Optimal Foraging of Whelks in the Intertidal Zone

Many biotic communities are characterized by foundational species that transition from a food source to habitat as they increase in size (e.g., trees, coral reefs). Given the importance of these species to community diversity, understanding how size mediates interactions between these species and their predators is an important step in understanding community dynamics. Mussel beds provide both a source of food and a refuge for many species in the intertidal zone. Although previous work has suggested other keystone intertidal predators (e.g., sea stars) may be limited in their ability to prey upon large mussels, the impact of size on whelk feeding has not been explored. We used optimal foraging theory to examine how the relationship between whelks (Nucella emarginata) and mussels (Mytilus californianus) changes with mussel size.
Larger mussels offer more consumable tissue per mm and whelks are able to consume a wide range of mussel sizes regardless of their own length. However, field observations suggest whelks actively select their mussel prey. The size distribution of consumed mussels collected in the field differed from the observed distribution of mussel sizes. Growth studies in lab and field conditions revealed that whelks grow more quickly on smaller mussels. Together, these experiments suggest that although whelks can consume large mussels, they actively select prey to maximize their own growth as predicted by optimal foraging theory. However, future environmental change, such as ocean acidification, may weaken mussel shells, thus impacting this relationship and leading to community changes.c