The author states that what helps Sanibel attract a variety of shells are the currents.
1. Currents play a significant role in bringing shells to the shores of Sanibel.
2. The movement of water currents helps transport shells from various locations and bring them to the beaches of Sanibel.
3. These currents act as natural collectors, gathering a diverse array of shells from different regions and depositing them along the shoreline of Sanibel Island.
Therefore, the currents are highlighted as the factor that contributes to the diverse selection of shells found on Sanibel's beaches.
