An Exploratory Study In To Parasite Induced Shell Shape Variation In Littorinid Species

Littorina saxatilis is one of the most phenotypically variable of the littorinid species, demonstrating phenotypic plasticity both within and between geographically distinct populations. A large number of studies have been conducted to ascertain the effects of extrinsic factors on shell shape variation. Research has show that variation can occur in response to location on the shoreline, exposure levels and predation pressure. Recently a number of studies have been conducted to investigate the effects of parasitism on littorinid morphology. Findings showed that parasite infestation strongly contributed to shell shape variation. The aims of this investigation were to; investigate whether parasites caused variation in shell shape, to examine the extent of any parasite induced variation using unparasitised littorinid populations as a comparative measure, and to examine the role of gender in shell shape variability. Specimens of L. saxatilis, L. littorea and L. compressa were collected from 3 UK shorelines at; Filey Brigg, the River Mersey and Anglesey. Shell shape variance was explored using extended eigenshape modelling. Results indicated that a parasitised population of L. saxatilis demonstrates a higher level of shell shape variation than an unparasitised littorinid population. This occurred on both on an inter- and intra-species level. Interestingly males from the parasitised population showed a higher level of general shell shape variation, whereas females showed a higher degree of localisation in their variation. The physiology of the process by which parasitism can change host shell shape is still not fully understood, though it is likely to be dependent on a number of factors, including the species of both the host and parasite, the time of infection in relation to the host life span and the gender of the host. Based on the findings of this study there is a broad scope for further research into littorinid host-parasite interactions, as well as gender based morphometric differences.