Why do many freshwater and land snails not have a free-swimming veliger stage?

Many freshwater and land snails have suppressed the free-swimming veliger stage as an evolutionary adaptation to their non-marine environments.12

In marine habitats, the veliger stage allows for wide dispersal via ocean currents. However, this strategy is disadvantageous in freshwater systems, where such currents would primarily result in downstream dispersal. Furthermore, tiny embryos face significant osmotic challenges when developing in freshwater.2

To overcome these hurdles, these snails have evolved reproductive strategies that bypass the free-swimming larval phase:

• Brooding: Many freshwater prosobranchs retain and brood their young inside the female until they reach a crawling stage.12
• Encapsulation: Pulmonates and other species typically attach egg capsules to rocks, vegetation, or other surfaces, allowing the larvae to pass through the veliger stage protected within the egg mass.12
• Direct Development: By providing a food supply within the egg capsule, the young emerge as crawling juveniles rather than as immature organisms that must forage for plankton in the open water.1

These adaptations protect the developing young from environmental stressors and desiccation while shifting the focus of dispersal to individual movement or accidental transport by other animals.12