Tenacity of Attachment in Two Species of Littorinid, Littorina Littorea (L.) and Littorina Obtusata (L.)

Here we present data concerning the ability of two littorinids to remain attached to a slate substratum as a dislodging force is applied vertically. Specimens of Littorina littorea (L.) and L. obtusata (L.) were attached to a simple pulley and sandflow system which applied force to the animals at a constantly increasing rate until the animals released from the substratum. Three aspects of tenacity were measured: force required to remove the winkles from the substratum; time taken for removal; and final displacement at removal. For L. littorea the mean force required (1.09 N), the mean displacement (8.9 mm) and the mean time taken (185 s) were all found to be significantly greater than those recorded for L. obtusata (means = 0.60 N, 7.5 mm and 102 s, respectively). However, when animal size was taken into account there was no significant difference in either force or time required for removal, although L. obtusata showed a greater displacement than did L. littorea. Animal dry flesh weight was not a useful predictor of any of the tenacity variables. Rugosity of the slate substrata was shown to have a significant effect on all tenacity variables, greater forces being required to remove both species of winkle from smooth substrata than from rough substrata. This also generally held true when animal size was accounted for. The results of these experiments, although insufficient to identify the mechanism(s) of attachment, do discount the mechanism of ‘muscular grip’, since greater tenacity would be expected on rough substrata. Whether winkles were immersed or emersed had no significant effect on any of the tenacity variables. Recorded tenacities suggest that many L. littorea and L. obtusata would be dislodged by the direct forces created by breaking waves.