How did gastropods evolve from ocean habitats into freshwater and land habitats?

The evolutionary transition of gastropods from marine environments to freshwater and terrestrial habitats was a complex process driven by the plasticity of their body and organ systems. This migration occurred independently multiple times across various gastropod groups.12

Adaptations for Freshwater

The shift to freshwater required overcoming significant physiological challenges, particularly regarding reproduction and dispersal. Because the marine veliger stage—which relies on ocean currents for passive transport—would lead to downstream-only dispersal in rivers, it was largely suppressed. Instead, many freshwater prosobranchs evolved to brood their young, while pulmonates began attaching egg capsules to stable surfaces like rocks or vegetation. Furthermore, many freshwater pulmonates are hermaphrodites, allowing for self-fertilization, which enables a single individual to colonize and establish a new population in an isolated body of water.3

Adaptations for Land

To survive on land, gastropods developed several critical strategies to prevent desiccation:
* Respiratory Changes: Land snails evolved a "lung"—a vascularized portion of the mantle cavity—to breathe air, often protected by a small respiratory pore to minimize water loss.45
* Moisture Conservation: Many species secrete mucous coverings or calcium-impregnated seals (epiphragms) across their shell apertures. Others rely on behavioral adaptations, such as remaining in moist niches or burrowing into soil during dry periods.3
* Shell Reduction: In humid environments, some species evolved into slugs, reducing or losing the shell to conserve energy and calcium, as the shell's primary role in preventing desiccation becomes less critical when moisture is abundant.3