grazing on lichen communities are important, but so far completely overlooked, as vectors for lichen dispersal. This opens new ecological perspectives and questions the traditional view of an entirely antagonistic relationship between gastropods and lichens.     

Population genetics of freshwater snails

Freshwater snails have attracted the attention of biologists for a long time, because they are intermediate hosts of schistosomes, agents of bilharziases. However, population-genetic studies of freshwater snails have been undertaken only during the past decade, covering topics such as the relative roles of genetic drift and gene flow in subdivided populations and the roles of extinction and recolonization events in determining population structure. Other studies in freshwater snails have investigated the maintenance of sex and the evolution of selling, widening a debate restricted mainly to plant populations. The possible role of parasites in freshwater-snail population genetics has also been investigated.     

The growth of juvenile snails in water conditioned by snails of a different species

Summary Two species of freshwater snails, Physa acuta and Lymnaea sp. aff. columella, were collected from Asabata marsh, Shizuoka Japan. Individuals of both species inhabit the same plants. Individuals of P. acuta are more abundant than those of L. sp. aff. columella. Experiments were conducted to examine the effect of water conditioned by snails on the growth of 10-day-old juvenile P. acuta snails. Juvenile snails in water conditioned by L. sp. aff. columella grew faster than those in water conditioned by P. acuta or only lettuce. The effects of water conditioned by P. acuta differed among the litters. The results suggest that juvenile P. acuta snails experience accelated growth in the presence of L. sp. aff. columella. The freshwater snails interacted through resource competition as well as through substances disolved in the water.     

Water regulation in aestivating snails

Summary Aestivating snails form abundant lamellate vesicles in the cells of the mantle collar, an epithelium known to regulate the rate at which water is lost from its surface. Since lamellate vesicles are much reduced in hydrated mantle tissue of recently stimulated animals it is tentatively concluded that the vesicles, and their contents, form a barrier to water movement within these cells. X-ray microanalysis of unfixed thin sections shows that there is a concentration gradient of ions within these cells in aestivating animals which is not present in stimulated snails.Financial support from the National Research Council of Canada and the Atkinson Foundation. Toronto, is gratefully acknowledged. The work was performed during the sabbatical leave of one of us, using the facilities of the Department of Zoology and the Faculty of Medicine Electron Microscope Unit (supported, in part, by grant M.A. 4038 of the MRC of Canada), University of Toronto     

Polymorphic snails on varied backgrounds

The general matching of shell colours to background colours seen in terrestrial gastropods implies that, as in insects, visual predation is an important factor which has a long term directional effect. Polymorphism in snails is associated with background heterogeneity, but the causal relation of polymorphism to heterogeneity is not obvious. Predation could maintain polymorphism if predators are frequency dependent in their choice of prey. However, the appropriate predator behaviour does not depend directly on background heterogeneity. An indirect contribution could be that the heterogeneity serves to lower the signal:noise ratio during the predation process. Background heterogeneity could have a direct effect if the background provided specific elements mimicked by the morphs. The remaining diversity could aid the process by lowering the signal:noise ratio. Polymorphism could be maintained if there was frequency-dependent niche selection on the part of prey. Background diversity would then be directly involved. It is necessary that there should be independent control of numbers in the different niches. In warm conditions the niche selection could come about because darker morphs, which gain more radiant heat than paler ones, will move to more cryptic sites by seeking the shade. For morph frequences to approach equilibrium values closely it is necessary for the alleles controlling the polymorphism to exhibit dominance. Differences in received energy could make pale morphs disadvantageous compared with dark ones at low temperatures but advantageous at high ones. In spatially varied temperature conditions a polymorphism could be generated without predation. Density-dependent selection is again required.     

Sinistral snails and gentlemen scientists

A single gene mutation can cause a complete left-to-right and right-to-left inversion of the body axis. This mutation exemplifies a large class of so-called maternal effect genes that regulate key events during early development. This principle was first established nearly one hundred years ago, principally by the work of amateur naturalists.     

Yersiniae in pond water and snails

A Yersinia enterocolitica serotype 9 was isolated from pond water; Y. enterocolitica-like bacteria were also isolated from pond water and from three species of snails (Lymnaea palustris elodes, Helisoma sp., Oxyloma retusa) from the Edwin S. George Reserve in southeastern Michigan. There was evidence for biochemical stability among some of the organisms over a period of years. There also was evidence of transmission of these organisms to snails from the water.     

Genital divergence in sympatric sister snails

A pattern of greater divergence in mating traits between sister‐species pairs with overlapping ranges than between allopatric species pairs is expected if reinforcement commonly contributes to speciation. Few large‐scale comparative analyses have addressed this prediction, especially for genital form. Here, we show that penial morphology follows the predicted pattern in 40 robustly identified sister‐species pairs in the marine gastropod subfamily Littorininae. Further work is needed to exclude other processes that may contribute to genital divergence between sympatric species, but the clear pattern we observe strongly suggests a role for genital form in reproductive isolation in this large clade.