How does sex reversal occur in Crepidula slipper shells?

Crepidula slipper shells are sequential hermaphrodites that begin their lives as males. Their transition to the female sex is determined by their proximity to other individuals.123

These snails typically remain male as long as they are positioned near a female. However, they will transform into females if they become isolated or are placed near another male. In some instances, these snails stack themselves in piles, where the younger individuals on top are males, those in the middle are hermaphroditic, and those on the bottom are females that have lost their male reproductive organs through degeneration.34

The transition is influenced by the social composition of the group; for example, a decrease in the number of females in a stack can induce males to assume female characteristics, while an excess of females may retard this transition. Ultimately, the degree of maleness or femaleness in these gastropods is likely controlled by a combination of environmental and internal factors.4