Adhesion in the life history of the Australian sea urchin, Holopneustes purpurascens

The Australian sea urchin Holopneustes purpurascens undergoes a substantial increase in body size before risking the transition from its initial host alga Delisea pulchra to the much preferred kelp, Ecklonia radiata. It is thought that smaller urchins lack the adhesive strength of larger urchins and are unable to enmesh themselves in the fronds of the more rigid E. radiata, and therefore must settle in the softer D. pulchra. Using a number of key morphological and adhesive traits we aimed to determine the role of temporary gel adhesion in this shift. While larger urchins had more tube feet, a larger disc surface area and a greater adhesive coverage, there was no significant difference in the detachment force required between large, medium and small urchins on both host plants. Detachment force was significantly less for urchins on D. pulchra than those on E. radiata and is a finding suggested for further research. Laminar flow flume testing indicated that the smallest urchins adhered better under high flow rates, declining as size increased. Overall we concluded that adhesion is a substantial contributor to the ontogenetic shift of H. purpurascens in conjunction with one or more alternate mechanisms.