Influence of body size on tube feet morphology and attachment capacity in the sea urchin Holopneustes purpurascens (Temnopleuridae)

Body size is an important feature of how species interact with their environment, and can influence predation risk, requirements and type of diet consumed, habitat suitability, and susceptibility to environmental pressures. Echinoderms, such as sea urchins, often inhabit areas subjected to wave disturbance and tidal flow, and are able to withstand such forces using a unique method of gel adhesion that allows individuals to adhere to the substratum in these hostile environments. We assessed attachment capacity in relation to body size for the Australian sea urchin Holopneustes purpurascens, which lives enmeshed in two distinct types of algae, attached by tube feet. Relationships between the tube feet, which occur around the entire body, and body size were examined. The number of tube feet, tube foot disc surface area, and fully attached footprints increased as individual body size increased. Our results also demonstrated that all tube feet were functionally adhesive, enabling full enmeshment in the host alga. Our findings indicate that the attachment capacity is positively correlated with body size and that H. purpurascens display both morphological and adhesive characteristics that enable a size-driven host plant distribution. This change in attachment capacity may play an important role in the ontogenetic shift observed in H. purpurascens, with future research suggested to explore the relationship between attachment capacity and adhesive tenacity within this species and other echinoderms.