Which of the following is a natural behavior that contributes to zebra mussel spread?

The correct answer highlights the behavior of zebra mussels in attaching themselves to boats and equipment, which is a significant natural factor contributing to their spread. When zebra mussels attach to submerged surfaces, they can be transported unintentionally from one water body to another via recreational and commercial boating activities. This ability to cling to hard surfaces allows them to travel with humans, thus facilitating their introduction into new ecosystems where they can establish populations and disrupt local environments.

Zebra mussels do reproduce rapidly, but their method of expanding populations through human interaction via equipment is a more critical aspect of their spread. While other options may involve elements of their biology or ecology, they do not encapsulate the direct human-related vector as effectively as the attachment to boats and equipment does. Understanding this behavior is essential for managing and preventing the further spread of zebra mussels in aquatic systems.