Size-related aspects of arm damage, tissue mechanics, and autotomy in the starfish Asterias rubens

Arm damage is a widely reported but superficially investigated aspect of th e biology of the starfish Asterias rubens L. In the present study, the inci dence of arm damage was surveyed in populations of A. rubens at two sites i n the Firth of Clyde, Scotland, and three sites in Gullmarsfjorden, Sweden. The mean (+/-SD) incidence across all sites of individuals with basal arm damage (resulting from detachment at the basal autotomy plane) was 19.69 +/ - 8.86%, the incidence of those with distal arm damage (resulting from ampu tation at more distal levels) was 7.74 +/- 10.01%. The mean incidence of ar ms with basal damage was 5.28 +/- 4.12%, of those with distal damage 1.83 /- 2.45%. There was a significant negative correlation between size and the incidence of basal damage at all but one site, but no significant correlat ion between size and distal damage at any site. Mechanical tests on specime ns of the aboral body wall from the basal region of the arm (which included the autotomy plane) and from a more distal region revealed that with incre asing body size there was a significant increase in yield stress, ultimate stress and Young's modulus (stiffness) but no significant change in yield s train and ultimate strain. There was no significant difference between the relationships for basal and distal specimens. It is hypothesised that in la rger individuals increased mechanical toughness replaces autotomy as an eff ective antipredator strategy. Using two methods to induce autotomy, a signi ficant positive correlation between size and the delay between the onset of stimulation and arm detachment was found; this may represent a size-relate d decline in the efficiency of the autotomy mechanism through the relaxatio n of selection pressure. Since size is an unreliable indicator of age in A. rubens, the trends identified herein can be interpreted only tentatively a s age-associated phenomena.