Social environment mediates habitat shifts in a range-restricted giant limpet

Abstract

The individuals of many species shift habitats at different stages in their lives. Sometimes, these habitat shifts coincide with an ontogenetic transition. These ontogenetic habitat shifts can allow species to access nutritional resources, avoid conspecific aggression or predation, or secure breeding opportunities when it is advantageous to do so. The Kermadec Islands giant limpet Scutellastra kermadecensis is a protandrous hermaphrodite endemic to Rangitāhua, the Kermadec Islands. These limpets have an unusual habit whereby small limpets (piggies) piggy-back on the shells of larger individuals rather than living on the rocky substrate. We investigated whether the ontogenetic habitat shift between the piggy-backing and rock-attached limpets was determined by the availability of free space on the rock or whether the ontogenetic habitat shift was a response to the properties of the surrounding limpet population. We found that the available rock space did not influence the size at which an individual transitioned from being a piggy to being rock-attached. Furthermore, larger rock-attached limpets were more likely to have piggies, they had more piggies, and the piggies were larger. Overall, our results suggest that Kermadec giant limpets are motivated to piggy-back by the properties of the social environment rather than space constraints. The piggy-backing behaviour may be a mechanism to avoid bulldozing by larger limpets, to access grazing opportunities on the shells of larger limpets, and/or to monopolise breeding opportunities with larger rock-attached females. We discuss the repercussions of this life-history strategy for this extremely range-restricted species, with reference to how these populations may be monitored and maintained.