Why do Crepidula slipper shells form stacks?

Crepidula slipper shells form stacks primarily due to their unique reproductive strategy as sequential hermaphrodites.1

These marine snails begin their lives as males and later transform into females. When they aggregate in stacks, the individuals at the top of the pile are typically younger and function as males, while the larger, older individuals at the bottom have transitioned into females. This stacking behavior facilitates internal fertilization, as the male-phase snails are positioned to mate with the female-phase snails located beneath them in the stack. This process ensures efficient reproduction within the population.12