All plants and animals bind toxic metals to specific proteins, to minimise their impact. Unlike cadmium and mercury, lead has no such protein and we have a poor understanding of its toxicity in the animal kingdom. Amongst snails it is commonly found to be the least toxic though some claim it to be the most closely regulated of the major toxic metals. Our research has been examining the evidence for a regulatory mechanism for lead. Garden snails (Cantareus aspersus) have a long-term of store lead in their shell, rather like the bone of vertebrates, but with these invertebrates we can easily separate this storage site from the sensitive soft tissues, to analyse its movement through the animal.
Our recent work shows that lead uptake may be determined by growth or cell turnover in the soft tissues. Interestingly, this differs between populations: snails from the Mediterranean Basin have very different dynamics to those of southern England. Most significantly, our work finds little indication of a specific and evolved regulatory mechanism for lead in Cantareus.
Charles Darwin noted the significant role that sexual selection might play in defining a species and it is now clear that this has contributed to the speciation of several major groups. Sexual selection theory is best developed and most easily tested for species where the sexes are separate and the genders vie with each other to promote the survival of their genes. This becomes problematical when male and female gametes are produced in the same individual and when they are swapped during copulation.
The garden snail Cantareus aspersus is such a simultaneous hermaphrodite, where a large individual may have an advantage over competing partners. A larger partner is obviously successful, indicating a fit genotype, but it may also be favoured because of its capacity to provision the fertilized eggs. We have attempted to link size to resource availability, and in particular, the need for calcium to support egg production.
Unlike slugs, snails are iteroparous – they will breed several times during their life – and the large, highly mineralised, external shell may be an important part of this strategy. Beyond its protective function, we have suggested the shell may serve to facilitate repeated reproductive events - with every clutch of eggs, as much as half of the calcium in the soft tissues may be donated to the brood. The shell may thus represent an important reserve that facilitates the production of large broods and allow for short intervals between copulations.
Dr Alan Beeby
Tel: 020 7815 7912
Email: beebya@lsbu.ac.uk
Dr Larry Richmond
Tel: 020 7815 6229
Email: richmol@lsbu.ac.uk
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